Understanding Short Term and Long Term Fuel Trim (a SD Premium video)

okay fades for the old tram this is an introduction to it we're going to be doing this again in I believe it's section four oxygen sensors fearing operation what's the title what section for you throw that at me real - so - oxygen sensors in fuel terms so this won't be the last time you hear it I want to try to stay away from the variables with oxygen sensors for this part I just want to introduce this to you and I mean we'll cover it pretty good but there's certainly more more to it when it comes to whether or not an oxygen sensor is reporting accurately or whether the sensors bad can definitely mess up these numbers in these calculations but for now I think it's important that you get a good idea of whether or not a car is running rich or running lean or running normal and I want to introduce this to you for that reason um so I have some bullets up here as far as what each fuel trim does short term fuel trim long term fuel trim and I think the first thing you need to understand what's going on with the trim is to get a little bit of an insight to what the oxygen sensor is doing and I'm the best thing that I can do would be to just draw you a picture and talk about it for a second and we'll just jump over to here and basically when you're looking at an o2 sensor that's producing a signal you're going to see an oscillating signal like that um and there is a lot of theory that goes in behind the o2 and again we'll do that more later but for now understand that this dotted line that I drew right here that numbers above this dotted line which by the way is 450 millivolts this would be a rich signal and numbers below this would be a lean signal and notice I use the term signal and not mixture I'm careful in my terminology because an o2 sensor can report the wrong signal and so if it's reporting the wrong signal the mixture is going to be opposite I don't want to get into that right now we'll do that part later just assume for now and these come this conversation that we have a good reporting oxygen sense so first thing would be is why does the signal look like that um it doesn't always look like that the sensor produces its own voltage so this sense of the signal you're looking at is basically in a range of around 0 to 1 volt with a typical amplitude or mid max voltage of around 200 millivolts at the bottom and 800 millivolts at the top not uncommon to see one hundred and nine hundred millivolts in that range but that's the o2 producing the voltage chemical reaction between the amount of oxygen in the exhaust compared to the amount of oxygen in the atmosphere so an oxygen sensor actually is sampling both areas if there's a lot of oxygen in the exhaust um there's obviously a lot of oxygen outside there's not a real big imbalance and so the sensor produces low voltage another way you can remember that a low voltage would be a lean condition is two L's or an association low is lean low voltage is a lean sigma um when there's very little oxygen in the exhaust which would be a rich condition that that sensor is going to have a bigger imbalance between the oxygen and the exhaust of the oxygen outside and it's going to produce more voltage up to near a bowl so numbers above 450 is interpreted by the computer as rich numbers below 450 is interpreted by the computer as clean again sensor produces its own voltage however the sensor signal looks like this because the computer is driving it to look like that if you were to thread say a single wire zirconia Oh to into a small-block Chevy old carbureted engine that our engine teacher has mr. Kaplan has that engine on the stand if you were to thread an o2 into that it would produce a signal but what would the signal look like um it would be basically a flat line of X amount of voltage to depending on how ribs are lean that carburetor box it would produce a voltage granted there's some other other theory in here that sensors need to be hot to work and we'll get into more of that later but um it would produce a signal so why does the signal in in my red draw and why does it look like that well it looks like that because the computer is reacting to it right you've heard these terms before possibly and it was called open loop and close loop on in closed loop and again I have this in section 4 so I'm kind of getting a little bit ahead where we we are right now but in closed loop the oxygen sensor is being used and in open loop it is not you guys don't need to write that down again I have that in section 4 we'll do that again later but closed loop simply means computers going to look at the o2 and respond and react to it so if the kid oh two reports a rich signal computer sees that signal is rich and it's command is going to be to take fuel away and that's going to drive it back lean so the next pass through because the computers take and fuel away that o2 sensor is now going to drop lean computer sees a lean signal and wants to add more fuel to bring it back riche so what is causing this ah rich lean rich lean swing is the computer is making that o2 signal look like that ok so let's say that we want this vehicle operating at or near stoichiometric um the computer is going to do that by switching the o2 it's actually called an o2 switch in some reading material they'll call it an o2 switch it's not really able to detect how rich or how lean the mixture is on a on a narrow band o2 like this one is um it just shows where stoichiometric is and the computer is going to move it a little bit above a little bit below primary function is for catalytic converter efficiency okay all a cat needs certain gases to do its job and on the rich side of stoichiometric what gases are we producing eight C's what else Co is the main one I need so the reduction bed which is for NOx emissions needs Co gaps - for that chemical reaction to pull that back upon so a cat needs a little bit of co for that NOx reduction bed to do its job on the lean side of stoichiometric what gas are we producing that the cat needs oxygen so that cat needs oxygen to do that its oxidizing for that oxidizing bed of HTS and co so we need to be moving back and forth to keep the cat hat um our programs in our cars are not driven by fuel economy they're not driven by power what are they driven by primarily when you're cruising down the road what is the engine computer primary focus is emissions okay we can get better gas mileage if we run a eighteen to one air fuel ratio uh are we going to get better gas miles sure but what's it at the sacrifice of emissions right so um you know you guys are into programmers and tutors and yeah you can you can put a program in a car and totally eliminate the o2 and get better gas mileage but what are you going to be producing more emissions in there not 49 state legal right they're made for track only and what else do you have to do is remove the cap because the cat's going to meltdown oh you can't do it so primary is cat so think about that computer is driven to keep this mixture where it's out and now a little bit more with the mixture and this can be a little bit complicated so bear with me I want to pick one set of parameters for one particular engine okay because understand that every engine breathes differently depending on volumetric efficiency of that engine every engine is going to breathe differently um bore and stroke a number of valves per cylinder intake runner design ought to name a few is it is it naturally aspirated is it turbocharged supercharged every engine breeze differently so let's say we have one particular engine and what we're trying to do is match the amount of air with a specific amount of fuel to put me near stoichiometric this 14 7 to 1 ratio written in the program for that vehicle is going to be how the engine reads so the engineers already figured that part out how much does this engine breathe and we'll have this written into a program so let's talk about some other variables as far as breathing goes are we going to breathe differently at sea level compared to 10,000 feet of elevation so barometric pressure is a factor as what is it going to breathe differently with zero degrees of air temperature as opposed to 90 degrees of air acceptance so again another parameter that we have to worry about um so what I want to try to do is simplify this and say we're at sea level and we have seventy degrees of outside air and the car is at idle with the throttle closed and let's say in my hypothetical vehicle that on that particular engine that we're going to be operating let's say this is this is a just kind of load and this is Benjen load and this is rpm and there's different blocks ourselves GM used to use a it's called a 16 cell block learn I don't know if you've heard that term before coming out of dance class but what it basically was was you had different areas of a learned memory on fuel correction so I'm just going to put us in one block right now now what I'm talking about is this idle block right here so this load is going to match this rpm and let's say in my hypothetical engine again sea level 70 degrees outside air we've already calculated volumetric efficiency so we already know cam design and by lift and all that other stuffs already being calculated a wing not to mention fuel injector nozzle size the fuel pressure behind it is also a factor in here - lets all Ben figure it out and let's say that one millisecond shot of fuel that means that fuel injectors going to turn on for one millisecond is going to provide the correct amount of fuel for that air that's coming to this end did you follow all that where are we as far as fuel trim goes we are at 0% correct that's where we are right now that's ideal that's factory that's where we want to be 0% correction means that ideal number that they wrote from the factory always well ok from that point now that's going to put me in stoichiometric from that point where do we want to go with that mixture a little bit rich and a little bit lean from stoichiometric what might that look like had you looked at an injector pulse maybe graphed it on a screen as you might see one point one and point naught would one point one put me up just a little bit richer than stoichiometric as far as time the fuel injector firing would point nine put me maybe a little leaner than stoichiometric and so what drives that injector pulse or what drives let me let me say this again what drives that injection pulse which is going to drive this Oh to you guys follow its the injector opening closing that's driving that o2 up there is the short term fuel trim commit that sort term fuel trims number one job is to do what make sure that that o to sensor is switching back and forth across to look at what will that look like you're looking at a known good car you see sort term fuel trim and you're going to see positive and negative from zero in an ideal situation maybe four or five percent positive let's call it five percent negative five percent it might not be that much maybe it's plus 2 and minus 2 in any case that sort term is going to be swinging up and down that sort terms command when it goes positive what's that command to do add field sets in and fuel in the floormat of turn the injector on for a little bit long when it goes negative what's the command take fuel away which is going to look like the injector pulse using just a little bit what's that going to do to the Oh to sing that right there that top picture Oh to is going to go rich and lean in response to the short term field trip so now let's do the same thing and let's say that we're at I don't know half throttle half load and let's say up in here by the way this bottom number to call that one millisecond down there but it'd be a zero percent correction let's say up here where it's seven milliseconds I'm just hypothetical number here so we have 7 times more fuel but what else do we have we have 7 times more air - so we're not running rich because we have 7 milliseconds of fuel now we're at I even we had one we're still it's doing in magic so what's the short term fuel trim number up here now is a ideal again is you a percent will call that seven milliseconds what's that going to look like to trim that number it's going to go positive and negative and it might be seven point two and six point eight maybe I'm off on that number a little bit maybe it's seven one and six nine who knows right it's moving back and forth again now here's the thing about injection pulse and I have people ask me all the time what's a normal injection pulse for that vehicle and my answer is I don't know I have no idea um you know think about the variables that I just spit out at you a few minutes ago about what it took to figure out what the injection pulse should be what if you change the altitude a little bit is that going to change that number what are the air temperature change is that going to change that number oh yeah and what if you put a different injector in that car or you change your fuel pressures at all all those numbers are going to change so what you need to understand is every single car is going to be different and you can't compare like a four liter Ford to say a 3.8 GM you're thinking well the displacements close so maybe they should have the same pulse but you don't understand they might be using different injectors what if Ford decided to use a smaller orifice in the tip of their injectors in there on time going to be totally different than what GM is going to be so what's a better number to look at to know if your pulse width is right field trip because if your fuel trims right your pulse what's going to be ready right we look at the fuel trim to know how that cars perform and so the first one is the short term short term is driven to keep that o2 up here and just like just like it idle down here off here this Oh 2 is going to be moving richandleight ok fine so let's put a problem in this now let's see what happens when there's a problem I think it helps to see a reaction to a problem with a fuel trim issue let's let's put a vacuum leak in this first let's see what what happens with the vacuuming and I'll just pick the i/o cell here for a second and let's say this da l o2 is happy and I pull the brake booster vacuum hose off of course you know to signal is going to go lean or get a lot more air in this engine than what's being calculated and that Oh 2 is going to react and it's going to show the computer leaned to dish what does it immediately react and want to do it's going to try to rich in the mixture and what's that going to look like to us our short term say over here was going at 0% a little bit pause a little bit negative from zero but now at this point that short term is going to start to add fuel and it's going to start on positive until that emma2 comes up let's say hit plus 30% by the way when you're looking at fuel travel and scan tool they don't give you plus some they only give you minus signs ok so no number in front of 30 is positive 30% it had at 30 percent more fuel before that o2 came back rich again and now what happens is the o2 start switching again and the o2 is moving up and down pasto a key metric we're protecting the cat giving the cat what it eats but the difference now is I'm now at 30% as a baseline and now to correct and add fuel from 30% goes above 30% and to take fuel away it goes - from 30% now wouldn't be a - it might be plus 28 but isn't plus 28 less than 30 so would it be taking fuel away so now my new area of operation for this fuel trim is now 30% and by the way trouble codes usually fall in about 15% is where you'll get a trouble code for a fuel trim this is where your lean exhaust and rich exhaust trouble codes come from right on fuel trim numbers that are beyond 15 to 20 percent of that rings all right so everything is happy but one of the things that happen with this vacuum leak is of course the customer is going to complain about poor gas mileage not to mention some other idling issues if you pull the brake booster hose off but it's pretty big leak um but definitely going to have some gas mileage is using you can understand why we're adding 30% more fuel than what would be normal at idle from a vacuum okay um if this was it if this was all the system did when you shut the car off and let it cool down restart the car and go through the process again um it would be lean for a long period of time until the computer until the o2 came back to life and the computer knows whoa that o2 is lean and and add fuel all over again so the process would have to repeat on every shutdown so where's the long term coming to play it's kind of in the name long term fuel trim is it's retained in memory it learns where the mixture was and what do you think the computer does on the next start it looks at its long-term memory and says okay this where we water so this is where we're going to start so let's talk about how the long-term comes into play so I deal with the long-term is going to be 0% an ideal long term fuel trim of course is 0% you're allowed a tolerance plus and minus 10% is considered normal I'm not going to even bat an eye at a negative 5 a positive 5 a positive 7 you with me a negative 7 I'm not worried about those numbers I'm probably not going to find a cause for that um there are times where you might want to look at that a little closer depending on symptom but in this case 0% ideal and a little bit above and below zouk a so we were good here and not what happens my short-term is at 30% and now if you understand the number ones over the long term you will understand what's going to happen next what's my second bullet say on that page number one number one job of the long term keeping shorter keep the short term as close to 0% as possible so is my short term at 0% no it's at 30% so yeah it was over here but we're looking over here now is my is my short term at zero no so the long term is going to try to bring that short term back down and what this is going to end up looking like is oh of course it's going to matter on the car on how fast this takes place every car is different in the reaction of the long term but GM and Chrysler are pretty quick at changing their long term and Ford's a little bit delayed and changing theirs but in any case of what's going to happen is this is going to end up counting up and as this one counts up this one's going to count back down and what's going to happen when these balance out is long term is going to be 30% and my short term is going to be back it's you now you might be thinking what kind of strange and I have a good analogy on why this happens which one is responsible for keeping that out to where it needs to be short term so if you think it's important to give the short term as much correction capability as possible here's another thick piece you're missing to the pub fuel trim numbers are limited they're not given ultimate control and by ultimate control I'm saying what would happen if a no to signal shorted to ground on the body of the car and that's reporting a lean condition we said blow is lean right what's the computer's reaction is to add fuel add fuel add fuel what if we didn't let how much fuel would that thing at you'd have raw fuel running on a tailpipe with you we have to limit these fuel turn numbers and so if you can think about it like this and I don't know force or every car is different on its limitations okay but if you can think about this as your long-term and this as your short-term we have limits and I'm just going to since I use 30 as a number we'll use 30 as our limit okay I'll say that my limit for short-term is 30% and 30% down here negative 30 positive 30 and sometimes a long term is different sometimes it's not for argument purposes we'll just say that our long term limit is 30 and minus 30 as well and we started off in the mid before we want to be we want this short term to have movement positive and negative to be able to do what it needs to do to that o2 sensor when this vacuum leak occurred and put my source arm way up here does that sort term have any more control and correction at least in the positive range it's out of its limit so if I can take this and and realize this car is running lean and put this in its memory down here now branded out buried the long term but is that going to bring that short term back down to this midpoint and it does so does it now give the short term more control and that we could actually even run leaner and I can get more fuel into this car so one of the things you want to think about is this there's limitations the second thing is it's the total between the two that give you your total fuel trim see if you guys can answer this question I got a long term fuel trim of 30% and a short term the fuel trim of 0% are we at our limit on either one can you tell me that right how do you know that so you say long terms at its limit how do you know that what if I've seen cars with a 50% long term what if I did this what if I said the long-term was at 30% and the short-term was at 15% and this car hadn't been cleared and it's been driving for a while could you answer that question now and the answer is you can your long-term is at its limit how do you know that number one jobs of the long terms to do what to the short keep the short term at zero why isn't the short term at zero on this call because we're at our limit on the long term and all this short term is probably bouncing positive and negative from 15 but is there still control for fuel for the cap this car not that this is any diagnostic advantage but you can use this information to know where your fuel trim numbers maximum is what if you saw a car that was 30 percent here and 30 percent here you'll see him you just got a really really big Vaculik or really really bad lean condition or possibly even another to problem but this would be a completely out of control system most likely in this case if you were to see that a good-looking o2 would look like this you'd be down here 50 millivolt flatline on the o2 if you saw those kind of numbers so just a little bit of insight on how these things react to each other um let's expand on this just a little bit more watch what happens now everybody okay don't hesitate to ask a question all three PD so everybody can hear it and we'll talk about all right so we've in our scenario we've compensated for it so our sort term i'll i'll rewrite the numbers our short term fuel trim with our brake booster hose remove ah is back down at zero and our long term is starting if we shut this car off we restart the car where's the computer look first yeah if you're a kid and you burn your hand on a stove it's your long term memory that says I probably shouldn't touch that again all right it's not your short-term memory it's your long-term memory is a good motivator right oh I don't know that's a good analogy but long-term memory says where were we running we were running what rich earlene easy to say rich isn't it I tricked you on purpose it's a rich command this cars running lean but we got to get those numbers down everybody does that you see 30% fuel trim this cars running lean that's a rich command in reaction to one a lean condition but the memory says we need to start 30% more fuel so what's going to happen soon as you start the car possibly even before the other twos even warmed up and active and ready to go the computers already changed that the old curve to be 30% more rich than it was previously so our emissions improved with this long-term memory that we don't have to wait for the process to take place again it's already there okay so you start the car what's computer looking at first long-term okay now this is where a problem might have especially in your interpretation I know in particular on the ASE l1 test question there was a question like this and it showed a a car lean condition after the technician fixed it that we had a short term fuel trim command and a long term fuel trim command they show the long term was 25 and short term was negative 25 and they asked you what's wrong with this car is it running rich is it running lean is it running normal what's the problem this was an ASE l1 test course the answer is the car is fixed and it's running normal they should never contradict each other if they're contradicting each other the problem is either gone or somebody fixed it and didn't wipe out the memory so here's what happens we start the car what's looked at first long term long term memory is saying what what kind of command is that add fuel okay um in the process of uh setting the car off we we've maybe earth maybe didn't even shut the car off if I plug the brake booster back hoes in that I just that I just fixed what happens what's the o2 signal look like let's all get a repeat of up here oh we're happy now right we're happy being that the o2 is happy it took 30% more fuel to make it happy but as the o2 happy as the emission is the cat happy is the cat being fed yes what happens when I plug that vacuum leak up what's the very next thing that happens what's the command LT fuel trim is 30% that Oh 2 is not a flatline rich now right because the memory is stood still looking at the long term to know where the base is short-term is of course is still into play but what happens now short-term is looking at this number and it's saying whoa Oh - you're too rich now so what's immediately the short-term was at zero right it's been corrected for long terms at 30 what's the short-term I'm going to mmediately start to do take it away that's right it's going to start on negative and once it hits that point it's going to pull that Oh to back down now it's happy what if you took a still capture digital photo of that moment in time what do you think you might see you might see the short term and long term short terms at negative 30 and long term is at 30 that's what you might see at that brief moment in time does that make sense that's what you'll see that was an ANSI l1 test question so remember that when the cars run normal the technician didn't wipe out the memory you've probably heard of people telling you make sure you wipe out the memory after fixing a car ah that would be one reason what memory are you wiping out that long-term memory okay everybody okay it gets worse not really it's not really but if you if you were able to absorb what I just threw out you you need to understand that my little generic cell block up here ah of these 16 cells this is an older one yes you have a question um this uh this block you have to understand that every single cell or load and rpm combination let's redraw and again older GM's use sixteen cells you think today's cars use more probably I don't know how many different cells they operate for fuel trim but this let's put a bunch of them in there just for just for a hypothetical uh Theory purpose let's say that this is engine load and this is rpm if the memory is wiped out what is each cell filled with and that would be you know if you're here on here that would be one that's going to put you in that block or if your load is here your rpms here if you're a audible you probably be in this block what do you think every block is filled with if you wipe the memory out zeros right that's our percentage of fuel trim correction UH one of you guys mentioned a binary number it would be 128 let's put this back Vic goes class where he talked about 256 possible positions that just was computer binary numbers it's the same thing why 128 if you're talking binary what 128 is zero by the way why one time why do you think they picked 128 s0 if you looked at 255 from 0 to 255 you have 250 256 possible computer language zeros and ones right binary numbers what's right in the middle of 256 zero percent okay so don't sweat those numbers it was just older GM's that use them we'll plug those in later in section 4 but in any case whether it be 128 across the board or 0 across the board that is what you're doing when you wipe out the memory right our car that we made the vacuum leak on again let's say this car was normal everywhere else so you'd have zeros across the board on this call what is the one cell that had relearned a number item and it was 30 percent so that means when I go back to that cell let's say I pulled that car up out of the idle block where am I now I might be in I might now be in this block what that block say for memory zero rooms so John an idle your own two is happy you got a vacuum leak still roll back and you raise the RPM just a little bit what's up what's the O to do immediately those who immediately drops leaned again and all your fuel trim numbers change so the last part of this is to understand that every cell has a learned long term memory in it okay so just because I relearned the idle cell everything's back to normal by the way when you're long-term at 30 in short-term at negative 30 are these going to balance out yeah because what's the long term driven to do keep the short-term at zero the short-term is way down there so what's the long-term I'm going to do is going to start taking fuel away these are going to balance out basically when we're done at zero and zero what if this car had been driven for a long time with a vacuum leak do you think it would just affect the idle cell or would it affect a good portion of those bottom cells because that's where our engine is operating in and that we have to relearn all of those sounds how do you reload the cell you got a drive in that cell you're not driving in that so it's not relearning that stuff have you ever heard anybody tell you it can take 500 miles of driving to relearn a car yeah because maybe you never get into that block and if you never get into that block you're not relearning it not that it's an issue it will eventually relearn itself I don't panic about resetting the memory on cars in fact for fuel trim numbers I actually want to see the learning process that tells me I fixed the car if I get a car that comes in it's got 30% fuel trim long term and I fix a vacuum leak what's the first thing I want to see when I'm done I want to see the short term go very negative if I see short term go negative 25 while the long term memory was third did I fix this car that we left with 5% on the long term is 5% accan a considered normal is this car going to come back with a check engine light on now I'm done I'm happy I want to see that so do I want to wipe out the memory right away now so a little bit of insight on fuel trim I mean obviously we're gonna do a lot more with this later on in this section actually I'm going to show you how to recognize what type of lean condition you that and this last black line I drew on the screen here uh is key what's going on down in these cells compared to up in these cells which be higher load higher rpm we can actually identify what type of lean condition that we're dealing whether it be vacuum leak low fuel pressure 30 mass airflow plugged injectors we can actually change our rpm while we're watching fuel trim numbers and get a good idea of what type of vacuum leak we're dealing with when we have leaders okay everything that I just described to you would be the flip side of the car was running rich if you had a leaking fuel pressure regulator whatever you're sucking fueling through the vacuum hose what do you think the reaction is going to be immediately with the short-term field trip it's going to take fuel away so what are we gonna see we're gonna see negative numbers everything is there it's just the opposite okay last comment don't forget if you see negative fuel trim numbers that's a lean command the car's not running lean it's running rich if you see positive fuel trim numbers that's a rinse command that car is not running rich it's running lean one last variable co2 could be live at the o2 is lying everything's wrong does that make sense because is everything reactive to what the o2 is doing then if the o2 is reporting in accurately oh twos can can skew and they can read on the rich side all the time we think the computers going to do take all the fuel waiting it's gonna max out both fuel trends they're both going to be negative all the way how do you think that car might run it runs good cold because the other twos not used we're not in close loop runs good at wide open throttle runs horrible all the other time low power won't get out of its own way puts it to the floor runs beautiful those are the symptoms what about the other other end where the o2 is is sorted and reads lean all the time what kind of symptoms might you have from that what's that yeah I'm Way too much fuel so under load it might not run bad because car is under load need more fuel rich is good but uh all other times I don't low-speed and yes horrible black smoke fuel mileage is gone right this what I just described to you is what happens when you have a fat o2 right so an o2 is really not needed to make the car run good however when you have a computer system that is set up for one it can really mess the car up bad can you put a program in the car you guys that are tuner guys love to do this on your Honda's eliminate your own twos and eliminate your cat car runs beautiful and it does no argument here Oh two is not needed to make a car run good right but when you have a program that's designed to have one you cannot simply remove it and just expect the car to run good because what's a computer driven to do make dado to move if it ain't moving it's going to keep try and how's that car going to react not very well okay little intro to fuel trip alright so we're going to do a fuel trim review we talked yesterday about how it works how it functions and the first thing you want to know on the screen is our sort term fuel trim to the left this star attitude to the right I have the engine rpm point up and long-term pulled out this is on a 2008 Dodge Caravan and this vehicle does have a little bit of a lean condition as noted by the nine percent positive and I know we already found it has a little small leak on the EGR stem area so it needs a needs your bow but we can still review this process with this car um one other thing to point out here is the voltage range on this OH - on this Chrysler you can see it's two and a half at the base 342 at the top this is a different subject I actually have this in my book and it's a two and a half full bias that Chrysler uses on their o - s which change the signal I'm not going to go into that right now just imagine two and a half as your lowest lean and 3.2 as your highest rich somewhere in the middle of this is stoichiometric so it's still a basically zero to one moto - ignore the numbers for this demo for now we're going to focus on our fuel trim so you can see short terms moving up and down doing exactly what we described which is altering the pulse width which is going to change what your o - looks like the reason the O tube looks like this is the computer is adding fuel taking fuel away and you can see that on the short term fuel trim command the command goes rich computer is going to add fuel it's going to drive the other - rich when the command goes lean the computer takes fuel away and it's going to drive the O to lead so what we want to do first I'm going to make a vacuum leak but we're going to watch the reaction first thing that's going to happen short term is going to start adding fuel right away and your notice the long term is going to follow behind it so once the short term watch CEO to watch the long term I'm going to pull it brake booster vacuum hose off so really big vacuum leak as noted by the auto-tune dropping leaned and immediately what you see happen sort terms started to add fuel incorrect Oh two is still lame short term still adding fuel and what you're going to notice is this long term is going to start correcting it's going to start adding fuel because it wants this short term to be lower than what it is we want this short term down at zero near zero we should see this long term correct some cars are a little bit funny all the car just go that sucks okay we'll try that again start that back up we might lose our down a little bit notice the short-term back at zero why because we're an open-loop we just went into closed as noted by the short-term increasing I want to see this long-term correct behind it again long-term memory some cars are different than others in how this reacts it might take a little bit of time hopefully this won't stall and it'll stay running for our demo here short terms maxed out at 33 looks like that's the top number that the computer is going to allow for a short term you notice our Oh 2 is still we still have not seen our long term correct yet you just business looks like it dropped out a closed loop momentarily there ended when a shorter term came back down there we go long term is correcting now you see how long that took some cars reacts faster what's the long term try to do number one - over the long term chemistry tournament zero is try to bring the short term back down so you're going to start to see this guy increase eventually these are both going to bury and of course the car stalled again this might be just a little bit too big of a vacuum leak got to start back up just waiting to close loop computers recognizing the others lien adding bill I want this long-term to get a little bit higher before I plug this vacuum because I want you to see the opposite effect while the term started coughs more now this is a leak the computer can't compensate for it's just too big so with short-term buried at 33 long-term still counting Oh two is still lean I'm going to plug this vacuum hose up now let's see what it does hopefully knots the whole car in light all right as soon as I plug that vacuum Anika what's the first thing you notice happen is the o2 went rinse why because the command was 30% here 24% here notice short terms now what going into the negative numbers long terms starting to count back down short terms now moving RIT saline again Oh twos now moving rich and lean again couldn't really see the positive fuel trim negative long term sorry you really couldn't see on this car the long term fuel trim stay positive in the short term go away negative I missed it I don't know if I saw it see if we can go back and take a look at that moment it was so quick this only got down to negative nine point two the short-term before the long-term started to correct well in some cars what you will see is that as I described yesterday which was negative 25 percent on the short-term positive 25 percent on the long-term and that's a car that's readjusting and fixing itself couldn't really see that on this one but it happened so fast which is why we missed it but unfreeze this next thing I'm going to do now is I'm going to add fuel to the intake and we're going to watch this picture first thing that's going to happen o2 is going to go rich short-term should now go negative long-term is going to react behind that so add some fuel propane go to new intake low twos full rich again this computer is not liking what I'm doing see how the fuel trips or turn went way negative then jump back up to zero now it's going negative again that's keep the computer dropping in and out I open it close little long term coming behind trying to balance out the short-term Oh - still rates alright I'm setting the fuel off now what's the first thing that's going to happen long term was at negative 33 soon as I set the fuel off with co2 do now why zo2 lien because the command is still negative 33% so o to lean sort terms now correcting adding fuel watch this number these are going to be opposite each other now short term is positive long terms negative what's looked at first long terms looked at first the command is issued Oh to reiax computer dusty oh - on the short term you just start to balance out like a pause this negative 22 positive 12 getting pretty close to what I described so again as a review number one job the short-term keep the o2 moving up and down number one job in the long term you keep that short-term as close to zero as possible hopefully I did a good job of showing that reaction this fans a little bit funny in how it reacts every card has a little bit different timeframe that the long term will react but a pretty good example of those two characteristics short term long term fuel trim on a 2008 Chrysler Town and Country

Channel: ScannerDanner

Views: 387,213

Rating: 4.9012113 out of 5

Keywords: O2 sensor operation, Rosedale Tech Pittsburgh, Auto Tech, Tech, Engine Performance Diagnostics, mechanic school, rich, lean, auto repair, Paul Danner, auto tech training, training, How to, STFT, DIY auto repair, LTFT, scannerdanner, car repair, fuel trim, what is fuel trim, how to read fuel trim data, short term fuel trim, long term fuel trim, vacuum leak analysis, low fuel pressure, dirty MAF sensor, lean and rich conditions, understanding st and lt fuel trim

Id: oRX2V6_a3dc

Length: 49min 45sec (2985 seconds)

Published: Sun Apr 01 2012