The Trainer #67 - When Fuel Trims Won't Trim

welcome to another edition of the trainer today's topic diagnosing system lean and system rich DTC's it's coming up next stick around the PIO 171 and pio 174 generic dtcs that is the system lean DTC's are among the top ten diagnostic trouble codes that technicians have to face on a routine basis how do these occur well a time that the ECM is additions to fuel control are beyond what the program threshold is in the software well then that code will set in other words if they're having to make additional Corrections to the base fuel trim calculations then that's what's going to cause these codes to set now the other side of the coin is the system rich and that's when the base fuel calculations are having to be subtracted from in order to maintain fuel control and if any of that stuff that I just said to you is not making any sense well I want you to go back and take a look at the trainer number 65 where we talk about fuel trims and how they what they mean what they do and how the ECM calculates those so we're going to focus on diagnosing these two codes and provide some helpful tips and techniques that might make finding the solutions to these problems a little easier for you let's start off talking a little bit about the system lean DPC's first okay the same is going to apply to system rich but in Reverse right what I want to focus on here in the beginning is making sure everyone understands what we're mean by fuel control when the ECM is in fuel control now in my definition in my mind fuel control is established when the ECM is able to see a constant feedback from the oxygen sensor that's staying right around the middle of its range in other words it's constantly switching back across zero across that point four five zero a millivolt range and we know that the content the air fuel ratio this fed to the engine is correct now if it starts to stay on the high side where we're looking at too little oxygen the rich condition or the low side were too much oxygen or a lean condition well then the ECM is going to try to correct for that to get it back to the middle now how is it doing that well it's getting a measurement of the air and there's - there's two ways that can happen and we'll discuss in a moment and then once it knows how much air is going to the engine that calculates the amount of fuel to add to get that proper ratio and it's doing so under a few assumptions the fuel pressure feeding the injectors has to be the specification the ECM is expecting and the flow rate that the injectors are providing has to be that within that ECM expectation if neither one are then the amount of fuel passing through at that calculated pulse with that injector pulse width is not going to be correct is it imagine a leaking injector for example the pulse width calculation may be correct for the amount of air coming in but because we have leaking injectors the amount of fuel is skewed it's not meeting the expectations that the ECM is expecting is it same is true if we have a restriction in the fuel system say a low volume supply or a clogged injector the pulse weight may be correct but the amount of fuel being entered is skewed because it's not at specification so that both of these conditions can cause the system lean system rich codes that we're going to troubleshoot what I want you to get in your mind is it's not so much about adding fuel or taking away fuel you have to keep in mind that maybe that's not right to begin with and you have to eliminate that as a possibility now if you've been tracking our videos for any length of time you know one of the very first things that I encourage people to do when they're tackling any kind of diagnostic trouble code is to be familiar with the conditions and the criteria that the ECM is looking at in order to set that code in other words how is the ECM testing the system to see if it's working the way it's supposed to to see whether or not it should set that code now in the case of system lien and system rich codes the ECM is watching what it's trim Corrections are and if a continue is that you have to add a correction due to a seam lien condition or take away the correction because of a seemingly rich condition it's going to continue to do so trying to get that back in center as I said earlier but if it reaches a certain threshold whatever the engineers programmed into it and this will vary from make to make if it reaches that threshold and still can't get that fuel system under control well that's what's going to cause the code to set so we know what the computer is looking at and that's how we can also base our tests so once we understand what the ECM is looking for we can construct our own tests to mimic what the ECM is doing in order to find the cause and correct it now the other thing I want to point out when you're getting ready to tackle these codes is to make sure that you look at the freeze frame data take a look at that freeze frame data because these are constantly monitored codes constantly monitored situations we need to know what conditions the vehicle is operating under when the code set for example if you have a system lien code at idle cold that could point to a different cost than a system lien at high crews under a high load same with the system rich codes so it's vitally important that you don't clear anything you don't disconnect anything you don't clear that freeze frame data and so you had a chance to review it maybe write it down take a snapshot of it whatever you need to do so that you can go back and see exactly what the conditions were when the code set now there is one scan tool on the market the automotive test solution scan tool that has this test built in actually lays it out in a graph that you can see when these conditions occur another thing that you can do is to perform just a routine test drive set up your own route one you can use for every vehicle that you take that allows you to drive that vehicle under a variety of conditions idle highway crews section where you can actually do a wide-open throttle test and graph that in term a with your scan tool that you can go back to the shop and review that in more detail common graph items you would want to include pids that you would include RPM engine temperature intake temperature short and long term fuel trims for both banks of course and mass sensor so equipped what that reading is and calculated load or the load pit these are all items you can use to help you determine what kind of issues are going on and what the causes might be now before you can be successful in diagnosing system lean or system rich codes there's one more thing you need to understand and that's the type of system that you're working on are you working on a speed density system or are you working on a mass airflow system we'll talk about the speed density system here in a moment probably still the most common is the mass airflow system this actually started overcoming speed density over the last several years because of what the MAF sensor is capable of doing it can actually measure the quantity of air without any calculations necessary by the ECM so we provide this direct input for the amount of air we have an exact number we have a reliable figure to use now the ECM can move right onto its fuel calculations and just take care of that but sensors don't always tell the truth do they if you've been doing any kind of drivability Diagnostics for any length of time you've probably run into more than one situation where the sensor was the culprit because it was lying to the ECM mass sensors are no different they can also be the source of misinformation rather than correct information as they age the numbers may be skewed if the film or Hotwire elements become contaminated that's going to skew the figures and there's a couple of ways that you want to check to make sure that the map is reporting correctly probably the easiest is performing a volumetric efficiency test that's where you take data from the road test that we mentioned earlier you plug it into a calculator and you find out what the ve rating of the engine is that's a topic really for another time but lots of information already online motor acecomm if you want to learn more that's one you can find out at the map sensors telling the truth or not and certainly like any other sensor it's got to have good power and grounds so we can check those with our D vo women make sure that is correct if we suspect that we're getting a Mis reporting from that sensor now that's not the only way that we can get misinformation in the computer what if we open them backdoor to the engine what if there's a tear in the intake boot or leak in the intake gasket or cracked vacuum line anything like that that's allowing air into the engine and it's not passing through the MAF sensor we call it unmetered air and what the result is is there's air being added the engine that the ECM doesn't know about well it's basing his fuel calculation based on what the mass sets are told it isn't it so any air that we add additionally it's going to cause that overall mixture to become lean and then you're going to start seeing those positive Corrections from the ECM and attempt to bring that back to Center vacuum leaks are not always the easiest to spot sometimes they're hidden sometimes it can be internal best thing I would advise you to use is your smoke machine to hook that up to the intake and help you locate the sources of those types of leaks and how do you know if it's a vacuum leak and not something else we're first free stream data is going to indicate that the issue is a viable or very low speed why because there's very little air going the engine already isn't there so when we add any additional air it has more of an impact than what does say if we're cruising down the highway at 80 miles an hour one way you can check very easily is to watch the fuel trims of idle of course they're probably going to be correcting way positive and then bring the engine speed up to about 2500 rpm let us stabilize for a minute if you see the fuel trims trying to get back down to a more normal or they are at a more normal level or there you go that's one of the greatest indications that you have some source of unmetered air vacuum leak getting into the engine then causing your system link on a map equipped system now speed density systems were in common use back in the early days of obd and obd2 and then like I said the mass sensors kind of started taking over because of the simplicity and the ability of that sensor directly measure the airflow but now we're seeing a lot of OEMs they're going to turbocharged power plants and the speed density system is once again making its way under the scene now as I mentioned the speed density system relies on a variety of sensors primarily the manifold absolute pressure or map sensor to provide inputs to the ECM that it then uses to calculate how much air is coming in and as I said already on the other systems if there's a problem with any of the sensors providing these inputs then of course there's going to be an issue with that calculation and that could be there is cause of your system leaner system rich codes some of the other things with the map sensor we talked about vacuum leaks vacuum leaks will really have no impact on fuel trims they won't be the cause of a system lean code on a map system why because whatever vacuum these are getting into that intake manifold the map sensor is going to see that more commonly what you'll see is a higher than normal idle speed it's an IO speed you can't quite get under control because there is air getting into the engine that's not going through the throttle plate so that's one thing you want to keep a look out of okay before we go with this episode of the trainer is a few other things I want to leave you with when you're diagnosing system lean system reg codes that I've seen texts overlook number one is the fuel delivery system you know as I said right at the very beginning the ECM can only calculate how much fuel to add the pulse-width specifically for that injector if the injector flow rate is as it should be and the volume of fuel being delivered and the pressure is being delivered at is as it should be so we're all taught to check fuel pressure but how many of you are making a routine check of fuel volume that's vitally important and usually what you'll see is conditions that occur when starts to sputter or kick or fuel trims start to go lean when the system is operating at higher speeds or under heavier load certainly not an idle wood fuel demand is low another common thing that is becoming more and more reported is issues with alcohol content of the fuel especially if you're over in the Corn Belt there's two conditions that typically happen we have the vehicle that is not designed to run on ethanol that ends up getting filled up with ethanol and because of the higher oxygen content the system actually has to make positive Corrections in order to try to get that oxygen and sensor where it wants to see it another very common is some flex fuel vehicles that are driven short distances use a variety of fuel with ethanol conventional gasoline and never really get to get a handle at least the ECM doesn't on exactly what's in the tank and is trying to burn so one of the things you want to check is the alcohol content pit on the Enhanced data of your scan tool if you see that kind of out of whack you know somewhere in the middle 30 40 50 60 % rather than 85 or 10 then then that may be an indication that he seems confused and if it's running conventional gasoline but it thinks it's running on alcohol well then you can have a system lien condition because the computer is just factoring in the wrong type of fuel so make sure you check that as well actually checking alcohol content is probably not a bad thing to do as a part of your routine system lien system rich diagnosis just to help eliminate that as a possibility and it's not hard to do there are kits on the market that will allow you to do that very specifically we can exactly tell exactly how much alcohol contents in the fuel but again is a quick troubleshooting aid I just want to know is that pit on that vehicle correct I might do I have a high alcohol content in this tank or is it line is it being in misinforming the ECM so let me show you a really quick way that you can do that all you need is is a glass jar some water and a sample of the fuel okay to do this little test like I said there are kits that you can get you can get a graduated beaker or baby bottle from the local store and you can do these tests more details you until exactly what the alcohol content again is again Mike I just want to know if I'm dealing with a flex-fuel vehicle that's given me a 50 or 60 percent alcohol content but I suspect that the system lien condition is being caused by a pig that's not correct then then I want to go really just a really quick test to see whether I'm right or not so all we going to do is we're going to take a jar and I'm going to take some water and I'm just going to put a little bit in the jar maybe fill it up to about I don't know an inch at the bottom right about there now there's you I can see there's will on the on camera as I can there's a little Ridge on the jar there but just to be on the safe side I'm going to just make a little permanent marker mark there and then I'm going to take a fuel sample that I collected and I'm just going to pour that in to the jar on top of the water now of course the water and the gas are going to separate but if there's any alcohol in the fuel it's going to mix or be absorbed rather by the water and that should cause the water level to rise the more alcohol the higher that rise should be so we're just going to shake it up a little bit look like quite a good tight seal there and then I'm just going to let it sit and settle out we'll come back in a couple of minutes and see what happened because you can see on this particular sample the water level the base level is still right about at where I marked it originally even if there's as much as a 10% the increase here may be marginal now if this were e 85 then we should see a marked increase we should see that water level raising up even further as it absorbs the alcohol out of the gasoline so again let this sit I'd say three to five minutes let it get nice and stable check where you are if we're like doing the situation we talked about earlier you know we saw out of whack pin on the flex-fuel vehicle and then we wanted to make sure whether it was dealing we had alcohol in a tank or we didn't this is a quick way to tell this is obviously a 10 or or just commercial gasoline almost everything now I somehow call in it right so again this is the quick test you can do it more precisely if you want to but I would make this a habit on every vehicle that you're doing a system drivability or anything you think might be fuel fuel related issue whether it's a system leaner system rich code well it's going to out do it for this position in the trainer I hope you found the tips helpful I'll see you next month [Music] [Music]

Channel: Motor Age

Views: 57,777

Rating: 4.9163346 out of 5

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: rNf9CGJ9iIU

Length: 18min 15sec (1095 seconds)

Published: Fri Jul 07 2017