The Trainer #66 - Tackling Misfire DTCs

there are some diagnostic trouble codes that we deal with on a daily basis in the shop isn't there among them the P0300 series of codes the misfire codes if you find yourself having a hard time chasing down the cause of these misfire concerns well stick around I've got some tips and techniques that just might be of help to you in this edition of the trainer you know tracking down the cause of a misfire can be a frustrating experience especially if that misfire is intermittent or is not a a causing a completely dead hole dead cylinder so I have a few tips and techniques I'd like to share with you that just might help uh so let's go ahead and get started first let's talk about what exactly is a misfire in my mind a misfire is any condition that allows the C CER or cylinders to uh contribute less than the expected amount of push to the crankshaft now what do I mean by that if all eight cylinders are running the way they should they're all contributing an equal amount of power aren't they if a condition occurs that allows one or more cylinders to contribute even a little less than it should then it's going to apply a little less push on that crankshaft isn't it and the crankshaft is actually going to momentary slow down may not be a huge amount not something that you're going to see show up on the RPM gauge on the instrument panel but it does happen in fact that's one of the techniques that oems use to monitor for misfires they watch the crankshaft speed and look for those momentary slowdowns now the threshold of when a misfire code is actually recorded by the ECM varies from manufacturer to manufacturer so you can have an engine misfire that you can feel in the seat of your pants but it not caused the check engine light to come on or a misfired code to be Set uh we've all had that experience now how do we identify the cylinders that are misfiring well if we do have a check engine light on hopefully we won't have just a plain old po300 the random misfire code but we'll have a cylinder specific DTC that we can use as a start be careful though not all of those turn out to be the culprit I have had one one or two occasions where it was an ad Json cylinder in the firing order that was actually causing the problem so be sure that you take a look at the cylinder that's mentioned but if everything looks good take into account it could be one of its neighbors in the fing War you may want to move your investigation over a little bit and make sure they are okay as well the other thing I want to make sure you do too if you do have a DTC uh make sure that you record the Freeze Frame data that you take a look at that Freeze Frame data misfires are a continuous monitor that means that they're constantly being kept an eye on and why because we got to keep that catalytic converter safe right we got to make sure that we don't send uh too much raw fuel pump into that thing that could cause a problem so we don't want misfires to occur for that very reason but we need to know what conditions are occurring when that misfire is happening it's not static is it it could be happening in idle it could be happening at a steady Cruise it could be h happening under partial throttle under a load condition we need to look at that Freeze Frame data and make sure that we have that recorded so that we can duplicate that misfire for our testing purposes and verify that we fixed it once we found the cause now as my old friend G Trulia likes to say you don't want to wipe out Freeze Frame data and clear the codes right off the bat because it's like pouring Beach on the crime uh clent crime scene you won't be able to go back and check into it later on you you can check for misfire counters in enhanced data if you have that on your scan tool many of the oems Now list that as a PID and you can see which misfire or what cylinders have been counting up in their misfires or how many have been recorded in that data stream that's one other option for you uh you can check mode 6 too in fact nowadays I think probably every car is coming into your shop for the most part is a can vehicle right a controller area network system and they all have it in there I believe it starts with A2 is the test your looking for that's cylinder number one A3 is number two A4 is number three don't tell me why they numbered it that way it's just the way it is and there are counters in there as part of those test results that you can look at to see if there are cylinders that have been recording up even if there's not a DTC the counters are still going to show which cylinders have been recording misfires okay uh if you're dealing with a prean Ford Ford was nice enough to put that in very early on in mode 6 in fact right with the start of OBD2 so you can go go back I believe it's the test ID number 53 of course you can go on to motor.com you can find a lot of information there that will help you use mode 6 to identify these misfiring cylinders for your diagnosis now there are some other tools that are available these are more specialized tools and if you have a scope you can use these not only to help you identify the misfiring cylinder but also to help you determine what the cause is the product on the left is the a misfire detector uses a pressure sensor called the first look to actually look at the exhaust pulses in the exhaust stream while the engine's rank to help identify that weak cylinder or cylinders and the one on the right is the automotive test Solutions ire detector but what I like to call it to your attention here though is that the screen here is very similar to the Ford IDs misfire detection uh tool that a lot of scan tools a lot of Manufacturers have also Incorporated again so if you have that ability if you have the enhanced data without available make sure that you check that out and use that as a tool uh if you don't if you're using strictly Global ob2 then again mode 6 for can Vehicles it'll always be in there okay once you've determined which cylinders are causing the misfire the next thing to do is to narrow down or narrow down what is the cause of the actual misfire now here are the things I want you to consider when you're doing that first again go back to freeze frame and consider what are the conditions that are under uh the engine's operating under when that misfire happened let me give an example um an engine at idle takes very little fuel doesn't it but if it's pulling up a grade with a full load behind it three quarter throttle and a lot of load then it's going require a lot of fuel what if fuel system isn't able to deliver the volume of fuel that's needed could that result in a misfire sure but will that lack of volume affect it at idle no it won't so looking at the conditions is already going put a few things at the top of your list of possibilities that you need to inspect and go after remember troubleshooting is all about ruling out as much as possible in the beginning of what it can't be so that all that's left is what it can be that's one approach me personally I like to use whatever tests I can in the beginning to wipe out as many possibilities as I can in the beginning uh rather than focus on any one particular pinpoint System test but we've talked about that before you can go back to our YouTube channel and learn more about the diagnostic process consider what cylinders are affected if it's a single cylinder that's not so hard it's got one coil going to it one ignition event right it's got one injector associated with it and it's only got one mechanical section of the engine affected by it so these are all things that you can home in and look at distinctly but if there's more than one cylinders being causing misfire then you have to kind of consider what these cylinders have in common what could cause a misfire in both those cylinders or more than one cylinder at a time for example if you have a dis ignition system on the vehicle and two cylinders that share a coil happen to be the ones causing your misfire well I think one of the first things I'd be looking at would be that addition coil and its circuitry right so that's just one thing you can do now also consider as I mentioned earlier any Factor that's going to cause less push in that cylinder can cause a misfire so it's not limited to an ignition system I mean all too often I see things online about texts that immediately go after the spark plugs the coils maybe it's that term misfire I don't I don't know but don't get that out of your system right off the bat mechanical conditions can cause a misfire ignition systems can cause a misfire fuel systems can cause a misfire any of these factors need to be taken into account when we're trying to determine that cause let's take a little closer look at at all of them for mechanical testing one of the very first things I like to do in any driveability concern is do a relative compression test and I'm not going to go into detail here lots of stuff on our YouTube channel about different ways you can do a relative compression test the whole idea is to do something very quickly that gives us an idea of the health of each cylinder if I see a weak one well then I'm going to focus more on that one using some of these more conventional tests like the compression or cylinder leakdown test and another way you can do it is look at the ignition waveform if you can access the secondary and you can take a look at that or access the primary take a look at that there's a lot of information in the burn line on both of those patterns uh me you have to be a little more expert to understand the factors that affect that pattern and it applies not just a mechanical but it also can provide clues to ignition or fuel related faults as well um there's some great resources again like that online and if you're um thinking about attending nce autom mechanica let me suggest that if it's not full already you check out Jim Morton's uh ignition analysis class he does a great job of presenting the elements that make up that pattern to help you learn learn how to use that for your Diagnostics another tool that's at your disposal is fuel trim now the first thing you're going to look at is the fuel trim recorded in the Freeze Frame data for the codes that you found stored in the ECM and consider like we said in a few trainer issues ago it's all about keeping the Catal converter healthy I've got a very narrow range of Fe gases that are allowed into that converter and I've got to keep those under control a misfire just kind of throws that all up in the air doesn't it either two things are going to happen either way too much air is going in or way too much fuel is going in way too much fuel is a bad thing because that's definitely going to cause that that catalytic converter to overheat let's take a look at that in a little more detail see if we can talk see if you understand what I'm talking about here now this is only going to work with math systems it's not going to work with a speed density system that uses the map sensor uh for for fuel control so if your vehicle's equipped with a math sensor mass air flow sensor take a look at those Freeze Frame uh stored data in the freeze frame the fuel trim total fuel trim that is and consider what I'm about to tell you let's see if we can make sense of this V8 engine cylinder number one hasn't misfire cause as of yet unknown but it has a mass airf flow fuel control system now how does this work well let's just recap some of the things we've already learned the mass air flow can physically measure the amount of air going to the engine does it distinguish between the left side and the right side of the engine no it doesn't does the engine does the ECM rather yes it does because it's assigned one as bank one and one as Bank two so in the ecm's mind it's dealing with two four cylinder engines got me so it's taking a look at the total quantity of air that came in total quantity of air that came in let's just use round number 100 lb of air and it's going to calculate how much fuel in pounds it needs to add to the engine as a whole in order to get the proper air fuel mixture well the first thing it's going to take a look at is bank one and Bank two half and half that means if I got 100 pounds going in each bank is getting what 50 pounds right 50 pounds so now the ECM says okay bank one I need to deliver this much fuel to and I've got four injectors to do that with so that total amount of fuel added to that 50 pounds of air in bank one each injector is going to get enough for 12 and a half pounds of air with me so far and that's the bank one total that the ECM is going to send in and then that when that combustion goes past the downstream oxygen or the Upstream oxygen sensor the ECM is going to report in how it did with me Bank One Bank two total amount of air divided by two that's how much is getting in each Bank ECM makes his calculation decides how long to pulse with the injectors to deliver the correct amount of fuel to each bank and it gets its report when that combustion goes by the Upstream O2 sensor okay so let's talk about first an ignition misfire when ignition misfire occurs did the error in that bank change no did the amount of fuel given to that bank change no what happened what did change well the fuel in cylinder number one didn't combust so now we have raw fuel going into the exhaust and all that unused oxygen going into the exhaust and it's heading towards the catalytic converter now that raw fuel in in that converter is going to cause a lot of overheat that's not a good thing so one of the things that the OM uh may do or the ECM may do right off the bat is as soon as it sees that misfire occurring and knows this is going to be a catalytic converter damaging conver uh issue is to turn off that injector so no further fuel is being added to the problem the mixture of that air and fuel is going through the exhaust some of it because the exhaust is so hot is going to react just as it would have if it were in the combustion chamber but there's not going to be any contribution to power right because there's no there's no compression it just expands in the exhaust but a lot of it is not going to do either it's going to go past that oxygen sensor the oxygen sensor may see an increase in the oxygen content and may start reacting as if the uh fuel was lean or there was a lean condition and then you might start seeing some positive Corrections in the fuel trim but you're only going to see a small amount if any single digits so when you're looking at that Freeze Frame data and you're comparing Bank One Bank two and you can see that bank one is kind of showing a few points more positive correction than Bank two then I would tend to lean towards an ignition problem on bank one okay what if uh what if we looked at the fuel system okay again did the air change going through the system no that's still the same what about the fuel well the fuel did change didn't it because Cinder M1 didn't get any if the injector totally failed and there's no fuel going into that cylinder then cylinder one didn't get any fuel at all and it's supposed to get 1/4 of the total that's going through that uh or the air rather is supposed to get all of that fuel in order to uh combust correctly right to have the right air fuel mixture so essentially we shorted the air by 25% you with me four cylinders divided by four 100 divid by four 25% is what cylinder one represents so again no fuel but now I got a whole lot of unuse oxygen none of that's going to react in the exhaust because there's nothing to react with right and and this O2 sensor is definitely going to react to aan condition and I may see fuel Corrections up in the 20s as a result because I'm essentially 25% short so that's one thing that you can look for there on the on the fuel trims again depending on the OM and the strategy that the ECM uses as soon as the ECM sees that misfire it may just go ahead and shut it all off and you may see the same on both it's going to make an adjustment it's going to understand hey this ain't happening go in open loop and totally throw fuel control out the window because of that misfire but if you can catch it on your Freeze Frame just to recap if I go into a freeze frame and I know I've got a cylinder Miss po301 for cylinder 1 and I can see see that the fuel trims for that bank are increasing 3 to 7% more than Bank two was running total fuel trim then I may think well maybe that's that's telling me it's an ignition related event and if I definitely if I see it up in the 20s I'm thinking fuel okay is apply across all the cases no this is not a 100% check this only and you'll be good to go it's a tool just like some of the other tips that I've showed you so far and I've shown you in past editions of the trainer it's just one tool one step one other thing to look at to help you narrow down the possibility of what's causing that misfire to occur can you apply that to other engine design certainly this is an inline six of course but as far as the ecm's concerned isn't this the same as a V6 aren't these divided into two Banks sure even some four-cylinders are divided into two Banks if there's a downstream o or excuse me an upstream oxygen sensor a separate catalytic converter for uh any given pair of cylinders whether it's two out of four three out of six or four out of eight then that's a bank and that's going to have its own fuel control strategy so what would happen if number four had no fuel what kind of loss what kind of numbers would I expect to see there well one out of three that's 33% so now I may see total fuel contractions fuel Corrections way up there on a four-cylinder that's you're probably see anywhere near that high cuz that's way outside of the specs you may just see a system lean code set along with the misfire here's another tool that you can use to help you diagnose the cause of a misfire especially if it's a single cylinder misfire uh if you suspect there's a problem with an injector a bad injector you verified electrically it's okay it's getting the right power it's got Good Grounds it's being pulled to ground but maybe there a mechanical issue with it well why don't you just swap the two swap that good one or that suspect one with its neighbor that you know is okay see if the misfire moves again I'm saying just go right off to the bat start swapping parts around do your do your other tests first but that's that is a still it's a valid test it's a valid diagnostic tool same with if you're dealing with a cop coil and you suspect it's a bad coil you want to see if the misfire moves with the coil just just swap them cylinder cylinder see if the misfire moves if you've gone through the basics and again especially with single cylinder misfires and you verified that everything related to the fuel system is okay everything related with the ignition system is okay and that the mechanical the cylinder is sound there's no problem there don't forget that all of these events have to occur in the right amount and at the right time and and one of the neatest ways to be able to check that now is using an in-cylinder pressure transducer to look at the mechanical condition of the engine I can then map both the ignition event and the injector event and I can see when they're occurring I can see if they maintain a steady occurrence uh I can uh look them up in the OEM service information see when they're supposed to happen and compare them to what I'm getting actually happening on my scope uh it's a very very strong tool and again I'm not going to go real deep into pressure transducer Diagnostics here today just wanted to make you aware of it lots of resources on our YouTube channel about using this tool as a diagnostic Aid uh and all our alternatives to making sure the ignition timing is correct right you can take a look at the cam shaft and crankshaft patterns if you have a known good one that's one we covered a lot of those topics and I encourage you to check our YouTube channel out for additional resources well that's going to do it for this edition of the trainer I hope that the tips and techniques that I share with you today are going to be of help to you when it comes to misfire Diagnostics and you know what there's still time to join me at nce auto mechanic in Chicago July 26th through the 29th a lot of the topics you've seen me cover here in the last six months of the trainer and even beyond that are going to be handled by some of the best trainers in the industry at this event and it's all offered at no cost to you mechanical repair training and let me stress that again no cost to you you just have to cover your room and board and your travel to the event at mcari place you can find more information online or in the description for the video below I'll see you next [Music] month

Channel: Motor Age

Views: 61,201

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: 4iYBzP3kUjk

Length: 21min 46sec (1306 seconds)

Published: Tue Jun 06 2017