The Trainer #65: Fuel Trim Explained - What It Is , How It Works, And What It Can Tell You

#fueltrim #p0171 #p0172 #systemlean #systemrich #oxygensensor
Most technicians understand the concept of fuel correction and what we mean by “closed loop” operation but just in case…
Fuel control is the concept of maintaining the proper mixture of fuel and air in the cylinder for efficient combustion. Incoming air is measured (directly or indirectly) by the Engine Control Module (ECM) and then the ECM chooses how long to open the injectors in order to add the proper amount of fuel to that air charge. The original injection time is based (in part) on the known flow of fuel through the injector but that original number is not perfect.
So a feedback sensor is used to tell the ECM how it did. These are the conventional oxygen sensor or the wideband oxygen sensor. Based on this feedback, the ECM adjusts the next injection event by increasing or decreasing the injector on time as appropriate. This process is continuous and results in the Short Term Fuel Trim (STFT) numbers you see on your scan tool.
Ideally, the ECM will adjust the on time to cause the STFT to swing from a positive number to a negative number with each adjustment, with an average approaching zero. If the adjustments the ECM has made are insufficient to do that, the ECM “learns” a more permanent correction called Long Term Fuel Trim and adds that factor to its internal calculations. If the total amount of correction (STFT + LTFT) exceeds a programmed threshold, the ECM will record a “system lean” or “system rich” DTC and turn on the Check Engine light.
But do you really understand the diagnostic advantages these Parameter Identifiers (PIDs) have to offer? To the knowing eye, the trims can point to the cause of that lean or rich system DTC. They can also be helpful in determining the cause of a misfire, or even identify a clogged exhaust.
Watch this month’s edition of The Trainer to learn more!
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Due to factors beyond the control of Motor Age, it cannot guarantee against unauthorized modifications of this information, or improper use of this information. Motor Age magazine (UBM Americas - Automotive Group) assumes no liability for property damage or injury incurred as a result of any of the information contained in this video. Motor Age recommends safe practices when working with power tools, automotive lifts, lifting tools, jack stands, electrical equipment, blunt instruments, chemicals, lubricants, or any other tools or equipment seen or implied in this video. Due to factors beyond the control of Motor Age, no information contained in this video shall create any express or implied warranty or guarantee of any particular result. Any injury, damage or loss that may result from improper use of these tools, equipment, or the information contained in this video is the sole responsibility of the user and not Motor Age magazine or its corporate parent, UBM Americas.

[@blabaduplatinum1]

Look old man, I’ve been a senior master ASE tech/mechanic for over 25 years, after listening to you going on and on about your diagnostic theory I feel the need to say…… thanks. I’m still learning. Just subscribed and I’ll be going through all your previous videos and checking in for new ones as well.

[@royrobinette7833]

Give me a carb any day and get rid of O2 sensors map sensors maf sensors. All to confusing for me.

[@pinrockie]

You run into people that are true Teachers/Trainers in life, AND you Sir are one of them. Thank You

[@MotorAgeMagazine]

Thank you for those kind words - I'm glad you find the content helpful.

[@randybauer2998]

11:30 Thanks for this excellent video. I need some of your expert guidance. 2011 Hyundai Sante Fe engine failure126,000 connecting Rod bearing failed. Warranty engine installed by Hyundai. 6000 miles later engine lacks power. Finally I get a check engine code PO106 defective map sensor. Replace it with a OEM Hyundai map sensor . Check engine light goes off.No change in running condition . Vehicle is towed to the dealer Dealer. Dealer diagnosis, 2 defective CATS melted and coming apart inside. I claim the ECU was not communicating with new engine or updated when it was installed. We were probably driving vehicle for many miles with a faulty map sensor( making the engine run lean or rich) but the check engine light never came on and when it finally did it was to late the cats were compromised. My other reason the CATS failed is when the original engine blew up it sent engine parts ( rod bearing, connecting rod , piston and engine block)coolant, oil and fuel right into the CATS compromising them. It’s now a battle with the dealership trying to get them to replace the CATS under the engine warranty. Dealer checked the ecu with a basic scan And says its working. No check engine light currently on and no codes For catalytic converters . The dealer also ran a compression test 140,160,160,160. Not sure if this imbalance would affect the AFR in each cylinder? Any advice Appreciated Thank You

[@ernestkerley6282]

Yes Sir, Indeed !!!

[@apegues]

Thank You 🙏 Your videos are keeping this “Old Timer” relevant in today’s Automotive world, you have no idea how much this helps me, 45 years of wrenching and I’m learning more now than ever.

[@ruckus724]

Great explanation. Best 20 mins I’ve spent on KZbin in my life.

[@tgoodm1]

Looking forward to the next one. And many thanks for tackling my fuel pump waveform question a few months back.

[@ktate2002]

This lecturer cannot be more perfect in substance and delivery! Outstanding!

[@sollykhan2385]

why is there only 311 comments ? this channel is LEGENDARY, or is it above the heads of most 'TECHY'S ' ?? keep up the great work, BLESS 👍😍

[@jamesedwards8308]

Wish I had found this site a long time ago. THANK YOU.

[@kurthamblin1240]

This is a very comprehensive explination of fuel trims and how short term and long term fuel trims relate.

[@robertferrara4529]

New sub explained in an incredible easy to comprehend explanation, Thank you. At 70 I can still find well explained info.

[@movingforward6099]

Just bought a blue driver scan tool and wanted to learn. then i found you! Awesome channel ! I'm a new sub. Thanks for the wisdom !

[@stevej6892]

That was one of the most useful 20 minutes of my life! Amazing presentation both in terms of content and delivery style. You have a new subscriber.... (with a new Creader VIII diagnostic tool).... Thank you :-)

[@billpawelczyk5855]

One of the best field trip backsplashes I listen to ready for the next class thank you

[@Stunna_ECN]

Been studying your videos past few days thank you for your wisdom and knowledge I appreciate it

[@samr.m.7709]

Pete, thank you for another excellent video. I agree 100%, fuel trim, or in my neck of the woods "mixture adaptions" are a vital in engine diagnostics. I am looking forward to next month!

[@rayneedaebluz]

Well said Sam R. M.

[@neilray8416]

Wonderful instruction. Thank you for sharing you time and knowledge.

[@SurferSandman]

This was great at helping me understand and diagnose issues with my fuel trim.

[@jamieward6951]

Very informative. Explains everything very thoroughly. Excellent tutorial!

[@Sean-xm5sp]

Absolutely Outstanding videos You are an amazing instructor! Thank You Thank You Thank You !!!

[@khumbuzadibandlela]

Great explanation of fuel trims using hand gestures. I now have a better understanding

[@soaringmotors6434]

Excellent Content! Thank you so very much for the closed loop explanation.

[@TimmieTennis]

Very nice presentation from the ground up- helps so much in understanding how it all fits together!

[@smorrow0816]

Just found your channel. You are a gifted instructor!

[@billyr9840]

Nice ! Awesome explanation of fuel trims Pete !

[@MotorAgeMagazine]

Thanks Billy, and thanks for taking the time to comment.

[@KD-sj7iv]

Thanks you for this video, you explain things very well!

[@bryant3483]

Great method in your delivery approach to understanding fuel trims . Well done sir ✅👍

[@13371138]

Awesome explanation, thank you!

[@mckoylach1622]

Great and very helpful lessons. Well elaborate. Thanks

[@richardmcwhirter9396]

Thanks again Pete another well explained lesson,Much appreciated

[@MotorAgeMagazine]

Thanks Richard....

[@michaelcastillo7883]

All the explanations I’ve read on atmospheric pressure/barometric pressure were not helping me understand it but now that I’ve let go of trying to understand it, your explanation of it at 4:06 just expanded my mind enough and everything clicked into place. Finally makes sense to me! It feels like a needle was removed from my shoulder

[@MotorAgeMagazine]

Glad it helped you!

[@vicjanwa]

love your classes and easy to UNDERSTAND. BESTTTTTTTTTTT

[@granthughes395]

Hey I’m just a parts salesman who uses a scanner daily this video has helped me tremendously understanding dtc’s. In instances were I would’ve sold a spark plug, maf sensor or 02 sensor for a trouble code, now are instances of bad vacuum lines and air leaks that are more apparent seeing and understanding now. Thanks again great video

[@scaracci24]

Thank you for taking the time to educate yourself and not just blindly sell parts

[@kamakaziozzie3038]

good point. Too many salesmen just want to sell without fully understanding the purpose of the part.

[@richardkrajewski8609]

Great video. I learned it slightly different but, basically the same in that the computers response could be compared to an old style tv. The short term is like a fine tuning and the long term is like changing the channel when the fine tuning reaches its limit.

[@1EagleDave]

Always the very best love you videos all of them period.

[@balam8743]

Best video on KZbin period. Thank you Sir!

[@MotorAgeMagazine]

Thank you!

[@enriquev8832]

Thank you. You got yourself another loyal subscriber!

[@rodneyconstable3262]

Fantastic Video! I subscribed to your channel. Thank you for making these highly educational videos.

[@le_Bizarre]

many thanks, a good explanation and a good heart

[@martinparra5099]

THANK YOU FOR TEACHING US VERY VERY CLEAR

[@travisrosa6628]

Excellent job, thank you sir

[@hubercats]

Very nice tutorial. Thank you!

[@paulferrara9079]

many thanks, well done video 👍😎

[@aminullahfaizy6154]

Thank you sir all of your videos are awesome thans

[@ozstickman]

This has just made it easier for me to explain fuel trim to a customer. Got a Holden Commodore that’s it stft bouncing between 30% and 130%. O2 sensors are shagged! Not codes either.

[@richardrhoads538]

I have tried numerous times to explain this to rookies. I must say that I would not make a very good teacher. That is why I refer them all to either watch your videos or find out more info on their own. Believe me also that you can also teach an old dog new tricks. Thanks again, this is a great video. Keep them coming.

[@petemeier2764]

Thanks for the kind words! I'll do my best!

[@matteo3893]

This is great content thank you

[@nickayivor8432]

Great tutorial thank you 👍 Good Afternoon Pete Meier motor age Take care and have a great day 👍 From Nick Ayivor from London England UK 🇬🇧

[@Stickboy75]

Super. Thank you. VERY well explained to this DIY.

[@stuarthunter6657]

I can always tell when somebody knows what the subject is he or she is talking about, that is when I begin to understand also. Thanks for being that here, well said and explained.

[@MotorAgeMagazine]

Thank you Stuart!

[@mike-yp1uk]

Always learning something . good info

[@mikechiodetti6737]

Thanks Pete.

[@mikeoneill7360]

Exceptional, thanks.

[@romanjaspe4172]

Thanks so much Pete

[@jkingsrd1]

Awesome video thanks.

[@7110A33]

Thank you, great video.

[@essy184]

Indeed, very helpful.

[@kringle02]

You just explained ,very simply and articaly what I have tryed to explain to young people I have worked with. Most of them and customers when you mention 02 sensor they think that it can't be causing there problems .Not to mention like you said you can go from a bad 02sensor or bad coil and end up with a destroyed catalytic converter all because of a simple problem and then the crying begins and how stupid what ever car company who built the car is .A car being worked on right now 2005 Mazda tribute . With 3 kats ,it started with a misfire to bad coils ,was driven quite awhile like that .Needless to say ran scan .short term and long term fuel trim1 all at 0 .and a open loop fault With all upstream 02 sensors. And a whole lot more .just by looking at every thing on the scan tool kats are blocked EGR tube and EGR blocked

[@DylanRabier]

Good video Pete. Thx

[@jgeorges3061]

Pete,hello thanks for the info and all the videos u put on waiting for the next one. cheeeers

[@MotorAgeMagazine]

Thanks for taking the time to comment!

[@s.davidtrout3056]

That was very well said I learn a lot

[@Good-Enuff-Garage]

Excellent video and informaion

[@mikser6666]

Best explanation ive found

[@MotorAgeMagazine]

Glad you found it helpful!

[@simonac688.]

Wow awsome tutorial " Sir " 👍👍

[@djentyman4002]

I just fixed my car with a P0171. Did almost every test and came to the conclusion that my fuel filter was the culprit. It can literally be a million different things. Fuel trims are your best friends. Along with 02 data

[@DevonWayne]

I've been assuming that's it for me also. However, fuel filters dont exist for me SUV. Wtf!? No auto parts store or even online. Super weird

[@robertmedina6875]

@@DevonWayne some only have one fuel filter built into the fuel tank/ pump and are only replaced when dropping the fuel tank? I don't like that design yuk

[@danielandujar5479]

@@robertmedina6875 Robert tengo una ??? Tengo una Pilot 2003 Te hago esta ?? Porque no creo que el Don del vid conteste.. Mi problema es el bank #1 no funciona Mientras el bank #2 si.he cambiado lo que Trabaja en in lado al otro y sigue con lo mismo. Desconecto el bank #2 y tiro a prender y enciende Con el bank #1 conecto el #2 y deja de funcionar. Hoy Hice otra prueba. Desconecté el plug del Injector #1 bank #1 y la unidad falla lo conecto y recupera. Le desconecto la corriente a la bobina y el cylinder se queda igual. Está bien weird esto. El short trim del bank #1 está en -27. algo% Se que lo hace la comp pero de donde viene la señal para ese comportamiento????? Ahh! Y la miras y el motor está como si fuera normal quieto en tres cylindros👣 Tiene nuevo todo lo relacionado con.... 👀👃👀Acepto comentario de cualquiera que interese en ayudarme. Pero que tengan LOGICA please!!!! Thanks👈

[@camerongriffin1820]

I’ve tried everything. New o2 sensors. Smoke test and no vacuum leaks. New injectors, new fuel pump, and still getting the po171 and short term fuel trim at like 30% at idle.

[@archlab007]

I hope EVs,. with less moving parts don't have to deal with all this silly feedback mess. but we all know how that goes the manufacturers will overcomplicate it somehow.

[@antiochiaadtaurum3786]

If your not sure how rich (or lean) the ratio is, would unplugging the MAF sensor be a way to reduce harm to the cat? Would it bring it back to default mix? I've just had a new cat fitted today, yet now see a prior 'too rich' code and a Maf voltage too low code. So am now concerned that the new cat will burn out fast, which would be far from ideal, given the expense of the replacement.

[@stevenmchenry9276]

Great job

[@stopfuckinabout]

Very good explanation two thumbs up

[@MotorAgeMagazine]

Thanks!

[@AT-fi9dj]

Just subscribed. He’s the real deal

[@MotorAgeMagazine]

Thanks! Share our channel with your friends! 2021 is going to raise the bar.

[@TheJosephayal]

Best vid by far

[@bme7491]

I just had those codes show up on my 2002 Toyota. Ran the MAF procedure from their Service Bulletin and saw that it was out of spec. Replaced and fixed. Apparently it was under reporting the air weight.

[@donaldlucas8688]

You Sir, are a wonderful mechanic!

[@raiderman28]

excellent explanations

[@bricekoirokpi9954]

Thank you so much sir🙏🙏🙏

[@ernestomonge7726]

EXCELLENT THANKS

[@clems6989]

Great Job !!

[@stuzman52]

Very nice explanation Pete on fuel trims and how they work. I've got a question for you that I'm really not understanding when it comes to AFR sensors. On a typical Zirconium Dioxide sensor, (narrow band) as you know, the waveform it going up and down creating a sine wave like pattern. When the mixture is rich, (waveform high), then the cat cleans up the NOx. And when the mixture is lean, (waveform low), then the cat cleans up the CO and HC. And the cat can't clean up all of these gases at the same time. So, with the narrow band sensor cycling up and down creating the rich and lean conditions, the cat is able to clean up all of these gases. Let's say that a vehicle is cruising along at a constant speed on the freeway and this vehicle is using AFR sensor(s). These sensors can read very wide air/fuel ratios and I'm inclined to believed that the air fuel ratio is held at 14.7:1 for the best emissions. But, it would seem obvious that the computer has to be adjusting this mixture rich and lean for the cat to work as above although on the scan tool, it would appear that lambda seems to hold steady at 1. Could you add anything to this Pete of how the cat is cleaning up the gasses when using an AFR sensor? Thanks Pete for the video and keep up the good work!

[@DylanRabier]

Good question sir. I think a lot of people are wondering about that 😉

[@MotorAgeMagazine]

That is a great question. The feed gas range, compared to lambda, is very small - If I recall it's something like 0.995 to 1.005. The AFS sensor does switch, but it does so in current, not in voltage. Here's a video that does a great job of explaining how it works: kzbin.info/www/bejne/sJvIl3eoe62SbMU

[@stuzman52]

Thanks Pete and I figured that the sensor had to be switching although in small amounts to take care of the cat. And yes, I was aware that it was adjusting the current in mA. Thanks Pete for the reply and link hope you have a great day!

[@chrisczarnik3439]

Great vid.👍👍👍👍

[@Ed-ip2sg]

Obviously great info and well presented. Just read the comments. I’ll subscribe and join all the other binge watchers.

[@oldmanmtb1443]

Damn dude awesome video...

[@StationRussification]

Long Term is the window of time the adaptive memory dictates the fuel trim, Short Term is live data calculating to learn the parameters for the adaptive memory to update the Long Term. The O2 sensor signals lean below .45 volts & rich above .45. The Engine Controller only cares about that threshold & uses that to determine fuel trim. .. in other words the computer calculates the % of time that the O2 sensor is past the threshold of .45 volts & does that each time the threshold is crossed.

[@kdeh21803]

I'm chasing a stubborn P0507 Code on a '05 Grand Prix Series 3- 3800 Non-Supercharged. I also have a LTFT of 25 and a STFT of 16 at idle. Which usually means a vacuum leak... I've smoked the engine... I see no leaks..... I've switch out the TB and the MAF with one known to be good. I've replaced the PCV and it's gaskets. The Intake manifold gaskets have been replaced recently. All the "usual" things regarding this code I have done..... I just can't seem to find the cause............................other ideas, that why I'm watching this EGR video......?? BTW.... the engine seems to run fine... until the CEL comes on and it goes into Limp Home Mode.

[@hrfb9696]

I have taken a picture of the obd2 screen but don’t know how to share them with you. Is there a way I could email you the result. Thanks

[@4wardnthought234]

Great video. Thank you for the wisdom. On occasion, when I've been holding steady speed for some time - like 60 mph for 15 min - my short term will stop carrying. It will level out at one or two percent and just stay there steady until I change speed. Is this a sign of a problem?

[@MotorAgeMagazine]

You have to consider total trim. What is the long term trim doing at the same time? How are you monitoring short term trim? What kind of car are we talking about?

[@4wardnthought234]

@@MotorAgeMagazine 2003 Nissan Xterra, 4 cyl KA24DE engine. Ltft is holding steady at 0 or 1%. Sometimes the shorts holds at 0 and long will hold at -1%. Another time 0 and +1%. Another time the long will hold at 0 and the short at -1%, and then another 0 and -1%. At other times the 1 holds at 2 (+ or -). In any case, one of the fuel trims will hold at 0 the other fuel trim at positive, or negative, 1 or 2. And it will stay there as long as I don't adjust my speed. Short stops fluctuating altogether. I'm just a do-it-yourselfer on a low budget, so I just have a cheapo code reader from Walmart running through an app on my phone. It doesn't supply a ton of info, but it seems to be accurate with those stats and shows a graph on screen in real time.

[@MotorAgeMagazine]

@@4wardnthought234 I think that's your problem. The short term should swing more than that and never stops swinging. You may be experiencing a slow data refresh rate on the code reader you're using. If there are no DTCs, no drivability concerns and numbers this low, I wouldn't worry about it.

[@johnmitchell2741]

Sir this is by far the best explanation on fuel trim I've watched yet .You have made it so understandable . Thank you.I now fully understand the processes 🤓

[@lielyncher755]

I just had a flashback to my "salad" days of 1998. Motor age and motor magazine were like gold to me.. Sadly all the knuckle headed so called techs, shop managers, and owners I've been around were damn near illiterate. Frickin morons. "Dude what you need that vantage for", types. Before subscribing to these mags every issue I got my hands on became part of my personal library. Repeatedly read cover to cover. What a journey it has been. You're either eager to learn everything you can about this biz with a smile on your face, or the parts cannon will break your vertebrae in no time and your tech career will become a fantasy.

[@invain143]

Thanks for the lecture sir..

[@richard1835]

Thank you

[@MrJGarcia13106]

Thank you so much for this informative video. It helped me troubleshooted my car. After after changing the spark plug and cables, motor mount, MAF sensor, Throttle body postilion sensor, MAP sensor, Air intake valve, new filter, new thermostat, upstream and downstream sensor, PVC valve,fuel filter, new gas pump, timing belts, alternator belt, ac belt, water pump, intake seal was replaced, the car still has an abnormal vibration? I used my Actron CP9695 and it does not show any code. short fuel trim fluctuates between -+ 3, one oxygen sensor reads 1.2 volt and the other .665. My miles have gone down to 9.6 miles per gallon. I used to get 22 mpg. I have a Hyundai Elantra GT 2.0 and has 65,000. What else can I check to solve this issue.

[@MotorAgeMagazine]

I think it's time you took the car to a professional shop. I"d have to have this one in front of me to be of any further help. Good luck!

[@mackchannel6348]

Great video. I have a 30 year old pre OBD2 Toyota Celica, and it's occasionally running a bit lean (I have a wideband AFM not hooked into the ECU). Giving me food for thought. New plugs, distributor cap, and ignition wires. Air Flow Meter and O2 sensor come to mind as possible cuplrits--could be they're telling the ECM it needs to send less pulse to the injectors.

[@oxygen454]

First test I did had max LTFT and nearly maxed STFT at idle. Suspected vacuum leak. When throttled up to 2k rpm, the LTFT and STFM dropped to near zero. Owner drove the car for another week...now the car is showing maxed trims at all engine speeds and runs really rough... I wonder if the excess fuel killed the CAT? Or maybe I didn't get the car up to normal operating temperature due to having to keep foot on the throttle for so long to keep it running. (2010 HHR 2.3L)

[@pauls4995]

Informative, but maybe explain in simple terms. Is Lean + or - on the LTFT? Is Rich + or - ?

[@jokekelleey2071]

The fuel trim is one of the main indicators that you should look at just like the O2 sensor if the O2 sensor is giving you a good signal and you need to know what that looks like it should be going up and down real quick because the engine is actually puffing in air in other words it's skipping a cycle and pumping in air to heat up that catalytic converter

[@matthewpetersen4417]

So I did just run into these codes, and I watched my fuel trim, LTFT was a plus 55, I had a bad throttle body - she was letting air through somewhere, changed it and I have about from 8+ to 0 LTFT. My next question could that cause p0420 p0430 on new cats.

[@hemedocx]

It would be good to show some examples.

[@sunsetracing3809]

Hi thank you video help me a lot, I still have a question, I working on my 1996 Miata, I think the car is not engaged to close loop, how I can make sure she is in open loop all times?

[@MotorAgeMagazine]

It's the other way around. You want the vehicle in closed loop operation as soon, and as much, as possible. You can see the status on any Global OBD II certified scan tool.

[@Charkatak]

Thanks for the explanation as I am trying to figure out if my car is running properly. The RPM just goes under 700 when idle and sound of the engine of course changes. There is also some fuel smell coming from the tail pipe when doing 10+ min idle. After driving for some time, parking and smelling the exhaust, it doesn't smell at all. I did capture live data(15 min drive) and if I give you the link, will you be able to help and look at that data?

[@MotorAgeMagazine]

I can try...be sure to inclde make, model, year and mileage.

[@Charkatak]

@@MotorAgeMagazine Thanks! Just sent you 2 links with car info to Facebook link you had in the description.

[@MotorAgeMagazine]

I don't really see anything in the data except some strange readings from the MAF sensor. Cleaning very rarely fixes one that has become contaminated. However, I also see some strange PIDs on the data list you sent. I've never heard of Bank 3 for example. You might want to look at the information with a better quality tool first.

[@Charkatak]

@@MotorAgeMagazine Thank You! I will try another scan tool and/or the app. I don't know if MAF was contaminated or not, but as preventive maintenance I cleaned it to see if it was the reason my vehicle went below 700 at idle(at times). Could you please tell me about strange readings with MAF you saw, so I can troubleshoot and look at my options?

[@kingzach5000]

Love it

[@albertcassler8763]

What I found confusing was the term fuel trim. Note, to trim, means to me to cut back, as in trim the bushes. So I was only thinking in black and white, instead of like adding decorations to the Christmas tree...duh. so why not have the best of both worlds when it comes to fuel trim. Trim up or trim down, whatever the case maybe. Now who's the boss, it's the mechanic calling the shots to make it all work like it should..!

[@timbo19751975]

No. Trim means to 'Shape ' in this case. Not reduce.

[@user-cm6oz4ze5k]

Please tell me how to what to do about code p06de AES techs don't know how to check and clear this code techs turned off engine light but it came back on.

[@jokekelleey2071]

In the map sensor is a vacuum operated which senses the different vacuum your throttle creates a vacuum because it's closed and when you open that the vacuum goes down it's kind of like everything is there to react to give it gas just like the throttle position sensor the TPS. And as you know the multi-port fuel injection is right there at the cylinder it's not that it changes every time but at idle the engine is revolving at 10 times a second that makes each cylinder which you could multiply by 6 so that's 60 explosions per second.