The Trainer #65 - Making Sense Of Fuel Trim

did you know that the system lien DTC's the po1 71s and the PIO 174 s are among the top 10 DTC's that technicians deal with on a daily basis well they are if I could come in at number 3 but if how you can understand how to repair these DTC's if you don't understand what costing the set in the first place it all has to do with the fuel control that the ECM is trying the exercise try to keep that mixture perfect in those cylinders when it gets out of whack code set misfires occur all kinds of drivability issues can happen it's all about the fuel trims that's the topic of this edition of the trainer everybody thanks for watching this edition of the trainer now in this series that we started back in January we're setting the building blocks to develop our skills as drivability technicians we've talked about how the ECM thinks we've looked at the resources and modes of global obd2 and how we can use them as diagnostic tools and now we're going to look at fuel trims specifically how does the ECM maintain fuel control where the factors that we need to consider when we are diagnosing system lien system rich codes or using the fuel trim data as another diagnostic tool for other drivability conditions that we're going to face so let's start off with talking about this numero uno the main job of the ECM is to protect this device the catalytic converter that's the last step in line to clean up whatever exhaust gases have passed through the engine isn't it and before they're released to the atmosphere this is the number that the OEMs are being held responsible for what is coming out of those pipes now this is a very efficient device as long as what's going in maintains itself in a very narrow range what I mean by that is that we all know that to get the right combustion we want to have what's called a lamda ratio of air and fuel being placed in the engine and properly combusted in other words you have to have the right compression we have to have the ignition happened at the right moment if everything works is the way it's supposed to be then the gas is coming out the spent gas is coming out our going to be maintained in that very narrow range these speed gases as they're called that are being passed in the catalytic converter if for some reason we have higher than normal oxygen content or higher than normal fuel content for whatever reason passing into the converter the converter will overheat and the substrate will start to melt and come apart in fact when I'm dealing with catalytic efficiency codes one of the first things I like to do is try to get a peak at what the substrate looks like if I see that it's been melted or overheated well I know right away the converters garbage is going to have to be replaced but I also know that I have to find out why that converter overheated and repair that issue because if I don't then the new converter that I install is going to have a very very short life and D so again it's all about the converter and what's going into it now we all know about this right the stoichiometric ratio lambda ratio goes by a couple of different names the 14.7 the one now this isn't always the case nowadays some of these vehicles are running much leaner than this and doing it very efficiently we're not here to debate that what I want you to understand by it is again GCM is charged with feeding the right amount of fuel to the air coming in and it goes by not volume but by weight and that's what a lot of technicians don't understand so let me kind of give you an illustration for that here you see at the top we have a couple of measuring cups two and a half cups to be exact two and a half cups of fuel weighs about one pound in terms of this ratio that we're looking at fourteen point seven to one now the fourteen point seven pounds of air that has to go with it well imagine one square inch that extends from the floor 50 miles straight up that's how we determine atmospheric pressure that's the 14.7 PSI that we're all familiar with you want to have another little imaginary picture in your mind of what that is we'll consider one square foot one square foot extending 182 feet straight up that column of air is going to wave 14.7 pounds so that's what we're looking at how much air max with how much fuel to give us the correct ratio to provide the correct feed gases to qu the catalytic converter that's right okay now what does the ECM need to know before it can add the right amount of fuel well of course it needs to know how much air and how much air by weight got into the engine now there's two processes that we use for that the older one is the mat or speed density system that uses a manifold air pressure sensor and others so that the engine you see em can calculate how much air got into the vehicle so it's not a direct measurement it's an inferred measurement based on the data provided by these sensors now where does that come into play for us as diagnosticians your ears should be Perkin up right about now because any of those sensors are not telling the truth then obviously the calculation that the ECM is going to make will be wrong and that can lead to drivability issues can it the other method that is commonly used is the mass airflow system now the mass airflow sensor does indeed accurately measure how much air is getting into the engine but just like I said with the map sensor the speed density system if the mass sensor is not telling the truth and we've all seen mass sensors became contaminated and misreported or had electrical issues what other case might be if it's not telling the ECM what's really going on then of course any calculations that the ECM makes is going to be wrong in the air right but so that's the first thing easy in these know how much air is getting into the system now here's something the ECM already knows it knows that the engine it's now in charge of is fitted with a certain type of fuel injector and it knows what that fuel injectors flow rate is and since it knows what the flow rate is under a given pressure and volume of fuel then it knows if it turns it on for X amount of time whatever that might be the pulse width that you see on your scan tool that that fuel injector will deliver a X amount of fuel and it calculates again all of this by weight so once it's measured the air it can then go through its calculations and determine how long to turn the injector on to deliver the correct amount of fuel more said about the sensors though what if there is something amiss with anything in the fuel delivery side what the fuel pressure is low where there's a restriction to that volume where the injector is sticking what if it's not opening properly or it's restricted all of these will affect the end results morning the ECM needs to know way or it needs to have a way to know when that happens and that's where we come to hear what the ECM isn't sure of there can be manufacturing variations on a brand new car so the calculations it makes are going to be pretty darn close but they're not going to be spot-on and then of course is the vehicle where's is there's a wear and tear age on the vehicle these parameters are going to change as well so we have to have a way to get some kind of feedback to tell that ECM exactly what happened after it made its calculation and it fired that injector okay and that comes from oxygen sensors they're either the conventional oxygen sensors that the older vehicles have or the wideband sensors that are very commonly used this is the feedback to the ECM of what really happened and let's start off with the conventional oxygen sensor conventional oxygen sensors I want to stress right now are not sensors they do not measure anything they react and they react to the oxygen content in the exhaust stream and then report that information back to the ECM and because they're react because you don't physically measure something we have to do something that will cause those sensors to react in different directions we have to get it what we call switching we have to get it moving across its neutral point that 0.45 zero millivolts that we were at four or five zero millivolts that you see on your scanned tool is the mid-range for the sensor what's his upper and lower limits point one on the bottom maybe point eight point nine a top and we're looking to go from 0.1 the lean side to 0.9 the rich side and get it to go back and forth we'll talk a little bit more of that in a moment of course the wideband sensors a lot more accurate they're actually providing feedback and are more sensitive to these ultra lean conditions that a lot of the engines are running nowadays but the aim is the same no matter which one is in play that's what tells the computer how it's doing so the ECM the computer can make its adjustments that's what you we see in the short term fuel trim and long term fuel come more not here in a second but again everything that we talked about so far that's that's implying that they're all working the way they're supposed to be working so as a diagnostic technician I'm my ears of perking alike well here's something else I got to make sure of before I condemn another part I've got to make sure that these feedback sensors are telling the truth and that they're coming online we're going to what we call closed-loop nowadays that happens very quickly back on some of the older cars even the obd one cars it took a while to heat up that conventional oxygen sensor good to get that thing online and reacting to the content and the exhaust stream and it wasn't unusual for them to go offline while you're sitting there idling at a stoplight because there wasn't enough heat in the exhaust to keep it fired up so we want to make sure we look at that pit on our scan to make sure that we in closed-loop make sure that we are indeed providing feedback to the ECM through these sensors now let's talk a little bit about the role of SPF T that's the short term fuel trim pit on your scan tool L TFT that's the long term field trip of course and the rear fuel trim rear fuel trim if you don't know about hang on we'll get there but it's diagnostic technicians it's all about what the total fuel trim is what's the total amount of correction that the ECM is making at any given time let's start off talking about short term fuel trim as I mentioned let's will focus on a conventional Action Center analogy here first as I said the oxygen sensor conventional action sensor doesn't measure oxygen it reacts to the presence of oxygen oxygen and the exhaust stream so let's just look at it this way I'm the ECM and I go ahead and I send out a pulse to my injector and when it gets through the oxygen sensor the oxygen sensor says Oh little too much action that was a little on the lean side so it's both its level drops I see that and I'm thinking to myself that okay I want to get that to go the other way because what I really like to do is keep this back and forth across a neutral point zero point so I'm going to say okay I'm going to make a correction and I'm going to add a little bit more fuel the next time around that's the short term fuel trim that we see on earth on our scan tool so I add a little bit more I wait for the reaction from the oxygen sensor it goes up now indicating okay you did good went a little too far that's a little rich so I say okay that's fine I'm the ECM I'm going to make another adjustment I'll take a little bit away and I'm going to move back down on my pulse width and that's going to cause the short term fuel trim to drop the other way so when it goes lean when the oxygen sensor goes lean short term goes rich when the oxygen sensor goes rich short term fuel trim goes lean and really be a little more clear about that the short term fuel trim is a number that's actually a percentage that's either added or taken away from the computers based calculation in order but turning on that injector the amount of time the injectors going to be on for that pulse width if you will okay everybody with me so far so the short term fuel trim is the percentage adjustment so when you see that number plus six that's 6% more than the base calculation minus six minus per six percent less than the base calculation and whole idea again is to get that oxygen sensor to switch back and forth across that border the 50 millivolt center between the point one and point point eight point nine on a healthy sensor and as long as we're doing that and the short term field trim is switching back and forth across its zero point then we're in a fuel control and everybody's happy now ideally I want to see the short term fuel trim to cross that zero point no more than plus or - 5% ideally if I see plus or minus 10 my eyes are going to pop up could be an indication there's an issue maybe that oxygen sensor is getting a little old it's not reacting as fast as it should and that's kind of given the wrong information to the computer so maybe that's why it's making such a wide switch and certainly if I see it outside that plus or minus 10% I know there's a problem that I want to take a look at now the computer knows that - and it still can do it and it can do something about that when it sees that it's trying to make the correction - that short term fuel trim and no matter what it does it's that oxygen sensor is staying down low or up high no matter how much short term it takes away or how much short term it adds it's not making any headway then the ECM knows it has an issue that used to take and make a more permanent correction - so what it's going to do is participate calculation now it's going to start to add a little bit more again a percentage and that's the long term fuel trim number that you see that's what we do or what the ECM does to accommodate for parts that wear and tear now there's going to be some changes that we need to adjust for so when it sees that it can't quite correct it with its with its brand new numbers that's going to slowly start to add water and fuel trim again until it gets those numbers to start doing the switch back and forth across 0 and then once it does whatever the long term fuel trim correction was needed to make that happen that's what its going to store in its memory for that particular load and RPM condition and that's what it'll apply going forward now there will be occasions where we just can't get it no matter how much we try the number of short term and long term just climb and climb and climb generally when either or both get to plus or minus 25 35 % those are typical thresholds that the manufacturers use to say ok that's just us too much there's a problem and if we're adding too much fuel that obviously we have something causing a what would be a lean condition and we going to set the whole system lien code the opposite if we're taking away too much fuel then obviously we have too much going in or a rich condition for some reason imbalance on the fuel side and and then we're going to start looking at setting a system rich code okay so that's basically the long term and short term fuel trim but we're not done yet okay again for diagnostic purposes I want to look at what the total of those two are and I want to look at them over time because the fuel monitor is a continuous monitor remember how we define that that's being run and checked all the time under a variety of loading rpm conditions and doesn't the fuel needs of the engine change depending on how it's being operated of course it does it takes very little fuel to run idle but if I'm pulling a trailer uphill and I put my foot to the floor I'm going to want a lot of gas real quick I'm sucking in a lot of air right so I'm trying to get more out of it that we need to look at the loading rpms is being operated under but that's the diagnostic number that we're looking at but here's the situation you might run into what if you add your short term fuel trim and your long term fuel trim and you're fortunate to have a pit for total fuel trim but they don't match where's that telling you well then you might be dealing with it we our fuel trim rear fuel trim is a correction that's made based on input from the octane sensor downstream of the catalytic converter and this is mostly later model cars I think the first thing is I started seeing was somewhere around 2003 2004 and up many manufacturers use this as a sec another method of maintaining fuel control because this is what's looking at what's coming out of the catalytic converter typically not a huge correction factor but if there's something going wrong or something happens to the front sensors on a lot of vehicles the rear trim will now become the primary source of feedback for the ECM and you can see that you'll either see big numbers if the rear fuel crimp it is there or you'll add up short term long term and see the disease anywhere near close to what your total fuel trim is telling you on your stand tool and or you can look at the rear Bank sensors there's there's be aware that you may have an issue with rear fuel trims and if you do it's probably because you have an issue with the front sensors on that on that Bank now what do you think of when I say lean or rich are you focused on just the amount of fuel going in well during the carburetor days we sure did because we didn't really measure the air going in that kind of took care of itself if we had a lean condition we running probably the fuel jet that was being clogged or just too small for the application if we were running rich float levels just to fall out all kinds of things that could cause that but we really didn't factor in the air so much what I want to stress here on this screen if you're dealing with and we'll look at this more closely when we actually do troubleshooting on system lean system rich codes is that all the things that we talked about so far play a role and whether or not that fuel trim is correct whether that fuel trim is being measured and executed properly it has to do with again to recap are we properly measuring the air going in is the ECM being told the truth is it getting an accurate picture if it's not then that's going to skew the trims is the fuel delivery side to spec even minor variants is there can impact the end result and whether we have a system leaner system rich situation are the feedback sensors providing the information correctly and telling the computer the truth so there's a lot of things in there that the ECM is relying on a lot a lot of devices a lot of sensors that is depending on for accurate information and if any of them don't provide that accurately that can affect your fuel trims so you want to make sure we keep that in mind another thing I want to keep in mind is that fuel comes aren't static like I said there they're changing over a variety of load and rpm conditions so one of things that we'll get into a little later on is I'll show you how to do what's called a lot of our contributors called flat rate test drive this is where we go out and operate the vehicle on a stone route under the same conditions as we do every other vehicle while we're tracking certain data pins and recording them graphing them on our scan tools that's the most ideal way to take a look at a lot of this data is in graph format then you can see changes under these different condition conditions and you're not trying to get a snapshot of the little individual snippets that you see when you just have a data pit screen showing so learn how to graph those on your scanned tool today's scan tools have come a long way and provide very accurate graphs for our diagnostic purposes I also want to make sure you understand that fuel terms have to be considered by bank and not by the total engine package now we all know v6 is VH we have a left and right bank but there are a lot of inline 4-cylinder so if you look very closely are also divided into two banks typically cylinders 1 and 4 and tell us owners 2 and 3 and they each have their own feedback sensors was conventional or more commonly wideband sensors in place on those engines and again I can't stress enough we have to look at the operating conditions occurring at the time that we're looking at the fuel cans they can vary quite a bit and we'll demonstrate that in future episodes in fact the next time on the trainer we're going to take a look at fuel trims we're going to take a look at that flat rate test drive and we're going to use that information and apply it to helping us diagnose the cause of another very common dtc this fire DC sees pampy mayer motor age magazine thanks for watching this edition of the trailer I hope you found it helpful and I'll see you next month [Music] you

Channel: Motor Age

Views: 143,896

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Keywords: #automechanika, chicago, motor age, the trainer, automotive repair video, car repair, auto how to, how to fix my car, auto tech, auto technician, auto service professional, engine performance, auto electrical how to, automotive drivability, obd 2, obd II, onboard diagnostics, auto computer, voltage drop, ohm testing, voltage testing

Id: FP3bu3UBz5k

Length: 21min 43sec (1303 seconds)

Published: Fri Apr 28 2017