Secret of Engine Problem Diagnosis- Fuel Trims Pt.1

greetings fellow do-it-yourselfers in this special video I am going to reveal my deepest secret and that is the understanding of fuel Trims and you will need a scan tool for this however you can use even the most basic of scan tools nowadays there's $100 scan tools from Actron available uh that you can easily get and you need one that will have fuel trim data short-term and long-term fuel trim data and preferably oxygen sensor dat and the idea is if you master the concepts that I will show you in this video about fuel trims you will absolutely dramatically improve your diagnostic capability far beyond that of your average weekend warrior or shade tree mechanic and to be honest with you even passed a lot of professional mechanics to be honest so let's go ahead and start from the beginning and talk about uh fuel induction computerized fuel control you have to understand that to then understand fuel trim once you understand fuel trim you will be able to actually think like the engine computer if not the engine itself and therefore given certain responses that the engine has to different stimuli or different inputs that you give it you will be able to very much determine a good direction it's a very very powerful ability to have so if you're the type of person that likes to to replace all kinds of parts and get all dirty and everything and use every wrench in your toolbox this will not be the video for you for you guys uh AutoZone is open till 9:00 believe me they love you for the rest of you that want to be able to get an absolute accurate diagnosis or at least a accurate Direction on where to go on most check engine light problems then you're going to have to master these Concepts so let's take a look all right and yes we are going to use my amazing artistic abilities in this exercise but uh let's start off here uh we've got an engine here and of course we've got air that comes into the engine uh we have fuel delivered into the engine and of course we have an exhaust with a uh usually an oxygen sensor we'll label that O2 and I am well aware that most new engines um don't use oxygen sensors now they have these things called air fuel sensors and if you're familiar with oxygen sensors the first time you run into an air fuel sensor it's a pretty interesting experience and probably an expensive one because you're going to almost certainly think you have to replace uh a non-responsive oxygen sensor O2 sensors and air fuel sensors or wideband oxygen sensors are completely different things I'm going to actually do a video on that um soon to explain those but know that for the concept of fuel trims which is the purpose of this video the concept of fuel trims is identical whether your car uses an O2 sensor or an air fuel sensor so that part won't really matter as much the concept of fuel trims is identical either way now we're going to go ahead and use oxygen sensor in these examples simply because I want you to master the concept of fuel Trims and if you most likely if you're watching my videos you're more familiar with oxygen sensors so we're going to use that as an example but it really doesn't matter for fuel trims so in your engine of course you've got the air coming in it mixes with fuel in a combustion chamber it blows up and then comes out as exhaust and the oxygen and the exhaust measured by an O2 sensor now the important thing to know in a computerized engine is that the computer is very very very particular about this air fuel mixture and that air fuel mixture is always well not always but always going to have to be what's called a stochiometric ratio or in stochiometry and that's going to be 14.7 Parts air to one part Fuel and the computer is going to be very very obsessive compulsive about this there's only a couple times that the computer will not have that mixture that would be on a cold engine startup you need a rich engine for better startup um wideopen throttle is another example but there are some um exceptions to this rule but in general keep in mind computer is obsessive compulsive it is always going to try to make a 14.7 to1 stochiometric mix now the question is how does the computer do that how does it know well it's it's not from the oxygen sensor actually it's because with this air coming in there are going to be various input sensors to detect the amount of air coming in uh in many engines there's going to be a MAF or a mass air flow sensor also keep in mind that the temperature of the air has an effect colder air is more dense so there's going to be some type of intake air temperature sensor to tell the temperature of the air um many engines also have on the intake a manifold air pressure or a map sensor answer and given all of these inputs here the computer knows how much air is coming into the engine if the computer knows how much air is coming to the engine then it knows how much fuel to add to stay in STO geometry and it of course adjusts the fuel by regulating the pulse width on the fuel injectors now after the combustion the oxygen in the exhaust is measured by the O2 sensor and part of the responsibility for the O2 sensor it's got two responsibilities one of them is to maintain optimal activity of the catalytic converter and the other one is to adjust the amount of fuel necessary if the combustion didn't exactly go as planned in other words a fuel trim it trims the amount of fuel added to compensate for any error from these sensors or more likely from any unmetered air any air that entered the engine that wasn't measured by these sensors a vacuum leak or by some type of fuel delivery problem leaking fuel injector something like that so um let's go ahead and look at this in a little more detail because we need to understand how the computer uses this oxygen sensor to tell whether there needs to be some adjustment to the fuel so we have our air coming in in mixed with our fuel and of course it combusts and the combustion is sort of validated by the O2 sensor now if you're familiar with an O2 sensor you know that it has a wave form to it like this um from 900 molts to 100 molts an oxygen sensor is basically a battery that once to emit about a volt and in the presence of oxygen however that voltage is in inhibited so if there is oxygen present in the exhaust in other words a leaner exhaust then the voltage is going to be at the lower end of the scale and if there's very little oxygen at all in the exhaust then that's a richer exhaust and the O2 sensor can emit maximum voltage but this doesn't make much sense at this point because if we think about it if the computer is trying to maintain a perfect stochiometric ratio then the oxygen sensor should be flatlined at45 Vols correct correct that actually is right so in your ideal situation this is what you would have now the problem is is that a catalytic converter is not able to optimally clear pollutants in this condition a cataly converter can clear some pollutants when there's a little bit of a richer condition and increasing the temperature and then there is a other pollutants that the catalytic converter is better at with a leaner condition so the computer is going to actually control fuel delivery to oscillate between actually let's make a dotted line at 045 volts but the computer and here's 900 molts and 100 the computer is going to oscillate between that lean and rich condition and that's why you get this waveform however if the engine is at stochiometry there will be a center line between those sine waves at 045 volts so that's why you have that oscillation with the oxygen sensor now there is our perfect world right very simple computer knows how much air is coming in it knows exactly how much fuel to deliver the computer oscillates a little bit Rich a little bit lean the oxygen sensor responds to that with this wave the computer sees that wave is optimal perfect world what is there to worry about well the problem is is this assumes that the engine is absolutely accurate at measuring how much fuel and air is being mixed it knows exactly how much air is coming in what happens as the engine ages or if the spark isn't quite as much or there's a variance in fuel pressure or there is a little bit of a vacuum leak so more air comes in but this air from the vacuum leak is not measured by any of the map or math or uh IAT well then that creates a problem for this system because now since the amount of extra air from this vacuum leak coming in is not measured that is not going to be compensated for by the fuel in the engine and there for the exhaust is going to be lean and that is going to start creating a response in the oxygen sensor where it starts leaning out and that is not good and the computer does not like that enter fuel trims to correct this condition so let's take a look at that all right now keep in mind that oxygen sensors aren't even functional until they get up to 700° or so so when examining fuel trims and doing the things I'm going to show you make sure that the engine is warmed up um again at cold start the engine needs to be rich anyway but it doesn't matter the oxygen sensors won't even be functional but let's take a look at a fuel trim uh let's go ahead and we've got our engine and we've got our um intake here with of course let's put a math sensor on here and then maybe there's also a map over here and of course we're going to have here an intake air temp so this engine has all of these inputs to measure that amount of air coming in the engine knows how much air is coming in all right but we've got a little bit of a problem right here is an intake leak so we have some unmetered air coming in through this leak all right and then of course what's going to happen is the exhaust is going to be leaner and that's going to be detected of course by the O2 with a lean trace the computer does not like that what the computer is going to do is it is going to see this and the computer is going to say hm there must be some other source of air coming in I don't care I don't know where it's from I don't know what's wrong maybe it's also because there's just not enough fuel delivery who knows who cares all I know is we are not at 14.7 to one we need more fuel to bring this back up to stochiometry and the computer will do that using shortterm fuel trim or or stft as you will see on your scan tool and what you will see in this case is a couple things if we look over time at your short-term fuel trim it is going to be shown as a percent and the way to think of fuel trims is as kind of a percent deviation from normal that is required by the computer with the addition of fuel to maintain stochiometric um stochiometric ratio so in this case uh what's going to happen is your short-term fuel which would normally be at zero with the vacuum leak your short-term fuel trim is going to increase it's going to start going positive and these numbers can be anywhere from 50 to 50 to 100 whenever you see your short-term fuel trim or long-term fuel trim which I'll talk about going positive that is a response to a lean condition conversely if you see the fuel trims going negative that's a response to a rich condition and that's going to tell the computer to reduce injector pulse length and reduce the amount of fuel delivery in this case with a vacuum leak with unmetered air entered we would see the short-term fuel trim increase it's kind of the equivalent of adjusting a carburetor for a lean condition sort of by adding more fuel that's what's Happening Here what we would see with the oxygen sensor is of course the oxygen sensor would be normal with the induction of a vacuum leak we would see it lean out but then as this short-term fuel trim increases it's going to add more and more fuel until we get back to normal once that happens that short-term fuel trim is going to level off and it's going to remember that setting for this condition on the percentage of fuel that was necessary to add to return a stochiometric ratio and that level that the short term is at is going to be memorized it is going to be learned by the computer as long-term fuel trim and what the long-term fuel trim is is that sets a new zero point for the fuel delivery let's say it's at 25 % more fuel in this case and the computer is going to remember that so the engine is by default going to add 25% more fuel until the condition is fixed so let's take a closer look at that so here we've got two graphs we've got short-term fuel trim on top and long-term on the bottom and let's put 0o uh 25 and minus 25 and the same thing here 0 25 and minus 25 and let's say we've got a brand new engine and everything's all great so short-term fuel trim is going to be at around zero long-term fuel trim is going to be at around zero uh down here is your O2 sensor Trace O2 sensor Trace looks great everybody's happy right then let's say we pull a vacuum line well when we pull the vacuum line O2 sensor is going to detect that extra lean exhaust that we create because the computer doesn't know pull the vacuum line it has no idea the oxygen sensor does detect it though so it's going to go lean out remember lower voltage it's detecting lean once that happens short-term fuel trim is going to respond immediately a short-term fuel trim is an immediate response and it's going to start going positive adding fuel until the oxygen sensor starts to respond a little bit and the computer's going to keep adding and keep adding and let's say in this case it's up to 20% generally you'll get a check engine light at about 20 to 25% on most Vehicles usually 25% or more and it's going to be a code for a lean condition we'll talk about that now this whole time that this has happening this long-term fuel trim is going to start going well we better start paying attention to what this guy's doing so he's going to be a little slow because he's a learned response but eventually that's going to start pulling up now once enough fuel is added where the O2 sensor returns to normal and the computer sees good we're back at stochiometry that percentage 20% is going to be memorized by the long-term fuel trim and that sets the new Baseline but that means we no longer have to call for the addition of fuel from the short-term fuel trim because the long-term fuel trim has now set that 20% extra fuel as the new Baseline so we're going to see the short-term fuel trim start coming down back to zero and of course the oxygen sensor continues on its happy merry way so the thing is is there is still something wrong with this car even though the short-term fuel trim is at zero and the O2 sensor looks great we're still adding by default 20% more fuel to compensate for that vacuum leak that we caused so that's the new Baseline and that's not a normal condition but the engine will run perfectly fine now let's do this let's go back just a little bit and we've got our long-term fuel trim establishing that new Baseline so our shortterm returns to zero and we plug that vacuum line back in what's going to happen well at that point because we're at a new Baseline and there's no adjustment to the fuel delivery from the short term what's going to happen is when we plug that line in we're going to immediately have a rich condition why because the computer's adding 20% more fuel by default because of the new Baseline set by the long term if we go into a rich condition what's going to happen is that short term trim is going to start to go negative it's going to call for the reduction of fuel because the oxygen sensor when we plug that in is going to start going rich and the short-term trim is going to start going down to actually minus 20% the long-term trim again slower to respond is a learned response and it's going to start going down now eventually we're going to reduce the amount of fuel till the O2 sensor return to normal because we're back at stochiometry that means that this new learned response which in this case happens to be back at factory setting 0% long-term fuel trim and then of course the short-term fuel trim will level off back to zero again so that's how the responses of short-term and long-term fuel trim are tied in with the oxygen sensor great how do we use this now to make diagnoses well we're going to give some examples of it and as a matter of fact the diagnosis can get pretty sophisticated if you really look at this so um let's uh give a actual example of how you would use this knowledge now to determine the cause of a check edine light a very common one let's do a p171 uh and I hope I got this code right an 0171 this is going to be uh Bank One lean um in these examples right right now we're going to do four-cylinder engines and that was also assumed before in my explanations with v engines there's actually short and long-term fuel trim for each bank and we'll get into that as a matter of fact it's actually easier to make a diagnosis on a v engine using fuel trims we'll explain why later but in this example we've got a for Banger with a po171 that's going to read something to the effect of oxygen sensor lean bank one something like that of course now to you you realize what that means is the oxygen sensor is reporting a lean condition and therefore the long-term fuel trim must be high enough that it set this code that's for you of course for your parts changer or your typical AutoZone guy the code means blah blah blah blah blah oxygen sensor blah blah blah hm you must need a new oxygen sensor we know now not to put an oxygen sensor in this car based on if we see the normal oxygen sensor trace and a short-term fuel trim at zero right so let's look at this engine here um we've got uh your throttle here so air is coming in here but we have a little a little vacuum leak not a big one just a little one but that it sets this lean code so we plugg in our scanner and we've got a p171 so we are going to go ahead and what's the first thing we're going to do well let's validate that code let's look at the long-term fuel trim and we see our long-term fuel trim is at 25% now what is the short-term fuel trim going to be without even looking well most certainly the short-term fuel trim is going to be around 0% because remember the long-term fuel trim has set a new Baseline so there's no longer the need for the short-term trim to call for fuel and we know our oxygen sensor should look like that if we have these conditions and you put an oxygen sensor in this car to fix it you're an idiot now let's take a really close look at this because one of the things is remember we don't know that there is a vacuum leak in this engine all we know by looking at the long-term fuel trim is the engine's running lean what other things could cause this couldn't a dirty mass airflow sensor cause it of course could a weak fuel pump not delivering enough fuel causing a lean condition caused this absolutely it could so how would we know that we have a vacuum leak based on this the answer is by thinking like the computer how would it respond if this were a vacuum leak and how can we simulate that to see if the responses are consistent with a vacuum leak so let's think and use our brains really quickly here we've got a little vacuum leak so we do have some unmetered air coming into the engine and all of this air here of course is metered air that's coming in through the sensors if the engine is at idle there's fairly little air coming in through the throttle so the vacuum leak plays a fairly significant role in contributing unmetered air to the total amount of air in the engine now let's say that we open the throttle plates and we get way more air coming into the engine we increase the engine RPM or um you could see on your scan tool an increase in the throttle position sensor however it is and we bring way more more metered air into the engine well now this vacuum leak plays a smaller and smaller and smaller percentage of the total air in the engine which means that less fuel trim is required to compensate for this so let's look at that on a graph so again we're idling the engine and we've got our long-term fuel trim at 25% but but let's add in RPM and here our RPM is low but then let's say we increase the RPM and then tail off If This Were a vacuum leak as we increase the RPM as I said before much more air is coming into the engine so that vacuum leak becomes less and less significant which means less fuel trim is required so the short term fuel trim as you increase the RPM is going to decline it's going to pull this long-term fuel trim down and we're going to see an improvement in the long-term fuel trim the more that we increase RPM and we may even see at full RPM that our fuel trim might even be at zero all right then as we let off the throttle and return back to idle we see that again at idle we're back so if we see this pattern we can pretty safely assume that a vacuum leak is the most likely cause we should turn our direction to a vacuum leak what's a good way to detect the vacuum leak well one way I do it is my scanner to short-term fuel trim what's the short-term fuel trim going to be well it's going to be at zero but then if I take propane and spray it around the engine and we're going to see that short-term fuel trim immediately take a nose dive as soon as that propane gets sucked into the engine through that part that's leaking so that's a great way to use your short-term fuel trim to detect where the leak is um some shops may have some more fancy equipment like smoke machines and things like that you could also use carburetor cleaner any of those tricks but if you set your short-term fuel trim on your scan tool you can get an instant response when you catch where that vacuum leak is because you're going to see the computer respond to that rich condition that you're creating using the propane or whatever

Channel: Schrodingers Box

Views: 1,434,395

Keywords: oxygen sensor, scannerdanner, eric the car, wideband o2, o2 sensor, p0171, p0174, p0172, p0175, lean condition, rich exhaust, rich condition, engine running lean, engine running rich, check engine light, engine stumbles, auto repair, air/fuel sensor, maf sensor, map sensor, p0300, p0301, misfire, scan tool, engine diagnosis, diagnose lean condition, fuel trim, ltft, stft, long term fuel trim, short term fuel trim, understanding fuel trim, engine performance

Id: 5WnM_NsOtd8

Length: 27min 41sec (1661 seconds)

Published: Fri Mar 07 2014