P0171 Case Study from Hell 2000 Trans Am Pt3

[@ScannerDannerPremium]

Up late tonight with a severe ear infection, trying to keep my mind off it. And your videos I find do the trick! I don't watch many KZbin vids, just don't have time. But I am enjoying this. I like hearing your thought processes. We think a lot alike. Anyway, at 17:47, when you disconnected the IAC and were commenting on the count not changing. Remember the IAC count is a command, not an input. So we have to view this a little differently, especially stepper motor designs. The only way the PCM can keep an accurate track on the pintle position, is to re-orient it's 0 count on each key cycle. The actual valve position and count command on the scan tool should be the same. However if the valve sticks at all, even for a moment, it then becomes out of sync. So the valve could actually be 30 steps out from 0, but it took an issued command of 50 to get it there. At one point I was seeing 100 on the IAC at idle, would have been nice to see desired idle and rpm pids at that moment. A hypothetical: RPM 600 Desired 700 IAC 30 and rising to open the valve to achieve 700. If the valve is sticking, you may this count go very high, lets say at 100 issued commands to open (counts) the valve finally moves. Idle is steady now. RPM 700 Desired 700 IAC 100 Hope I am making sense. So I haven't watched the rest of the video yet or the newest upload, but let me add one thing. I have read the some fuel control is based off of the issued IAC command. So could a sticking IAC cause fuel trim issues? If this theory is correct, then yes. However, if this valve is sticking, I would expect intermittent high idle, low idle and stalling complaints as well. Not just fuel trim codes and no other driveability problems. Keep up the great work!

[@ScannerDanner]

Oh yea, and when you unplug a stepper motor, you lose the sync as well, so you cannot rely on the count on the scanner to match position anymore until it resets the o count. (cycle the key)

[@SchrodingersBox]

ScannerDanner Danner, it's an absolute honor to have you on my channel. You will enjoy this series... sorry about the ear infection, but sit back and enjoy- this one is right up your alley.

[@rhkips]

At 17:47, the IAC was displaying as voltage, not counts. Is that an interpretation of desired steps, or actual voltage being applied to the IAC? I have to think it's either interpreted, or just completely false. There should be 0 or 5 volts, depending on if it's a ground-controlled or power-controlled. My poor little brain cell is struggling with what's actually happening...

[@ScannerDanner]

rhkips No, it is a stepper motor. Stepper motor displays are always listed as "steps", "counts" or "position". Each control wire on a 4 wire stepper is pulsed on and off as much as 30 times for second, to not pulsing at all (holding). So on all 4 wires you will see either 12 or 0 (not 5 like you were thinking). So a voltage display on the scanner for this type of control circuit would be useless because each wire has a different voltage level and would not indicate position. I like your thinking #rhkips about power and ground side switching. This one is different than other outputs. It is both! In other words it is a reverse polarity circuit for each wire. Sometimes it is grounding the circuit and sometimes it is applying power to the circuit. I wish I could post pictures here. I could describe it so much better with a picture. Hopefully I helped clear things up for you a bit. Lastly, remember that IAC position on the scan tool is a command. You are not looking at the actual position of the valve. The actual position is assumed and should be correct as long as the valve is not sticking.

[@rhkips]

ScannerDanner Ah, alright! I understand now. Thank you, Mr. Danner! I don't know why I was thinking 0/5V; probably because I was watching a bunch of your sensor diagnostics videos last night? XD

[@ScannerDanner]

As I'm now watching you test motor function (nice job btw, you have amazing insight for a DIY guy!) You can test both directions by simply cycling the key. You will need help though because if you do this test without providing a limit. It will wind itself out of the housing. It needs a 0 point (resistance to extending any further). I use my finger as the lmit

[@DENicholsAutoBravado]

Thanks for the tip!

[@johnwrench4speed]

This IAC is a four wire stepper motor. Test(s) 1) DC Voltage Test (using a two channel lab scope): Connect one positive test lead to IAC signal A(+), which is at connector position “D”, connect other positive test lead to IAC signal B(+) which is at connector position “B”, and connect ground test lead to known good ground. When cycling ignition switch on and off, both IAC signal voltages should switch between ground and system voltage as PCM positions IAC. This produces a square wave pattern. Test 2) Resistance Test: Connect to motor terminal windings . Resistance should be 40 to 60 ohms.

[@DIYdake]

Another Great Video ! Parts changers be damned ! Good logic going through all the steps and letting the data lead the way to the answer. The IAC is a bipolar stepper motor .... You can test this removed from the vehicle with an Arduino microcontroller ($20 or less) and an L293 motor driver board($10 or less)..... you can get both parts on ebay for less than $30 total ...... Simple program in arduino with built-in libraries to have in running in short order.... its really simple to use, easiest microcontroller to use for hobbyists. Keep the vids rolling .... learning alot here on your channel!

[@PineHollowAutoDiagnostics]

Hi Matt, AMAZING case study!! I have been on the edge of my chair for this one. Based on Parts 1 and 2, I would have narrowed this phenomenon down to three systems: 1) Air Injecton, 2) EGR, or 3) an inconsistent/incorrect MAF signal. But not IAC, since the car actually has no idle/starting problems whatsoever. At 5:16 you say the idle air control change would not affect the MAF signal. I do not follow your logic here. The MAF meters ALL the air entering the engine, including the air allowed in by the IAC past the throttle plate. So an added load (like A/C) would cause the IAC to open more, letting in more air past the throttle plate, resulting in higher MAP and MAF readings. My educated guess based on this: the MAF is not responding appropriately to the change in air flow. Now this could be due to two things: 1) a faulty/dirty MAF. 2) a crack in the air intake pipe between the MAF and the throttle plate (not technically a vacuum leak). However, you did eliminate the cracked air pipe after the smoke test of the intake manifold. One way to easily eliminate the MAF is to just unplug it!! The car will still run in closed loop, but use the MAP as its main fuel input for the A/F calculation. If the fuel trims are now normal, the MAF is indeed the culprit! Time to watch part 4... Ivan

[@area46241]

Your analysis and method of explaining are excellent. Yours and Danners tutorials helped me determine a stuck IAC on a '01 Kia Sportage today. It wanted to die at idle whenever it felt like staying stuck in it's rotational sleeve. After cleaning it an lubing with Marvel mystery oil (love that stuff) the valve "bounced"back and forth on its shaft (spring mounted) was not aware it did that but that gave smooth idle down back and idle up when needed and the shift quality improved the car had no codes awesome vids last five.... thumbs up!!!

[@SchrodingersBox]

Thanks, anytime I compared to Danner I consider it a huge compliment. I have come to find his channel is my favorite on all youtube and the only channel I watch now. I even became a premium subscriber even though I don't own a scope lol. but I have a lot of catching up to do- he has a ton of vids and I want to watch them all- twice!

[@trainer2018]

Matt if you go to the grid section for data on the Autoengineuity scan tool you can actually pick all the values you want to graph by recording a movie. Save the data, disconnect, and you can pick and choose whichever four pids you want to see relationships to each other. I just bought my Autoengineuity and figured that out. Keep in mind though the more pids you select, the update rate on the scan tool will slow down. Give me a shout if you would like to see a file of mine where I do this.

[@pamelavance648]

Watching Watching learning tired will have to watch part 3 tomorrow 😊 i know the video is older but this series totally hits it out of the park ..thank you for the knowledge you share 😊😊😊😊❤

[@jcsaves9725]

Personal issues 😂! I spent a lot of time binge-watching all these videos, which I'm sure are going to pay for themselves! 💸 You have a very informative adiuance as well, very helpful! Thanks again. On to part #4.

[@SchrodingersBox]

Yes i noticed you are having fun!!

[@jcsaves9725]

@@SchrodingersBox I'm leaning towards (vacume)😂, a vacuum leak. Do you know how much a smoke test normally cost?

[@polymetric2614]

I'm sure you know this by now given this is an old video but the IAC is usually a stepper motor which is an open loop motor - the driver tells it where to go and has absolutely no feedback as to whether it's actually going there or not. It's the same kind of motor used in 3d printers and that's why if you push a 3d printer out of alignment while it's working it'll keep going like nothing's wrong, just in the wrong place. of course if one of these kinds of motors burns out it won't move but you won't get any feedback. at least not with the simplest type of driver. a smarter driver would probably measure the resistance of both coils (there's 2 coils which is why there's usually 4 pins) but obviously this car doesn't do that. the way i'd drive the motor independently of the car would be using an arduino and either a dual-bridge motor driver or a dedicated stepper driver. there's stuff on youtube and all over the internet on how to do that. I know this was 2014 but it was definitely possible back then too. either way its incredibly easy now, you could probably get the parts to do this for under $20 total (12V wall adapter, knockoff arduino, wires, maybe a breadboard depending on what driver you get, and the dual-bridge driver itself). The driver I'd use is either a L298N (dual bridge) or a A4988.

[@joeb7558]

Matt, great video, great series, great Channel! First time comment and recent subscriber! I have to agree with johnwrench4speed about the resistance test on both coils in the IAC with a DVOM. Its usually very accurate in diagnosis on the condition of the part. Unless there is carbon build up inside it should be quick on its reactions or movements. Thank for all the useful info!!

[@bchrisl1491]

Just take a 9v battery and tie the two "low" leads together and connect to negative. With the other leads, alternately connect to the positive side of the 9v. You should see the valve move one step at a time. To make it move the other way, just reverse the connections by tieing the high sides to the negative and alternately connecting the low leads.

[@miayamia4382]

Its funny that now, 9 years later, in small EU country, diagnostic magazine published article about false DPF temperature values and misdiagnostic with Bosch diagnostic system. Solution - use global OBD mode. Pity they didnt explained it better but hey, you did :-) Thank you

[@johnbrass7196]

I had a p0171 (lean condition) driving 2k miles or so, with my 2001 Toyota corolla.conditions: 1 Full power loss at mid throttle when going up a hill , when the car was warming up. When the car was at running temp, the issue would not show up as often. 2. Intermitantly a bad idle, engine would shake at 760rpm, and sometimes would be fine at 860 rpm. 3. At freeway speeds, would run fine, no issues, accept for random lean conditions indicated with scan tool.4. No vacuum leaks (smoke tested)5. Injectors all cleaned, and replaced one, because spray pattern was abnormal.6. Fuel pressure looks fine 44PSI7. Both O2 Sensors look fine and reacting to changes.8. Scan tool indicated a random lean condition, all of the time.9. New spark plugs, and no misfiresThe solution:My Idle Control Valve was sticking, due to carbon build up. It's a magnetic rotary typeCleaned it, and made sure the bearings were lubricated.The car accelerates on hills, and has full power now, Scan tool shows no more lean condition. Thanks 4 this video!!BTW, this motor does not have a feedback mechanism, so it does not know if the motor had actually moved or opened correctly, so this issue throws you off.I am guessing the carbon build up came from the PCV valve, that is the only place it could come from. YES, my PCV valve had a lot of carbon build up on it, and I replaced it as well.

[@SchrodingersBox]

+John Brass I need data bro. Can't help with just a description of symptoms

[@DJDevon3]

I agree with BusJockey. Most motors that move forward & backward will do so by reversing polarity. So you can in fact put a positive and ground to those pins. I don't know of any reason not to try that. You just need to figure out which pins to apply voltage to and how much voltage. Really SUCKS that the car hasn't thrown any IAC codes. From the pins you show for the IAC it doesn't look like it has any active monitoring (no reference voltage) and would rely on any problems via detection of incorrect A/F. However you have incorrect A/F and there's no codes. Hey OBD-II isn't perfect. You're actually getting a really good taste of what I have to go through every day on every sensor of my OBD-I car.. I know your pain.

[@DENicholsAutoBravado]

Billy R added a comment to Busjockey1 that will be very illuminating on what you said, I'd come back to the video and take a look if you haven't already. I have a friend who I'm going to test one sensor at a time since it's OBD-1. Hopefully, logic points me in the right direction first. It's hard to start when it's cold, but only when it's cold. OBD-1.

[@SchrodingersBox]

No Devon, there is no way you can just add 12V DC to these pins- it is separate coils and the voltage alternates polarity constantly between these coils. In the next video this will be demonstrated very clearly. It not only is impossible to just run 12V to the IAC and get the motor to run- you can actually damage it trying!

[@techsavantlove]

***** I agree with nearly everything you've written here. On Matt's IAC, I think we're looking at two coils, one to move the shaft in one direction, the other coil to move it the other direction.

[@techsavantlove]

***** Sorry, but NO NO NO NO NO!. You can NOT put a pure uninterrupted voltage across the coil of an IAC (like Matt says below) or you will destroy the motor (you'll blow the s**t out of the coil).The motor is a very special type of motor called a "stepper" motor. It is very short PULSES of DC that turns it. In this case, each "pulse" turns the motor shaft 90 degrees. PLEASE don't ever put 12 volts pure DC across any stepper motor.

[@DJDevon3]

douglas lee Well that's good to know. Oops. :(

[@EdWeibe]

am working on my Buick right now with similar issues.....and the IAC will be one of the first things I look at.

[@trainer2018]

By looking at the pin out Matt it looks like to me you have two separate windings in the Iac. Here is my theory. Low means either a lower speed the valve moves on those windings and it switches polarity. This function might be solely used at idle. The computer then changes polarity to open and close the valve. Then High could mean a higher speed because there are more windings on the high function, therefore you will have more magnetic field strength and the motor will move faster. Never worked with that type of valve but I bet it works like that. It is also probably pulse width modulated, so if you put a direct 12 volts to it you will take the chance of damaging the valve because it is not designed for a full 100 percent duty cycle. Like Steve said, you need a scope to check this or a manufacturer specific tool that can test it by putting a modulated voltage to it. You could do a ohm check of the seperate windings though, not as good as a dynamic test but you might find something with. Matt can you show me the schematic how the Iac is wired up. Bet everyone could you better if you show the schematic . Good job by the way.

[@SteveRobReviews]

Now you know your stuff. Fantastic

[@trainer2018]

Thanks, I'm a junkie for this kind of stuff, always liked the challenge.

[@billyr9840]

Your right about it being Pulse Width Modulated, I was real nervous when Matt said he was going to put 12volts DC directly to that motor, POOOOOOOF! If it is Positive side switch in the PCM thru a transistor the PCM connects the IAC to + power and creates a ON/Off signal to IAC ( High = 12 volts and would be on, and Low = 0 volts and would off ) or it could also be Ground side switched or grounded thru the PCM and would create a OFF/ON signal where 0 volts would be on and 12 volts would be off. By doing this it creates that square wave on a Oscilloscope that John Wrench mentioned in his post. The amount of time that the PCM keeps this signal ON/OFF is called the Duty Cycle of that IAC/Signal. The PCM uses this for two purposes, one of them is very precise adjustment of the position of the IAC and the other is to control amperage flow so you don't burn out the IAC with to much or unrestricted amperage flow. POOOF! Direct 12 volts DC!

[@SchrodingersBox]

Billy R Exactly Billy- in fact in the info I read while researching the connector wires, it specifically states not to put 12V to the motor. I was actually already aware that IAC's are kiny and aren't tested through typical means, luckily I remembered that and looked up the wiring first. It would definitely have introduced another variable "oh, great so did I fry the IAC now?" lol

[@DENicholsAutoBravado]

Billy R Great comment Billy, and thanks for another great video Matt. How you manage to throw a little fun and humor in it all I don't know. I've been getting less sleep as this video series goes on because I can't stop thinking about everything I've learned! As hard as it might be on some people to hear, I think you can handle this. I'm glad you didn't directly find the problem, in the journey, there are so many that contributed and added greatly to my learning. This whole community is thanked by both of us I'm sure.

[@fivefortyeye540i]

Is that iac duty cycled? Which would explain its speed and behavior in which it extends and extract.. ECM is controlling the magnetic field.. It was working maybe not as expected.. So there is current flow.. So that rule out ECM not having control?

[@techsavantlove]

Corrections, Retractions, and Kudo's. Damn, I wrote all my comments before scrolling down the list and reading everybody elses. I HAVE to stop doing that. Just imagine, if we could open a shop, and have all of these brilliant people all working in one place! We'd own the freakin' town! I made several errors in my post. But it's not practical to go back and edit. BUT, I can tell everybody, that if you read ALL of the posts people have put up here in the last roughly 36 hours or so, about stepper motors, and then add together and synthesize what everybody has said into one big ball of knowledge, you'll know more about steppers than you could ever want to. There are so many people with good posts, it's impossible to name them all, but there are some true experts here. I learned a lot reading other people's posts. I did not know there were stepper motor testers out there for automotive use. That's REALLY cool, I ain't gonna forget that tip. Personally, I agree with 95% of everything that everybody else has written here on the subject, and the 5% I don't agree with I'm probably wrong about. I'm gonna go see if there is a part 4 video up, RIGHT NOW. Carry On............

[@jeffreynobles948]

Matt/Dan, here is a question for you guys. My 01 S-10 4.3L showed no vacuum leak at all when I did a smoke test (cold engine) a while back, but it did a week before under the same conditions. Saturday when I did another smoke test under the exact conditions, it shows smoke coming from the back of the engine. It seems to be hit or miss doing the smoke test. There is a plastic vacuum tube with a rubber connector back there that I'm assuming goes to the fuel tank from the intake for the EVAP. Also, I ordered a Delphi MAF sensor for it a while back, but when I install it, it shows higher LTFTs. If I put the original one back on they come down. Currently they bounce anywhere from 4.9-7.8 on Bank 1. Bank 2 is lower than this. The intake gaskets have been replaced by a buddy of mine last year when I had my hip surgery, so I'm wondering if he didn't install them correctly? I don't have a mirror, which would definitely help in this situation, and I should get one for the next smoke test, but smoke comes from the back of the distributor area. I don't know if it's an intake gasket leak, or maybe that EVAP connector is leaking, etc. What really puzzles me is why would a new Delphi MAF cause the fuel trim numbers to go up, with all things being equal? I guess I need to get a mirror the next time I do a smoke test to see if I can determine where exactly it is coming from. The trims never go above +10 with the original MAF, but they do with the new Delphi MAF. Matt, I definitely understand your frustration on your issue, as mine is just as frustrating. Any advice guys? Thanks.

[@SchrodingersBox]

I would need to see your fuel trim data curved against load at both immediately after closed loop and at full operating temperature.

[@SteveRobReviews]

Stuzman52 has a great series on the Pico. It was my friend ScannerDanner that influenced me into getting the scope. I would really like a new snap on Verus for Christmas just in case my daughter is reading the comments. Hint hint.

[@SchrodingersBox]

Steve Rob Hahaha, Yeah I would love one of those Verus' but it is out of the question given that I do not do this stuff as a career. But a scope- yeah, I see one in my future!

[@MrJcardimen]

If you're looking at a bad iac (or a faulty iac circuit/command) wouldn't you have expected to throw one of the iac codes by now, rather than the lean condition dtc's?

[@trainer2018]

Not if it Intermittently sticks, the computer logs trouble codes on a certain amount of time, if the fault happens to fast sometimes the computer will not see it and no trouble code will be logged. I speak of this from experience working on Cummins Engines with computer controls. Same thing.

[@techsavantlove]

The IAC on the TransAm is a Bi-Polar Stepper Motor, It is driven by a circuit called an H-bridge. The motor is not driven by pure DC. It is driven by pulses of DC. The internal resistance of each coil will *probably* measure in the neghborhood of 30 to 60 ohms (if the coils are not damaged). The rated resistance (spec) is probably 56 ohms. For each full 360 degrees of the shaft, each of the two coils are energized by a pulse twice. 4 total pulses result in one full rotation. It can be thought of this way: For each full turn (360 degrees), there are four "steps", or independent "events". The events are controlled by the PCM. All 4 of the leads going to and from the IAC are controlled by the PCM, they are not any direct grounds on the IAC. There can't be, because half of all the pulses have to involve a reversal of the pulse voltage That's part of what the H bridge does. Lets label the events A, B, C, and D. In event A, a short (very short) pulse of positive 12 volts is applied across one coil. This pulse turns the shaft 90 degrees. In the next event, event B, another pulse of the same duration is applied to the other coil. This turns the shaft another 90 degrees. In Event C, another pulse is applied to the first coil, but in the opposite direction of the voltage applied in event A. In event D, a pulse is applied to the second coil, in opposite polarity to that applied in event B. Each of the four pulses turns the shaft 90 degress. After four pulses, the same thing continues, as long as desired, and the shaft keeps turning 90 degrees for every pulse applied. It's kind of like one coil "pushes" the rotation of the motor while the coil next to it "pulls" the motor along. It is the constant fairly fast switching of various coils on and off in alternating polarities that results in the seeming continuous "rotation" of the motor shaft. At 135 degrees of rotation, when the shaft is perfectly halfway between a pair of steps, the electromagnetism of one coil is decreasing while the electromagnetism of the next coil in sequence is increasing, and for that one microsecond the two forces are balanced before one gets stronger and one gets weaker. This motor has 4 steps per rotation, but many steppers have much finer resolutions, like 96 steps per rotation. Steppers are commonly used in VCR's, CD players, etc. Also extremely common in industrial applications. Everything above is only half the total operation of the motor, because to get the motor to rotate in the opposite direction, everything above has to happen, but in reverse. As somebody already noted, on this IAC design the stepper motor can kind or "rest" close to the middle range of it's movement, When it rotates and linear motion occurs, it is because of pulses going to it, in one direction or the other. On a lot of Ford's though, the plunger is spring loaded, so the motor either pushes it open more or pushes it open less, but it's the constant spring pressure in the opposite direction that gets the plunger to move in one of the two directions --I think the normal state is closed, due to spring pressure, and that actuation of the motor has to occur for the plunger to "open". That explanation is a little sloppy,, but it's close enough to be used as a general Theory of Operation. These stepper motors NEVER use continuous dc current through them. Any current that occurs is always mere pulses of current. Some types of stepper motors are controlled through the use of VPW Variable Pulse Width pulses or Pulse Width Modulation, but on this GM, I don't think it is. The speed of rotation of the motor is determined by the frequency of the pulses, and the total rotation is determined by the number of sequential pulses. The distance the "pintle" or "plunger" moves is determined by the threading pitch and/or gear on the part the pintle is attached to, and how it interacts with the motor shaft (gear ratio). A stepper motor (including a stepper motor based Idle Air Control motor) CAN be tested for proper operations (sort of). The only thing required is 12 volts and a couple jumper wires. One end of each jumper wire has to be able to connect to each of the 4 pins or holes in the IAC connector (on teh IAC, not the harness). The other end has to be able to connect to the source of the 12 volts. The limitations, or the tricky part, of testing a bipolar stepper motor is that it is very tedious, and involves a lot of moving of jumper wires (or the use of quite a few jumper wires)."All" you have to do, is to use jumper wires to apply the 12 volt pulses to various combinations of the pins or holes on the IAC, one after another. The length of the pulses is not critical, because you simply "bump" the motor with each touch of a jumper to one of the motor terminals. A T-pin or needle type probe or something similar might make it simpler to be able to touch each contact. But all it takes is a simple "touch" to the contact. A very momentary touch, barely a flick. Holding the connection longer won't make the movement of the motor any different, but could overheat the coil. The resistance value of the coil is what determines the current flow during each pulse. The resistance is high enough (probably about 30 to 60 ohms), that each flick of voltage to the coil terminal won't endanger the coil, nor whatever is supplying the 12 volts. It MIGHT be possible to use a standard test light to make each "touch" I assume it would, but I'm not positive. But I've worked with bi-polar steppers and I have just used simple momentary touchs of the lead, without any protective series resistance, to create each pulse. But as I said above, each sequential pulse has to be applied to the opposite coil, and in an opposite polarity. So it's a lot of work. My initial reading has indicated that there are 320 possible steps in the TransAm IAC, so that's what, 80 rotations? So it is a very very fine pitch thread. (The 320 is my best guess, based on research, but I'm not positive about that number). If there is something wrong with what the IAC is doing on the BlackBeast, then it is either a signal (or wiring) issue or there is a problem inside the motor itself. (Matt already stated this). The only method that I can think of to monitor what is happening is the use of an oscilloscope. You want to be able to watch what signals (pulses) are sent to the motor, and what motion or rotation occurs as a result. There would be two pulse trains to watch, each 180 degrees out of phase with the other, and each pulse within one train would be of opposite polarities. The motor HAS to be working fairly well most of the time, or the car and the idle would be not be correct. So if there's a problem affecting the IAC, it must be pretty intermittent or pretty small, I would think. On an H-bridge stepper (in this case the H-bridge controller is inside the PCM), there is no "feedback" or "positioning sensing" that goes back to the PCM (as somebody else here already commented). The only thing the PCM can do is "assume" that if it sends X number of pulses, that those pulses have resulted in Y amount of rotation, and hence Z quantity of linear movement. If the motor is not responding properly, I assume the only inputs the PCM would have, to be able to see that, would be engine RPM changes compared to counts (steps) sent, air fuel ratio, and possibly a mismatch between Desired RPM versus Actual RPM. I can't think of any other ways the PCM would know if the IAC is not doing what it is commanded to do. The above statements are based on a combination of my already existing knowledge combined with a little reading I did this afternoon. It is not authoritative, but I think most of the details are correct, or close to it. I don't really have a handle on whether Matt's IAC is physically responding properly or not. I am kind of "tending" to assume that it is, because if it isn't and hasn't been for a while, then Matt would see it in RPM changes at idle, which he hasn't, as far as I know. If the RPM's, or at least the Idle Air quantity increases when the AC is turned on, that indicates the stepper is working and that turning the AC on is letting the PCM know that it should open the IAC more by sending more pulses. I THINK (but I'm not sure), that if the IAC has any meaningful problems, it would be noticeable in idle speeds or idle variations or float, or with a problem with engine speed upon hard decel conditions (on the road). But Matt has repeatedly said there have never been and noticable drivability implications of teh 0171 code, so that generally points away from the IAC. HOWEVER, during video 3 of the last round, it SORT OF seemed like the response of the IAC might be "flaky", or intermittent. If that's true (that there is flaky movement) then either the motor is being "sticky" moment to moment, mechanically, or one or both coils are having problems (broken wire just barely touching or conducting on a random basis), or the PCM is sending bad commands. Pointing to the IAC WAS a pretty good idea for Matt, because he did have a way of explaining that if the IAC was the problem, it would or could explain every tested quantity so far. I personally have no real thoughts as to whether the IAC is working properly or not. The only way I would be comfortable labeleing it as Good or Bad would be watching shaft movement or motor rotations while monitoring all the pulses on an oscilliscope.

[@SchrodingersBox]

Fantastic input bro, and extremely well explained. I really hope everyone sees your comment- dead on!

[@billyr9840]

Schrodingers Box Douglass, Where do you get this quality of info? I knew enough about the operation of a IAC, just enough to be quite close to being dangerous, but after reading your post it all makes sense. I'm not an electronic engineer or anything close but electricity and the control of electrical circuits have always fascinated me since I was a kid. For a amateur, I think I have reasonably strong understanding of electrical circuits and the material properties used in them and there interaction with mechanical systems. But you have made it all to clear as far as this IAC goes. To sum it all up the PCM pulses one of the two coils of the IAC to create a pushing force to rotate the armature shaft one step and then in the opposite polarity the PCM pulses the other coil to create a pulling force to rotate the armature shaft one more step and continues to do this until desired position is achieved. There's one thing that bother's me, and you just made me more curious. you explained that this particular IAC has 4 steps per rotation and that rotational speed and linear position are controlled by PCM regulating the polarity and the frequency of those pluses. We know the PCM commands ( as ScannerDanner has stated ) this IAC to launch itself forward to the fully closed position ( Pintail Seat ) when the key is turned on and then the PCM commands it to retract itself to it's desired anticipated idle position on start up! During the film, Matt described it as doing just that, It would open towards the close position fairly quickly and than it would retract rather slowly and would not retract all the way! Could this have been the PCM commanding the IAC to the pintal and then back to the desired IDLE Position! It only makes sense that the PCM would slow the frequency of those pluses down if it were only going to retract a short distance. I think this IAC was functioning, but as Paul Danner said if it stuck at some point or another it would become out of sync with where the PCM thought the IACs position was in compared to the reality of the physical position of the IAC! Not good for Idle control, You would think you would have Idle control issues. As a matter of fact, now that I am thinking of it, in Part 1 or 2, I forget, I mention to Matt that just off Idle I thought I heard the engine stumble, not once but twice in a row as he tried to put the engine under load! Even with the car Fixed, we hope, there is still a lot that can be gleaned from this case study! Anyone care to elaborate?

[@techsavantlove]

Billy R Billy said: "To sum it all up the PCM pulses one of the two coils of the IAC to create a pushing force to rotate the armature shaft one step and then in the opposite polarity the PCM pulses the other coil to create a pulling force to rotate the armature shaft one more step and continues to do this until desired position is achieved." PRECISELY! As far as knowledge source goes, I already knew about half of what I wrote, and I got most of the rest of it here: www.megamanual.com/ms2/IAC.htm I also came across this: homepage.cs.uiowa.edu/~jones/step/types.html I can see that it is an EXCELLENT article, but I have not read any of it yet. I LOVE a challenge in stuff like this, it's like a detective story to find just the right little nub of info, and so I've gotten good using the ole' Goog to find stuff. A good hint for ya here.....often you will find much better stuff, faster, by using Google Images, rather than just the regular Google. I do that a lot. Try it out, I think you'll like it. Billy wrote: "Could this have been the PCM commanding the IAC to the pintal and then back to the desired IDLE Position! It only makes sense that the PCM would slow the frequency of those pluses down if it were only going to retract a short distance. I think this IAC was functioning, but as Paul Danner said if it stuck at some point or another it would become out of sync with where the PCM thought the IACs position was in compared to the reality of the physical position of the IAC!" Yes, I think you have naailed it exactly right, there. As I wrote someplace around here, and several others have pointed out, the PCM does not get any DIRECT feedback about where the "pintle" or "plunger" ACTUALLY IS. All it knows is, it tells the thing to move X number of counts, and it then assumes the IAC does what it tells it to do. There is no "sensor" or anything that directly lets the PCM KNOW that the motor is not moving properly. As far as I can think of, the only indication the PCM would have of improper movement is any change, or rate of change, of engine RPM, actual measured idle speed, and Desired Idle versus Actual Idle. Also, Air/Fuel information (the O2's might tell the PCM that the Fuel Trims aren't quite right, which would be a hint to the PCM that maybe the motor isn't working exactly right). Also, as one or more people have pointed out, IAC's DO get dirty with carbon. There has to be a way for the PCM to "learn" this over time, and to be able to factor it in to what it is doing, just like it does with a lot of other sensors, as they "wear". Several people have pointed out two things: One, the computer is damn fast on this car (and VERY smart), AND that the stepper motor itself is extremely fast to move. Two, the PCM MUST be using an adjustment of injector open time to add a little extra (or a little less) fuel if it tells the IAC to open to a certain point to meet the right idle number, but if the idle does not go to that speed, the only other thing the PCM can do is add a slight amount of fuel. I'm 99.99% certain there is no "actuator" of any kind on this car that moves the throttle plate.So if it can't move the throttle plate, and it can't get the idle to go where it wants by adjusting the idle air quantity, the only other thing it can do is tweek the fuel (and the ignition timing). On older cars (in the 80's) there was a stepper motor that moved the throttle plate to adjust idle speed. So overall, I think your "understanding" that you wrote here is perfectly correct in every sense! And just to be clear, it is not that I definitely KNOW any of what I'm writing, (well some of it I know for sure), but a lot of it is just Inferring on a logical basis. I also STEAL a lot from what everybody else is writing, then I "re-mix" it into a different type of narrative, and just post that. On the IAC and stepper motor stuff in particular, that are at least six people here who have proven they know WAY more about it than I do. But as Steve Jobs used to say: "good artists borrow, GREAT artists STEAL". Be Well

[@billyr9840]

Well Said! about Great Artists Steel! Gotta remember that one! Going to check them sites out that you gave me , Thanks!

[@AutoFocus-se]

I was about to post on your previous video but then this showed up. The car was running great now! :) I can buy that there is no code for IAC. The IAC value is just a calculation. "One count represents one change in polarity, which equals one step of the stepper motor. The PCM counts the steps it has commanded in order to determine IAC pintle position." Anyway in my mind even if the IAC is fully open, its not a "air leak" its still measured by the MAF. But we also have MAP, MAF and load input. Its never easy this right? - I noticed when you plugged the IAC. LTFT was 24 which is opposite from the expected, If the plug is sealing? You can also do IAC tests with it on the car. Like a small vacuum leak and see if it can bring down the idle.. Or get this fanzy GM thing J-37027-A :D I need to sleep now.

[@techsavantlove]

Excellent point about the MAF. I had not thought about that, but it makes sense. The PCM is still going to get a MAF signal even if the IAC is only moving part of what it is supposed to, so the PCM could readjust it's thinking. Cool point, dude.

[@edwaggonersr.7446]

+AutoFocus Please bear with me, I'm new to all this sensor/computer stuff, a late comer. But you wrote: "Anyway in my mind even if the IAC is fully open, its not a "air leak" its still measured by the MAF." My understand of the MAF is that it measures incoming air not manifold air pressure. If I am right, wouldn't this be the correct observation:Anyway in my mind even if the IAC is fully open, its an "air leak" measured by the MAP. The MAP sensor not the MAF sensor.

[@AutoFocus-se]

+Ed Waggoner Sr. (Papatch) You are right, MAF measures air flow or mass. And MAP measure pressure in the manifold. And yes a air leak would affect the two systems different if it was one or the other. but in this case we have both. So what it comes down to is engine programming on what inputs it uses. So really all we can do is speculate, it was some time I saw this so I cant go in to details. Cheers.

[@SteveRobReviews]

Hum - resolved iac bad ? If you want to test the iac its scope time. Yours is like 1/2 forward and 1/2 reverse. To find simple problems you need simple tools , scope time. You have something up your sleeve . Waiting for the shoe to drop. Did I mention scope time. 😜

[@SteveRobReviews]

The pids you looking for I believe are there but not in alphabetical order , hard to find for me as well. Turning the ign off, on also resets iac. I'm still leaning towards the MAF . The data stream is corrupted for sure, none of this makes any logic to me , but I'm from Canada and we usually pour 1 gallon of maple syrup in the tank for a situation like this. Want some I got gallons of the stuff ?

[@trainer2018]

Steve Rob GM use digital mafs correct? I don't remember, I have Bus on the brain now more than anything else. Don't work on cars as much as I used too but I have been playing catch up. To bad Matt lives to far away from me, I would be more than happy to hook up my Scope and help him out if I was close by.

[@trainer2018]

Tell her to buy one from Autonerdz for him, Lol. Just jokin, LOL.

[@SteveRobReviews]

that's where she bought mine from but its only a 2 channel, for Matt for sure its a 4 channel the master kit as well. HA

[@trainer2018]

Didn't know you were a pico group member, what's your name on the forums? We might have already talked, Lol.

[@chrissraceporting7055]

Which obd cable do you use and also the scan and tune software that you don't need to pay a kidney do for

[@SchrodingersBox]

Autoenginuity.

[@macknumber9]

I'm a little confused as to how the IAC opening does not affect the MAF....I thought the IAC opening would change the airflow across the MAF. I feel little bumps in the rpms when at idle when my a/c compressor turns on and at the same time I see MAF readings go up then the idle comes back down and the MAF reading does as well. Would this be indicative of a faulty MAF or is it reading correctly because of what the IAC is doing?

[@SchrodingersBox]

On non-DBW cars it is Because the IAC bypasses the throttle - of is already measured by the MAF. It is already measured it just bypasses the throttle opening.

[@macknumber9]

@Schrodingers Box if MAF reading goes up when I open the throttle (the movement of the throttle plate changes allowing more air to flow and changes vacuum which brings more air passed the MAF.) Then wouldn't the opening of the IAC valve produce the exact same thing albeit at a much smaller rate? This is why I see the MAF reading bump up ever so slightly when idling and the a/c compressor cycles on....the small movement of the iac opening is affecting the airflow passing the maf just the same as the throttle opening but of course at just a much lower rate... Not meant for argument of course. I'm just trying to learn....because my live data shows this but I can not actually SEE any IAC data on my scan tool...I just hypothesize that if the throttle opening causes airflow change at the maf sensor then why wouldn't the IAC? IAC can be thought of as a miniature throttle plate as it performs same function but at a MUCH smaller level For example my live data shows in lbs/min and will sit there idling at around .5 lbs/min then all of a sudden I will hear the engine rev up ever so slightly and I will see the maf go to .8 lbs/min all while the TPS pid has not budged (stays at 19.6%)

[@SchrodingersBox]

When you open the throttle the IAC will close.

[@fivefortyeye540i]

Iacv Matt man I'm just can't see it not seating properly.. Wouldn't there be some sort physical evidence like carb build up on it,or inside the house itself?? Man I'm stuck on that maf sending bad incorrect signal.. It would be impossible to see a glitch with out scope..

[@SchrodingersBox]

The MAF is a great directiob 540, However we can eliminate it because of a couple things- First, there is inconsistency with normal/lean at idle where no change in airflow through MAF occurs. But the main reason is we can consistently turn the fuel trims way high by turning the A/C on AT IDLE. This cinches it- A load that is devoid of air intake past the MAF has to be responible for this. It has to be directly related to the Intake. It's either MAP or IAC and we showed MAP is working.

[@bchrisl1491]

A GM IAC is a stepper motor. It makes a full excursion from 0 to 255 counts, or 00000000 to 11111111 or 00 to FF counts. The PCM will set a DTC if it has more than 255 counts. Each step is 1/256th of a full excursion. So like a 6-32 screw, one revolution is exactly 1/32nd of an inch, one count of a IAC is 1/256th of its full range. It it is not moving each time it is commanded, the. The pindle is not positioned properly and the air opening is not correct. There is no feedback from the device. The computer commands and the steper moves. Also, take a rubber hose with a valve in the end to stopper the IAC chamber, then you can dial in an amount of airflow rather than closed or open as was your test.

[@bchrisl1491]

Do a google search for testing a stepper motor and I am sure you will be able to rig a setup for doing so. The dealerships used to have a jig to test them, but they are largely gone these days. Nobody tests anymore, at 190 dollars an hour it is cheaper to throw parts at it rather than spend the time figuring it out. That is a pisser but that is the way it is.

[@bchrisl1491]

You cannot test a stepper motor by just putting volts on its coil. It has to have two active fields with correct polarity to "step" one increment. It will not spin like a conventional DC motor.

[@bchrisl1491]

Most likely the stepper was in it's learning mode when you felt it step slowly. For some lite reading search PID controllers. This is the algorithm applied to this part of the system.

[@techsavantlove]

Matt, Please measure the resistance of the two coils in the IAC and let us know the values. I have a way to test the IAC, at least in part, I think, but I need to know the resistance of the coils.

[@SchrodingersBox]

douglas lee Ah darn, sorry I didn't see this until after I posted the last video. I had thought of doing that but I didn't know what values to compare against- I should have done that after buying the new IAC.

[@techsavantlove]

Schrodingers Box So I assume that means there is a part 4 that is up? I haven't actually looked tonight to see if there was a new Episode. I'll do that now.

[@ManuelAPerez-pp2yd]

I have been watching your videos because I'm a dys type of guy i have a p2096 code Lean fuel trim bank 1 i took my 2005 jeep Grand Cherokee 3.7 to get it inspected everything else pass NY inspection but for that code i have put new spark plugs new injectors checked for vacuum leaks and exhaust leaks myself and nothing took it to a mechanic shop and smoked it to find leaks and nothing they play the guessing game with me it could be the throttle body or the fuel pump i tested the fuel pressure everything is good i drop the tank and checked the pump everything looks good clean the throttle body clean the map sensor tested the purge valve and as your last video tested the IAC valve my code continue to appear i don't want to take it to jeep because i know I'm going to pay and arm and a leg and they won't fix the problem like they have in the pass with my boss vehicles spend money and the problem didn't get solve, any insight on what to try next?

[@SchrodingersBox]

I haven’t seen any data yet. what was the data you are interpreting that indicates you needed new spark plugs AND injectors? I don’t follow how you determined both were necessary- did you find an ignition failure or a fuel failure- or was it both? And I don’t follow why you are checking for vacuum leaks when you determined a fuel and ignition problem. what was the fuel trim indicating that you suspected a vacuum leak is present, especially while checking the fuel pump- which is it, a vacuum leak or fuel deficiency? What was your determination the fuel pump is fine? what was the voltage drop? what was the amperage? I am totally not following the process you are using here because you haven’t provided the data you collected to make all these various conclusions. please provide your data that you are using and why you suspect a jeep dealer will not use data to make a diagnosis and they will just throw parts and guess randomly at it without ever fixing it.

[@caduceus33]

This problem could be an outworking of the Heisenberg Inconsistency Theory.

[@SchrodingersBox]

I believe you are referring to "Uncertainty Principle"? Either way, agreed, at this point in the video, my attempts to make accurate measurements seem to be fouled due to some inherent nature of the system. It could also be wave duality (Schrodingers Box) in that my very act of taking measurements is leading to the inconsistency of the measurements. Luckily, a far more rational explanation is on the horizon- keep watching!

[@caduceus33]

Schrodingers Box Congratulations on the self control you have been exhibiting on this series! The kind of experience you have been going through might have led many mortals to be seduced by the parts changers side of the force!

[@SchrodingersBox]

caduceus33 Yeah it's kind of hard to put this series up to the public because basically it's not exactly showing quick and fast mad diagnostic skills or anything but it does show the limitations of a scientific process when variables are always introduced into a controlled environment. I really came within a hair of putting a computer in this car which would not have fixed it. So I think there is definite value in seeing how to handle situations like this one, so I kept it up. But trust me, this was frustrating as hell and trying to make good filming of it was really a challenge.

[@joeandgod1]

I have an unrelated question: Talking about a circuit. If all voltage drops at the resistor/load(therefore no voltage left on the ground wire side), how does the amps/current continue through the ground side if theres no voltage? I thought that amperage/current was the movement,flow or motion of voltage? But if theres no voltage(on the ground side) how can there be amperage/current(which is the movement of voltage) ? Can someone answer this.

[@billyr9840]

Voltage and Current are two different beasts! Think of electricity as water in a garden hose. If you think of Voltage as the ( Water pressure ) in the hose with the hose on at the source and off at the nozzle, and was at 12 psi. Think of the Current as the flow of the water in the hose which would be 0 gallons per minute with the nozzle off. When you open the nozzle all the way, the( Flow of water ) would increase rapidly to say 1 Gallon of water per minute. But the ( Water pressure ) would drop toward 0 psi. Electricity works in a similar fashion, When the Amperage ( Electrons/ Water ) starts to flow towards ground the ( Voltage/ water pressure ) starts to fall towards 0. The higher the ( Amperage/ water flow ) the lower the Voltage / Water pressure. Now if you start to close the nozzle the ( Water flow / Amperage ) starts to fall towards 0 but the ( Water pressure/ Voltage starts to rise to 12 psi / 12 volts. Ohms Law states that when Amperage rises Voltage falls and when Voltage rises Amperage falls. I hope I have helped you see why there is little to no voltage on the ground side of a electrical loaded circuit.

[@joeandgod1]

Billy R Let me ask it this way; Can a wire that has 0 volts have amperage/current flowing through it ? (Cause thats what the ground-side of a loaded circuit is)

[@SteveRobReviews]

Check out tst seminars on KZbin they have hours on voltage drop testing.

[@joeandgod1]

Billy R I just thought that amperage was "the movement or flow" of voltage. And since theres 0 volts on the ground-side of a loaded circuit, how can there be amperage,with no voltage being there. How can you have amperage on a wire,when that wire has no voltage,cause amperage is "the movement or motion" or voltage. In other words, you can have voltage without amperage(unloaded circuit),but you shouldn't be able to have amperage without having voltage.(which you do on the ground-side of a loaded circuit). Maybe Ill let this go.

[@joeandgod1]

Billy R O.K., Thats simple and it helps,thanks.

[@pablomarquez4431]

Wouldn't a iacv effect both banks.

[@mikeb5981]

Don’t question this man your waisting your time, I agree with you though.

[@pablomarquez4431]

@@mikeb5981 🤣

[@mikeb5981]

@@pablomarquez4431 I’m serious

[@mkuehn5450]

made it to 2:16 "iac opening causing a lean condition" care to justify why you think this is unmetered air? i'm going to file this with your comment "the freeze frame was lying too" response to area46241 in your prior video. you got the FF data from vin specific, not generic, i cannot vouch for the accuracy of your tools

[@SchrodingersBox]

I believe it’s because when the IAC does not open to the correct commanded position, the amount of air entering is not what is expected.

[@mkuehn5450]

@@SchrodingersBox i retract my crankcase breather statement from an earlier video when you verified it was only the oil fill cap. the clean air intake to the crankcase is a pipe cast into the passenger valve cover, drawing filtered air after the AFM, but before the throttle plate. exactly the same as the IAC. metered air. interested to continue watching, i really appreciate your method of verification of data especially when it contradicts what you're expecting. likewise for me when i saw 15" of engine vacuum but only 9% load in your freeze frame data @ 1800 rpm, only to learn you're in Denver. this changes what i'd expect to see @ sea level (here) as BARO is an important tool to verify MAF on certain models. resetting the KAM on fords, for example sets BARO from 29" to a factory default of 26" which i believe has a lot to do with your LTFT additives, subtracting fuel because of altitude may be a expected condition.until Fuel trim cells are written properly to compensate for altitude. i also believe MAP is primarily used to monitor EGR on a normally aspirated motor, and possibly KOEO BARO. so i continue watching, thank you for the content.

[@billyr9840]

Matt, I am counting! That was so lame! I get it. IAC counts, you need to get some sleep Dude! LOL! Going to bed, I'm going to start counting sheep so I won't start counting IAC counts! Yea Right, So, So, So Lame! LOL!

[@SchrodingersBox]

Billy R Lol, Billy, we will allllllll sleep better after part 4 ;-)

[@billyr9840]

I was just getting ready shut down the lap top and your msg pops up that you uploaded Part 4. After all that I had stay up and watch it! Glad you fixed it. Go to admit it was fun. Lot of good ideas hey! After all this you should be able to find some good direction for your channel with all the different directions we went in. anyway happy to see you redeemed yourself. Have a great night!

[@mikeb5981]

You beat around the bush to much and obviously don’t know what your talking about if you have to make 6 videos over 26 mins 🤦🏻‍♂️🤦🏻‍♂️

[@SchrodingersBox]

what a fool. it’s called advanced diagnostics. a parts clown like you would have spent thousands on this issue. So tell us what your interpretation of the fuel trims was the first time.