Short Term and Long Term Fuel Trims Explained

hey how's it going to you so first today we got to go over it your short term and long term fuel trims how they're calculated and how you can use them to diagnose issues with your car and this is going to be specially useful when you get no codes and no check engine lights on your car so for this video will be diagnosing an issue with this 2003 Ford f-150 with a 4.2 liter v6 engine but of course anything you learn in this video about your fuel trims you can apply to any car making model now the performance issue we have with this car is that when it's trying to get going from a dead stop it's experiencing a bit of a hesitation or lack of power and again since we don't have a code on our scanner that would point us in the right direction the next best thing to check is our fuel trims now before I show you the fuel trim numbers on this car though let's take a couple of minutes and go over to the white board and let me explain to you how they're calculated and what they exactly mean all right so in order for your catalytic converters to work properly and your car to pass emissions your air fuel mixture ratio needs to average exactly this 14.7 parts of air to one part of fuel and this is to be kept at this ratio under all engine loads or rpms and the way that this exact ratio is received under all conditions it through your car's computer your course computer basically controls the amount of time your fuel injectors stay open therefore controlling the amount of fuel going through your engine your cars computers will control the amount of air exactly entering your engine but it has some sensors that precisely measure the amount of air entering the engine usually through a MAF sensor which is this guy right in here now your MAF sensor and the fuel injector pulse width is what your course computer has in order to mix the correct air-fuel mixture but there are a lot of things outside your car's computers control they could affect their fuel mixture and cause you to have either a lien or a rich condition now in order for your course computer to be able to fine tune their if you'll make sure and also take into account any issues that may arise throughout of its control that may affect their fuel make sure what it uses is your oxygen sensors now when you have a correct if you'll make sure the signal from your oxygen sensor will look like this they will oscillate between about point one both to about point nine volts and it will look like this and it would average at about 0.45 zero volts and this is for your peak at or upstream oxygen sensor now let's something happens that causes your oxygen sensor to start putting out very little voltage in other words saying to your course computer that you have a lean condition and too much air too too little fuel in your exhaust gases so in order for your course computer to compensate for the extra amount of area that the oxygen sensor is sensing coming out of your engine it will increase the amount of time your fuel injectors stay open or your words your fuel injector pulse width and this will show as a positive percentage in your short term fuel trim so it will look like something like this and this will increase until your oxygen sensors start oscillating again between point 1 to point 9 volts and that's given if your oxygen sensors are working properly if you have a oxygen sensor that's bad and it's maybe shorted to ground let's say and it's only putting out point 0 volts to your car's computer this is going to increase but not because you have a problem with a MAF sensor or your fuel injectors or something but rather simply your oxygen sensors are bad now on the other hand if you have an oxygen sensor that's showing high voltage it's basically saying to your course computer that you have a rich fuel mixture if your mixture going to your engine and your car's computer will respond by shortening the time your fuel injectors are opened and this will show as a negative percentage in your short term fuel trim so basically I'd say it would be negative 20% here so in so many words if you have a positive number here your car is running lean if you have a negative number here your car is running rich and the job of your short term fuel trim is to make sure that your oxygen sensor is oscillating between lean and rich and averaging at 0.45 volts now if the problem that's causing your richer lean condition is continuous and it's affecting your engine's air fuel mixture for a long time if that's the case the information from your short term fuel trim will transfer over your long term future so let's say we're running lean and we were 20 percent positive what will happen is that you'll get 20 percent positive on the long term fuel trim and this is going to happen slowly though it's not going to happen as soon as this goes 20 percent your car's computer will have to run lean for a while in order for this to start increasing slowly but as this increases this will decrease until this goes all the way back and to about zero I think I forgot to mention what are some good numbers for your short and long term futures for the short term fuel trim anything from negative to positive 3% it's considered okay if in other words the percentage is going up and down between negative 3 and positive 3 that's good enough now your long term fuel trim you want to see the same city at a Dornier 0% you know if it's up or down a few percentage points not a big deal as a basic rule of thumb if you add these two percentages together you don't want to have more than negative or positive 10% now as far as what are some things that could cause a lean condition well on the air side a bad or dirty MAF sensor would do it and a bad oxygen sensor that's reporting lean when it's not supposed to vacuum leaks a very common cause for lean conditions and also an exhaust leak above or upstream of your pre keto to sensor they get where your exhaust manifold meets your solar hit like gasket if that exhaust gas get is leaking it's going to allow air to enter your exhaust system throwing off your o2 sensor making a report lean when it's not supposed to and on the fuel side dirty fuel injectors we keep enough air from entering your combustion chambers cause a lean condition a weak fuel pump and also restricted fuel lines like if you have a dirty fuel filter we do the same thing and as far as were some things that could cause a rich condition on the air side again a MAF sensor generally speaking map sensors when they go bad or get dirty they are responsible for a lean condition but our occasion I believe there could be responsible for each condition same thing again and although - since it is reporting rich when it's not supposed to warn engine if you have worn piston rings your engine is not going to be able to second enough air to mix with the fuel mixture therefore causing a rich condition also exhaust restrictions now I'm this on this one I don't have personal experience with this one exactly some sake this could also be responsible for lean condition like also an exhaust restriction the most common cause most common an example I should say would be a Clarke led converter and on the fuel side leaky fuel injectors bad fuel pressure regulators EVAP system like if the person is stuck open you know just going to be sucking in fuel vapors and are supposed to causing a rich condition also high fuel pressure inside your fuel pressure lines would do the same alright enough for boy action for now let's get to diagnosing our issue on this car alright so we're going to use the first story our agent and you want to run the engine until it's properly warmed up and it's in closed loop and that's a fact alrights ten minutes later the car is properly warmed up now it's time to take a look at our data so we'll just scroll down to our light data option enter and the first thing we're going to check is going to be our pre keto two sensors because if they're not working properly all our fuel trims are going to be off so here's a shot of our pecan o2 sensor on bank one this is basically saying oxygen sensor one one which is oxygen sensor bank 1 sensor 1 and as you can see this is oscillating between 0.08 to 0.7 which is what we want to see so this is working properly the next we'll go over to our sensor for bank 2 and here it is Oh two sensor - 1 which is Bank two sensor one and this one also is oscillating between 0.7 point zero a - about 0.7 so that's a good sign as well alright now that we've confirmed our o2 sensors are working properly and they're oscillating basically means that our short term fuel trim should be at or near zero unless things are really bad which I don't think is the case with this car because this car otherwise drives you know okay but if there is a problem with our fuel trims it will show in the long term field so here's a look at our short term fuel trim for bank one and as you can see it's between negative 3 to about positive 3 which is about normal and here's a short term fuel trim for bank 2 and this one as well as about negative 3 to positive 3 which again is a good sign and usually get our long term fuel trim for bank 1 and this one is at about 15% which is too high and it shows a lean condition this basically means that the computer has to increase the injector on time by about 15% in to get their fuel mixture to the correct ratio and here's a long term fuel trim for bank two and this one is even higher at about 17% which again means that we're running lean on this Bank as well all right so those are our numbers for a few trims and idle but let's raise the RPM to about 2500 to 3,000 and hold it there and see whether it makes a difference in a few trend numbers all right that's interesting spc it's gone down to about nine point four percent which is a bit of an improvement or let's do the same thing for a bank too all right also the numbers on this Bank got better as well all right so let's go over the number we just collected and see whether we can solve this puzzle so on this card on one background about 19% how to say on the other bank we had about 14.8% those numbers were positive therefore meaning we're having a lean condition at our computer has to increase the injector pulse width to allow more fuel to mix with the extra air that's in there but these numbers were at idle but what happened was that when we opened the throttle and increased our RPM these numbers decreased which is going to generally indicate a vacuum leak when the whole plate is closed your engine the piston is going up and down they're sucking in love a lot of air and they're putting a lot of vacuum inside your intake manifold if you have a vacuum leak your engine is going to be sucking in a lot of air through that vacuum leak but when the throttle plate is open you know there's an easier access for getting air inside the engine then the vacuum leak you have after the throttle plate so the effect of the whatever vacuum leak you have decreases as you open the throttle plate and therefore what and that's the reason why we saw the improvement in our long tail term fuel trims all right so the next step would be to go and find our vacuum leak and confirm our diagnosis but the only problem is I've been all over this engine with some starting fluid brake clean and even got my propane out and I just can't find a vacuum leak what I think is happening is the as it's very common on these fours is the upper intake manifold gaskets and orange are leaking and you can't really spray anything in there because they're leaking on the inside of the intake manifold but if I had a smoke machine we will be able to confirm it but unfortunately I don't but I might make a separate video showing you how you can make a duty sulfur smoke machine and find vacuum and evap leaks I want to make sure you stay tuned for that but you may also want to consider checking out these other related videos of which I've put links to on the screen you can click on they're also the links in the description box down below all right I hope you find this video helpful I'll see you guys

Channel: Ratchets And Wrenches

Views: 401,172

Rating: 4.9219394 out of 5

Keywords: Rathcets and Wrenches, long term fuel trim, short term fuel trim, what are fuel trims, P0174, P0171, P0172, P0175, positive fuel trim numbers, negative fuel trim numbers, vacuum leak effect on fuel trims, how does the oxygen sensor effect fuel trims, what are LT and ST Fuel Trims, what is LTFT and STFT, how to use fuel trims to diagnose performance issues on your car, how to fix a lean running engine, how to fix a rich running engine, what causes a rich condition, lean condition

Id: pIJdCZgEiys

Length: 11min 49sec (709 seconds)

Published: Tue Aug 09 2016