Test Your Lean P0171 / P0174 Diagnostic Skills-Pt4

[@ДаворДабић]

I follow this channel over a year, I watched almost every video (not all, not have time)....I'm automation engineer (by occupation) and I can tell you guys THIS GUY BLOW MY MIND OFF !!! I'm DIY guy, so I fixed a lot of my cars, most of GM's (used GM GDS). I can tell, the most of workshops doesn't know anything about fuel trims....and as a wise man said once (in one of previous videos) "Fuel trims are a powerful tool for diagnostic if you now how to read it"....Thank You for sharing with us Your great knowledge !!!!

[@SchrodingersBox]

Thanks for watching!! Appreciate the comment.

[@johndecowski8007]

I think it may be a substituted value problem as well.. I would unplug O2 check readings. I've seen a Saturn with substituted vales even when O2 was unplugged and never flashed a code. That car ended up being broken signal wire back to pcm. I think I'd check with scope or dvom at pcm. This is a really good case study you found. After you figure out the fuel trim issue you may find a maf problem! That would interesting. This is true evidence not to just throw parts at it!! Love your channel Matt!! I'm a professional mechanic and love watching your channel because I like looking for new ideas!!

[@SchrodingersBox]

Agree 100% and this will be easy to validate and will make for great video experience!

[@dan8718]

Great cliffhanger! Puzzling why the PCM reacts so quickly on one bank and so lethargically on the other. Can't wait for the videos after next weekend. Learning a lot here!

[@SchrodingersBox]

So am I- I think I know what's wrong thanks to some of the subscriber comments. This is a great case stuidy though and I love the way it developed so unexpectedly!

[@RK-dm2qo]

I know this is an old video, but my head just exploded.

[@spelunkerd]

When a case as complex as this one comes to a flat rate shop, you gotta have a little understanding of how difficult it may be for them. Not every shop has access to a scannerdanner to bail them out....

[@SchrodingersBox]

Yes the flat rate system can really eat you on this. I easily spent a couple hours tracking down the wrong etiology until I finally found this LTFT thing going on. I can easily see how anyone would have put O2's and maybe even a PCM in this thing after hours of tracking down diagnosis on "false" data that you would have no reason to expect is false!!!

[@bchrisl1491]

That is a good one. I stayed with you right up to the point you EXHAUSTED your hypothesis. At this point I am asking myself where does the long term fuel trim come from. I thought it was a summation of the short term trims. This all occurs in the PCM which is crunching data from the O2 sensors. The LTFT signal being observed is the signal from inside the PCM. I want to substitute the PCM at this point, but I really want to see where you go to figure this one out, as a PCM is an expensive guess. I am anxious to see the next installment.

[@SchrodingersBox]

Yers your thinking is right on- "where does the LTFT value come from". I am quite certain I don't need a PCM. What I am sure that is happening is the LTFT is a "substituted value"- but still begs the question- where is the PCM drawing that value and more importantly WHY is it doing that, especially considering the O2's clearly work? But I do have some direction for the next video based on this hypothesis. Right now, we want to exhaust (lol nice pun) any possibility that the fixed LTFT is actually intentional by the computer

[@jamesl197999]

When that test was done were both banks in closed loop. I know by looking at o2 sensor swithing it looks that way but the computer could be in open loop on the problematic bank and in turn the computer can be given a substitute value for the long term fuel trim. You may want to make sure that the code is erased beforehand sometimes the pcm will give substituted values or not act right when there's a fault, I have seen this before. Keep up good work.

[@SchrodingersBox]

James L Yes in fact my first thought was that somehow, Bank 2 must somehow be stuck in OL- I wondered maybe there is a separate CTS on each bank or something and the B2 CTS was somehow stuck "cold". But if so- why the STFT variation and even more so- why it isn't stuck rich if anything. While I agree we are seeing substitute values, I am not convinced it can be from OL on one bank. I am more inclined to believe the P0171 initial code which started the whole video may be preventing the reveal of the problem causing this new discovery.

[@GameBacardi]

Could there be a situation... if fault code is hanging on background, it change fuel trim to work as factory default ?

[@SteveRobReviews]

Fantastic - when I see illogical data I suspect you may be looking at a substituted value. If I remember correctly voltage readings can not be substituted. I would look at O2 voltage scan data not graph view. Since you have a fault shown on bank one does not rule out bank two as stated. I did notice an air breather on bank two valve cover ?

[@SchrodingersBox]

Love the substituted value idea. Yeah you are definitely on to something there. I think I will try to fix the lean condition to create a normal baseline then maybe something will throw when the 0171 is eliminated this revealing why there is a substituted value. Damn Steve- nice thinking. That's a solid direction!

[@SchrodingersBox]

Schrodingers Box Steve- Seriously dude- you are freaking genius. I did some research on this and the evidence is overwhelming you are on to something- if not dead on correct. As some others pointed out in the comments, using my scan tool in "generic" mode may well allow me to look past substituted values. I have read of substituted values long ago but never thought about it til now- and if I can indeed adapt my scantool configuration to see real data, that would be a game changer. I can't wait for the weekend so I can try some things in this direction!!!

[@alrifainidal]

very very very informative .. nice work i am learning a lot from your video

[@monomille1]

Since the LTFT is a remembered adaptation maybe you need to do a code clear or battery voltage interrupt to get the computer to unstick from prior adaptation.

[@SchrodingersBox]

I will definitely do that, but it still doesn't explain why it isn't learning new adaptation. I agree the first thing I need to do is try resetting though- well actually I need to fix the lean condition first to remove that variable and then ensure I get proper responses after resetting.

[@fieldsofomagh]

These videos are better than watching sherlock holmes and dr watson.The ltft on bank 2 compared with bank 1 and its slow response definitly indicates a malfunction of some kind.It seems that either the vacuum or propane is being inhibited from reaching bank2 and its effects are being nullified.As the stft covers the mid to upper range of engine operation and bank 1 has problems in this range,the ltft will be more affected than bank2.The cause, my dear watson,could be the weighted value stft array in the drive map gone bonkers.Mr Holmes you are a genius !!!

[@SchrodingersBox]

Tom OConnor LOL the only problem with your theory is we have definitive confirmation the vacuum/propane are not only reaching bank 2, but having a definitive effect on response both from the O2 and in fact from Short Term Fuel trim confirming "acknowledgement" of the induced variables. What is not responding is the LTFT- the question is, is because of a computer problem where the LTFT learn is faulty OR is it (more likely) the computer is intentionally preventing the LTFT and is instead using a substituted value to compensate for some other issue. Remember there is a P0171 on this car from the initial code- an O2 sensor code will prevent other related codes from setting. Methinks fixing the P0171 will allow the PCM to reveal why it is using a substitute data for the LTFT.

[@TexasPLNR]

My understanding and interpretation of your tests and O2 sensor swap is 1) O2 sensors are good and providing reasonable outputs to added fuel and air 2) STFT's are producing expected responses to added fuel and air, 3) LTFT Bank 2 is not responding or has severely inhibited response to added fuel and air. LTFT Bank1 appears to respond as expected to added fuel and air. Since LTFT is a function of STFT changes and some unknown update algorithm, it seems that a PCM firmware error may be the problem. Perhaps a flash update for your PCM is avaiable. I am not familar w/ Air Injection system on your vehicle but have seen cars w/ injection on the exhaust manifold. It would seem that this injected air would interfere w/ O2 sensor operation by artificially leaning the exhaust stream. Perhaps air is injected downstream of primary O2 sensors?

[@SchrodingersBox]

Tex, your thinking is absolutely congruent with mine in every way- except after doing some more research, I have found other explanations other than PCM firmware. I believe what we see on bank 2 is a substituted value for LTFT. Now, I don't know *why* the PCM would need a substituted value, especially considering we have O2 output and even STFT response. But I believe that something else is missing for the LTFT calculation and it may be because of the fact the car has a P0171 and thus may not be allowing other codes to show. But you are correct 100%- this is not an O2 sensor problem based on the data.

[@jnarez1]

It might be that the signal from one of the o2s is not making it to the PCM and the PCM is going of one o2. So maybe check the wiring from the o2 to the PCM. Also, might want to check the connections on the o2 for rust connection.

[@SchrodingersBox]

Agreed- I will be checking for exactly that possibility

[@02lowrider62]

After seeing no reaction of the LTFT with the propane the next thing I would do is clear the codes, it is very possible you are seeing substituted values like Steve said. Then retest with propane again. It also would be a good idea to look at generic obd to see what your data is. I know you don't own a scope but this is a perfect time to back probe both front o2 signal wires just to make sure scanner isn't doing something goofy. I also would disable air injection pump and take on road test there are case studies were they come on when not suppose to and cause a lean code ! You have a good one here but the way your going about it I have no doubt you will get it done .

[@satamanschmidt3428]

Matt, something that occurs to me is that we are thinking that O2 sensors are either good or bad. It's not that simple. As they age their response time lengthens. It takes longer for them to react to changes in the air/fuel mixture. I sort of knew this already but there was an episode on "Two Guys Garage" were this subject was in fact discussed for about 30 seconds right between the product placements and the open shilling of fuel additives. I know that you would rather swallow Drano than swap parts but your TA is 14 years old. The O2's might not be dead but they could be tired. I've put my flack jacket on so fire away.

[@SchrodingersBox]

Lol the O2 sensors are definitely not bad. We can easily see appropriate responses. My purpose of changing O2 sensor banks in fact had nothing to do with testing the sensors- it was really to see if there was a difference in the reading and data interpretation from the O2 sensors- and even then it was highly unlikely. The O2 sensors are working- the computer is just not showing LTFT adjustment for some reason and is using a default substituted value- and I believe there is a logical reason for this.

[@SchrodingersBox]

***** even were that the case, the problem did not move with swapping O2 sensor locations so a problem still clearly exists, though admittedly it may just be misidentified on the actual bank I suppose. But this is specifically addressed in the next video anyway where I do bank specific variables and confirm the O2 readings are on the correct bank.

[@michaelmillion183]

Schrodingers Box But, aren't the O2s responding properly? From what I saw, they were. It was the LTFT B2 that wasn't responding. Both STFTs were responding as well. So, IMHO, the mystery is why the LTFT B2 is 'stuck'.

[@johnwrench4speed]

Looking at part 3, I notice you have a K&N style breather on or near your engine. I was wondering what/where this was being used? Secondly, and unfortunately I have not seen every video on your channel, can you please list all improvements/modifications you have done to your engine, if any. Third, can you check monitor status in the global mode of your scanner and tell us their status (complete or incomplete). Finally, can you please tell us what the 8th character of your vehicle identification number for engine ID. Thanks.

[@SchrodingersBox]

Agreed with the mods the problem is that these mods were all done 10 years ago and only recently did this phenomenon start- previously it was perfectly normal by all counts. But agree as someone "new" to the car, the mods would all be suspect but they are not the issue- this is a recent issue as of just a couple months ago.

[@Pablo_Automotive]

This video reminds me of a case that Paul Daner in a Ford. While not the same case is similar as STFT not fit but then the O2 sensor was completely dead and it ignore PCM, then the PCM will reset the settings to see. Here is the link if you find it useful: Ford O2 Sensor Testing A and sorry for my English is very bad, greetings from Spain:-)

[@bbkmm1]

Do you have two coolant temp sensors, one for each bank? If the computer think one bank is not warmed up yet, it's operating in open loop until all sensors are up to operating temperature? I'm trying to think outside the box(O2) since your test eliminated the O2 sensor. How about any other sensors that's bank dependent? BTW, I'm now in the two percent because I looked like a fool as a parts changer. It hurts my pride. It also defeats the purpose of DIY. It may be cheaper to take it to a shop. Thank you and others such as Mr. Danner.

[@SchrodingersBox]

Qiong Jia Qiong, I was thinking exactly the same thing with the Bank 2 stuck in OL. See my next video!

[@michaelmillion183]

Yes. I thought of the same thing since I didn't see formally see bank 2 in CL. I believe I only saw Bank 1 in CL. I don't know for sure B2 is in CL. But, the STFT is changing, implying closed loop. From my experience in OL, FTs don't change at all. I'm trying to think why LT is acting so sluggish, yet there appears to be proper O2 behavior.

[@SchrodingersBox]

***** Correct Michael- indications are clear both banks are in CL despite the fact I cannot actually confirm a B2 specific loop status or temperature from CTS. All your thinking is dead on and in fact confirmed later in the series.

[@chrisi1909]

I would think it´s a PCM problem....Software ??

[@williamrockhill8018]

you could check pcm signal voltages at bank 1 and 2 at computer and its grounds

[@SchrodingersBox]

Signal voltages for what???

[@johndecowski8007]

While thinking some more... Are you in manufacturer specific mode or global obd on your scanner? The reason I'm asking because I was at a class a few months ago and the instructor emphasized using global mode because he says obd standard law prohibits substituted values and basically manufacturer specific mode they can give calculated values because it isn't under obd standard. I'd try both modes or even another scanner software so your not running in circles.

[@billyr9840]

John you are right. I read the same thing about that.

[@SchrodingersBox]

Yes I am familiar with that and I agree we are surely looking at a substituted value and that is definitely my main direction for when I continue.

[@jcsaves9725]

Very informative!

[@Sandbag1300]

That was fun! It appears we have to find out what within the electrical system translates short term fuel trim into long term fuel trim and test that. GULP. First, I would check Service Bulletins for your vehicle and see if there are any software updates needed.

[@SchrodingersBox]

Exactly- that is probably way beyond my capability. I have been trying to find that information and can't. It's a little scary that it would necessarily be in the PCM.

[@Sandbag1300]

Schrodingers Box I am guessing but imagine it is software code within the PCM that needs to be corrected or updated. Makes one wonder if the computer uses STFT or LTFT or both to make fuel trim corrections. Could LTFT only be an analytic indicator or a real driver of change? Great stuff you have uncovered.

[@SchrodingersBox]

Sandbag1300 STFT is the driver of change- LTFT "memorizes" that change to set it as the default.

[@Sandbag1300]

Schrodingers Box Are you saying that LTFT is used by the PCM to adjust injector pulse timing and length? On internet searches I really cannot find anything about PCM programming and the specific variables the programming uses. Information available is rather vague and white washed with repetition.

[@SchrodingersBox]

Sandbag1300 I am saying the LTFT is used by PCM to look and see what the injector pulselength should be. This default is set by the LTFT based on changes made by the STFT to bring the air/fuel back to stoichiometry. The LTFT does not change the fuel delivery- it sets the fuel delivery for that condition as a result of "learning" the correct delivery from the STFT. See my video "Secret of engine problem diagnosis: fuel trims" and it will be perfectly clear.

[@billyr9840]

I like the idea of substituted values your other viewers have suggested. It would explain the STFTs. As I stated in my early comments in part 2, that you may have an O2 sensor on B1 that was dropping out. Then after all that Injector Balance testing, pumping all that raw fuel into the cylinders and down the exhaust, you may have contaminated the B1 O2 to a degree and Killed it. When you switched the B1 O2 to Bank 2, could that be why B2 O2 now is flat. As John Decowski has suggested, I would check the O2 voltage at the PCM and I would Check it at the sensor it self to make sure what you are seeing is not substituted values. When you said we may have a lean condition effecting both banks then why didn't The B2 O2 detect that before you swapped them. I love a good mystery!

[@12vgs8606]

Yes, I was going to ask that question because manual injector balance seems to dumping quite a lot of raw fuel in the engine compared to the automatic one initiated by the scanner. Is it safe to put that much raw fuel?

[@billyr9840]

12vgs8606 For the most part, yes you are. But you should not over do it because you could hydro lock the pistons while cranking if you dump to much fuel into the cylinders. It's also a good idea when done with your repairs to change the oil in case you have any seepage into the oil sump/pan from washing the cylinders clean. The dry cylinder walls and thined out/contaminated oil could do some major long term damage to crank, cam, rings, and bearings over time if the oil is not changed.

[@JAIMEMUNOZPLASCENCIA]

if that donot be safe... scanners makers donot set that software in.

[@contagiousFX]

question was bank 2 even in closed loop status?

[@DENicholsAutoBravado]

By George, I might be on to something after ready 2/3 of the comments below. So, I wasn't self-actuated to think of the following theory, thank you to many. Early on in this series it was no longer P0171 and P0174, it was just a bank 2 issue. Since that issue remains, the PCM may be retarding the learning on bank 2 because it's in a little limp mode. It would be a pain if you have to diagnose the problem before the code sets again, but a "limp mode" my be preventing diagnosis to be completed.

[@DENicholsAutoBravado]

I also really appreciated the possibility that it may be a substituted value, and that OBD II world mode, versus manufacturer specific may catch it. I know almost nothing about this, but I learn from it having been mentioned and hopefully those who know more on it will correct me.

[@billyr9840]

DE Nichols Factory Specific can substitute any value the manufacture wants, genic mode is not allow by federal law.

[@billyr9840]

Billy R I meant to say generic not genic!

[@DENicholsAutoBravado]

Not allowed by federal law? Have a reference on that?

[@billyr9840]

DE Nichols I read a lot of different material trying always to advance my capabilities and understanding. I don't recall off hand where or what article I read that in, but if you want to sift through the EPA Rules and Regulations, Knock yourself out! Here is the link for the EPA regarding OBD Regulations - www.epa.gov/obd/ Believe it or not if you have the patience, there is some pretty interesting stuff.

[@AutoFocus-se]

That was a great twist. I think its interesting that the O2s and STFT almost exactly mirror each other. Like B1 LTFT is controlling both banks. Is the memory cleared after switching O2s? Up to this point you have only made changes (in air/fuel mix) that effect both banks what about make a bank specific change? To see how the computer reacts. Thanks.

[@SchrodingersBox]

That's a good idea- I can just pull an injector or plug. I will definitely do that, good thinking, but I expect we know what will happen. I was also thinking maybe somehow B1 is stuck in open loop- but if so, how does it have STFT response?

[@jonpaulrhinehardt7663]

Schrodingers Box-If B1 is stuck in open-loop because of an unforeseen problem, I don't think this would affect the out-put signal of the o2 or STFT. The STFT would be data that the PCM would ignore as if it were in open-loop. That is just my thought on this.

[@SchrodingersBox]

It would indeed affect O2 output because there would be no fuel trim adjustments to create the oscillating lean/rich condition creating the normal O2 sensor trace.

[@jonpaulrhinehardt7663]

Schrodingers Box What i meant to say was the o2 output would still control STFT, but the pcm/LTFT would ignore these values. I'm probably wrong, but I thought I would through it out there.

[@Sandbag1300]

This may sound silly but why not try a different scanning software - Autoenginuity. The problem with Bank Two may be a glitch in the scanning software you are using.

[@SchrodingersBox]

Sandbag1300 It's not a silly idea- in fact I actually think that's the problem- but not because of faulty software or a glitch. What is happening (I believe) is I am seeing a "substitute value" for the B2 LTFT because the computer is seeing a problem that is "hidden" because we have an existing P0171 code preventing other issues from being revealed. There is a way around this (using a modification to the software) which I will explain in next video. But basically, yes, I think you are right- I need to set the software to ignore any substituted values the computer is using.

[@Sandbag1300]

Schrodingers Box Could the fix be as simple as clearing the codes from the PCM so that the scanner will then pull LTFT data from the bank two tables that change based on O2 sensor input?

[@billyr9840]

Sandbag1300 Dude, I like that! If you clear the codes maybe Matt can capture enough scan data to show what the O2s signals are truly generating before a P0171 sets again! Excellent Reasoning!

[@SchrodingersBox]

Actually maybe- but what caused the problem in the first place? I agree if I clear the codes and reset fuel trim data and then fix the cause of the P0171 which may have been the issue all along that may indirectly fix this fixed LTFT. We will see. But in a way, I hate to see it go away, this presents a fascinating learning opportunity into the operation of the computer lol.

[@billyr9840]

Schrodingers Box Operation of the PCM! Now that would provide some really interesting material for a series that who knows where it good go.

[@jaberwocky6669]

Hmmm, the only thing that comes to my mind is a wiring harness issue for 02B2? Check the continuity?

[@SchrodingersBox]

If thats it, how is the PCM getting the O2 signal?

[@ob1Katoby1]

Schrodingers Box Well it might be getting a "signal" but not the O2 sensor signal. Ghost voltage, shorted to something else. etc. I would definitely load test the wiring from the sensor connectors back to the PCM looking for voltage drops. I have seen cars where it only uses one rear bank O2 sensor. The PCM assumes the other sensor has an identical signal. If you disconnect the reading sensor the second non-reading sensor might start to register a signal.

[@DENicholsAutoBravado]

I though about the signal getting to the computer wrong, and then I thought I was thinking better of it. I mean, The O2 sensor is reading completely normal. . . . but then of course it'd read normal. If it was wrong, the computer would correct for it and get "normal" to just be wrong. I'm personally left with verifying the wiring between the PCM and the O2. . . even if that isn't the problem, that's where I'm stuck.

[@chrisgmtech8421]

hello sir bank 2 is not affected may be because the vacuum hose is connected to bank1, thats why there is no effect in bank 2 if you put propane..

[@SchrodingersBox]

vacuum does not work that way. A B1 lean eliminated accessory vacuum as possibility.

[@chrisgmtech8421]

Schrodingers Box thank you for quick answer sir. I've watched all your videos and I've learned a lot. I love discussing about fuel trims. Thank you

[@SchrodingersBox]

Chris Umel Thanks for watching. It was a good thought you had for sure and certainly showed understanding of fuel trim variables but accessory vacuum will always be ported or at intake.

[@chrisgmtech8421]

Schrodingers Box you're welcome sir.

[@williamarnold958]

Schrodingers Box and all that I

[@leoeuro1543]

dont never trust check engine light...one lies they all lie...and who ever in charge of LTFt's needs to be fired ,not balaming no one just yet...this video just got way cooler

[@johnwrench4speed]

Another area to check for additional oxygen for only one bank is injector "o" rings or a leaking intake manifold.

[@curtisburdick2849]

Pretty sure the intake manifold would be covered during a smoke test that he performed as well as injector rings

[@curtisburdick2849]

Pretty sure the intake manifold would be covered during a smoke test that he performed as well as injector rings

[@DJDevon3]

Most of your videos leave me scratching my head thinking about it for hours and I usually come to the same conclusion. That don't make no god damn sense!

[@SchrodingersBox]

Hahahaha I'll never live that one down!! Yeah I am usually driving to work at my real job while trying to shake O2 sensor and LTFT graphs out of my mind. I can't even watch TV any more- I rewind 5 times every 2 minutes because I start daydreaming about fuel trims.

[@DENicholsAutoBravado]

Schrodingers Box My TV consumption has dropped considerably also. And when I do watch something I want it simpler, so if I'm distracted thinking about graphs and sensor read outs I won't lose what's going on.

[@DENicholsAutoBravado]

***** Your absolutely right, it makes no sense. So, no. 4 is false normal, if that's really the problem....how to test for that?

[@fivefortyeye540i]

Pcm update/or reprogram??

[@genehensley767]

Here's a good subject for you waste spark ignition systems please enlighten us Thanks

[@SchrodingersBox]

Gene Hensley Yes I am quite familiar with these- soon as I fix my damn trans am I will look into that lol

[@micromaster9629]

Huuumm I m also worry about this fact.Its a big deal for me.

[@chukachuka17]

Clogged bank 1 catalyst

[@SchrodingersBox]

based on what data?

[@satamanschmidt3428]

I just checked on Rock Auto. The O2 sensors for you TA are going for about $25@. Just in case.

[@billyr9840]

I've been hinting at that from day 1! I went back and reviewed the original film several times that got this series going and notice that the traces from the B1 O2 sensor wasn't keeping pace with the B2 O2 sensor even if it was spiking more than the B2 O2. After dumping all that fuel down the exhaust (Injector test ) then switching B1 O2 over to B2 O2 and now B2 O2 seams to be very lazy at best to dead! I'm talking about LTFT. But in any case it is great that we are getting all these intelligent educated theories. Some, with the limited amount of data, we have are pretty dam good.

[@DENicholsAutoBravado]

Billy R Trouble is, is it being lazy, or is it getting less fuel than desired so it appears to be reacting late? (In this case, I don't believe we have a fuel delivery problem so that idea has to be debunked now, but it may be of value at other times.) I too need to do some rewatching! The best stuff needs rewatching.

[@billyr9840]

DE Nichols I so confused! I'm going back to the beginning and watching everything again. I think I am even going to take notes on all the facts to try to make some sense of all this. I am also going to do a lot more research to see if there is another direction we can go in, because I am just plain stumped!

[@DENicholsAutoBravado]

I've done a lot of reading too. Just nothing helpful yet. Hey when you don't have the answer it makes you learn. Have you watched Matt's latest?

[@billyr9840]

Yea! Even more determined, all thou more dumbfounded!

[@wtbm123]

interesting

[@fivefortyeye540i]

Tsb for your car...

[@mtrltoolman]

now what wrong with this car? this is very interesting