Using Fuel Trim to identify type of misfire (can it be done?)

okay what we're going to do is a fuel trim review and what we're going to cover is how fuel trim relates to a misfire and there's a lot of debate in the field as to whether or not you can use your fuel trim numbers and determine the type of misfire that you have and so what we've done is we pulled up some scan data of the short term long term fuel trim horn and rx300 and what we're going to do is we're going to disable this spark and with that type of misfire you're gonna have raw fuel running into the exhaust not to mention the oxygen to that wasn't consumed and then we're going to pull a dual injector disable the injector and obviously we know fuel just outside or atmospheric air basically entering the exhaust so we're going to compare the two and see what our data looks like I'm going to do the ignition misfire first yeah just don't plug the coil the ejector is still firing I think that's an important variable to make sure the computers not shutting the injector off I can assure you that this fuel injector is still firing let's watch this number for a minute looks like we peaked at ten point nine and then it came back down as the computer adjusted the long term starting to add fuel in the long term which is bringing the short term back down into a normal range and so you can see that with our our ignition ignition misfire happening right now we have a total fuel trim of maybe around nine percent eight nine percent total fuel trim and that's positive so with an ignition misfires we know so far it does not go negative so some of the debate out there is well an ignition misfire raw fuel so people are thinking rinse and you think bill terms gonna go negative that is not the case you cannot make that statement when that ignition doesn't fire we're pumping raw fuel into the exhaust yes but raw fuel does not equal rich it's unburned we're also not burning the oxygen so you can see that with an ignition misfire essentially these fuel trim numbers are pretty much useless on these if you looked at this by itself and this car came in with an ignition misfire these wouldn't help me at all because they look pretty normal the car is got a misfire how is this giving you any direction at all I'm not sore I mean maybe you could say possibly that the type of misfire that we have is ignition related but it could also be compression too because compression is going to do the same thing as far as pumping raw fuel unburned into the exhaust so ignition misfire will plug this coil let these numbers stabilize for a second and then we'll do the fuel injector next okay with it the number stabilized we're going to do the injector now I'm going to unplug an injector and again we're looking at short term long term fuel trim just looking at it on bank two that's the side of the engine that we're dislike abling the spark and fuel do the injector now so now we know fuel misfire as you can see we what we would expect for it to go rates compensate rates and it certainly had to go more grits and reaction to this injector misfire than it did the ignition misfire but both ignition and ejector misfires that fuel trim went positive a lot more positive on the fuel injector I think one of the ways that we can maybe justify this is if you're not spraying the fuel in that cylinder then you're not taking up that space and there's going to be more oxygen coming into that cylinder than there would be if the fuel was there so that'll be one way we can explain the higher percentage number another way would be with a fuel I'm sorry whipping in an ignition misfire we're pumping fresh oxygen and fresh gas raw fuel into the exhaust and some of that can actually ignite in the exhaust manifold and we can use up some of that available oxygen in that process in a hot manifold I think both of those would explain why this is so much higher with a injector misfire as opposed to an ignition in this part but in either case here's what we know with both misfires the fuel trim command is going to go positive as you can see certainly more positive with a no fuel misfire as in this sir so maybe this might be some valuable information if you bring a car in and it has a dead miss and you see very positive military numbers maybe we lean more toward the injector than we would ignition so not discrediting this test by any means but what I'm trying to clarify is that an ignition misfire will not drive the fuel trim negative I know we want to think raw fuel is rich but we can't do that we're not burning the air in that cylinder either so it's good that o2 is going to read lean either way the only thing though about this would be you could plug in something else that would cause this and that would be an intake manifold runner gasket if that gasket was leaking and I can't simulate that but certainly that's going to drive your field trip positive and give you a signal stone or misfire so does this always say hey you got a bad injector no it doesn't but it's certainly more positive than an ignition or compression would be the reason you're seeing this trunk down right now a little fuel trim review here what's the long term doing long term is correcting trying to bring these short-term back down to zero that's what you just saw occur right there and that's why that sudden changed but hey that's our injector misfire umm what I want to do before we conclude this this vehicle is a wideband o2 and so what I want to do is the same process on a narrow band o2 car and see if we get the same kind of reactions okay this is our second car we're going to try to prove this theory a little bit better this is a two thousand four dogs beyond this is a narrowband sensor as opposed to that lexus that was a wideband one of the things you see right away the way this sort of tournament fuel term adapts there's a lot bigger swing in this briggs lean command on that short term kind of interesting we definitely need to do some wideband narrowband Oh to stuff we'll be doing that in the future so keep walked in but so different car we want to see the reaction again I'm going to go ignition first and we're going to look at the reaction of this with an ignition misfire let's do one thing before we do this reset our mid max forming here real quick and now we can use these scales and of course these look a lot bigger when we do that because this is a some updating min max scale I'm going to go ahead and sort the ignition this is a waste mark I'm just going to sort one plug wire out to be cylinder number two not that it matters okay that sonar to sorted out it's like we hit a max of 748 on the fuel trims so we see again right away an ignition misfire that has caused a lean condition even though you're pumping raw fuel into the exhaust you're not burning the oxygen either so that no two is picking up a lean condition ignition misfire so that right away is going to make the argument that you know an ignition misfire is going to drive the fuel trim negative that is a false statement absolutely going positive and I'm telling you guys from my experience what I've seen severe ignition misfires will drive your o2 really lean and your fuel Chum is very positive so keep that mine in fact if you look and in my book section five we talked about this one of the cases of a lean conviction or positive fuel trim numbers is a misfire and I have severe misfire listed as a pulse lean it's really not a lean condition the o2 is seeing extra oxygen from that misfire so again ignition misfire looking at about nine nine point six percent was the peak of that you can see we're more positive than we were before I'm gonna plug this plug wire back in [Music] we're going to let these numbers stabilize for a second and then we're going to do the inductor 1 [Music] I'm pretty happy with where we are that was pretty fast correction a lot faster than the other car we're back to a more stable range a little bit of changes here I'm gonna give it a second alright I'm gonna plug the injector now same cylinder fuel injector and unplug it certainly a lot higher just like around their car but now we got an issue and the issue is pause this for me Oh me are you still rolling Mike pause the screen practice I'll grab this cursor yeah bring this back over I want to pull this back on the screen right there okay don't suck far off because we'll lose our down in this picture we have a takes you and our H duties that we were in closed loop and I haven't plugged the inductor and as this is climbing up right at this point my sword term was about 20% so clearly like the other car more lean than our ignition misfire was but what happened is it went from closer to other loops the move your cursor bike over to get to this point right to where it's quits right to where from where we dropped by the way right right to this point right here watch this line what's this right here we went to open the so when you're an open-loop fuel trim is disabled and this computer as a default with a fuel injector unplugged it's recognizing that by its output state monitor that's watching the injector voltage it knows there's a problem dropped it into open-loop and so that makes this test invalid with a open in the electrical part of the injector now I'm not saying that that's always going to happen because you can have an injector that's still electrically firing and this would not happen if it was plugged up but in this case when with an electrical problem with the injector we're running an open-loop and so we can't use fuel trim again I just think that there's too many variables with fuel trim as far as using it to determine the type of misfire that you have is is it showing us more of a rig correction with an injector problem on both vehicles and I think the answer is yes so there would be some value to this but using it to say and pinpoint the type of misfire that you have I really don't think it's it's accurate enough to be able to do that you'll trim this fire that's what we got

Channel: ScannerDanner

Views: 108,150

Rating: 4.9073615 out of 5

Keywords: misfire fuel trim how-to, scannerdanner, Engine Performance Diagnostics, mechanic school, auto repair, Paul Danner, lean condition, auto tech training, training, Rosedale Tech Pittsburgh, STFT, rich condition, DIY auto repair, LTFT, misfire diagnosis, misfire troubleshooting, oxygen sensor operation, tomato sensor, fuel trim diagnosis, oxygen sensor vs fuel sensor

Id: G_89r12vZFw

Length: 12min 20sec (740 seconds)

Published: Wed Apr 18 2012