Occasional Misfire on same cylinder, not plug or coil - BMW M5 Forum and M6 Forums
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post #1 of 20 Old 8th February 2019, 04:52 PM Thread Starter
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Occasional Misfire on same cylinder, not plug or coil

I've searched around on this and the threads I've found usually contain myriad codes and other problems. That's not the case here, this one is a single-code-only issue on an otherwise pretty mechanically sound S62.

I'm occasionally getting a misfire during hard acceleration only on Cylinder #2. On the Peake this is a C5 code. I've swapped the coil with another cylinder to no avail and replaced all the plugs, the code still stays with the same cylinder. When I replaced the plugs they all looked consistent/alike so I don't think it's something mechanical/block/head related with just that cylinder (that's the hope, anyway).

So, what's left- Fuel Injector? Could that cause a C5 code? That is the ONLY code that the engine throws when this happens. Car will run normally after a restart.

171K, 2000 model, injectors have never been serviced.

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post #2 of 20 Old 8th February 2019, 05:01 PM
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If you know plugs and coils are fine. Check resistance on the injector and compression. Could be a burnt valve .... or anything really

Last edited by Al59751; 8th February 2019 at 05:02 PM.
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post #3 of 20 Old 8th February 2019, 06:15 PM
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You need to do more. The nature of a V engine is while one cyl is firing another is exhausting and the misfire detection may have a problem with that. It should not have but this car seems to have a problem, likely because it tries to be too smart with all those extra teeth in the flywheel. So when you decide to move a coil you should always move the opposite coil at the same time, and moving them back three positions seems to yield the best results for making the code move. Not sure why but it has been proven time and again on these cars. 8 and 2 are the pair so they should have both been moved to 6 and 1, just to rule out flukes.


That said occasional at high rpm sounds more like an injector, but could be coil or even over all power supply because coil is not 100%. Does not seem like vanos or exhaust gas poisoning but always keep that in the back of your head.


The first tests I would do on injector suspicions, is two different injector treatments. First a treatment with(because you are in the USA) techron. If you were somewhere else it would be a different product but the same chemical, it is specific not all injector cleaners are the same.


This product is a cleaner and does nothing to lube up the injectors, old worn injectors need a little lube to help them after they have fired a few billion times. After that product is run through and not in your tank I would treat the fuel with a double dose of Lucas fuel injector/upper head lube. I would double the dose for two tanks and then do regular doses.


I would keep an eye on the code and see if there was any change, because no treatment will fix a mechanically broken injector or a plugged injector screen, although Techron might help the latter, depending.


After that it is all in the details on what changes and how, because the second product also lubes the valve stems and that helps a lot if they are coated with carbon. If there was a change you could play the injector cleaning/rebuild lotto or replace the injectors with new. I always choose the latter but that is just me, the time it takes to change them out is the most significant cost to me.


Without going to the next steps after that, do that and I will type more then.
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post #4 of 20 Old 8th February 2019, 06:52 PM Thread Starter
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Thanks, this is super helpful and informative. If an injector is in fact leaning out (on either #2 or the opposite cylinder) is there harm in running the engine for hundreds of miles through multiple tanks of gas to get these cleaners/lubes through the system? Am I at risk of burning a valve here by continuing to run it or would the engine computer continue to intervene before damage is done? Is it smart enough to throw a misfire on a "kinda lean" condition on one cylinder? I don't know if I'm making sense here but I'm worried about running the car more, I guess.

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post #5 of 20 Old 8th February 2019, 07:24 PM Thread Starter
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I should add that as far as moving the coils, I only specifically labeled and moved the #2 coil. All of the rest were unlabeled and put back randomly on the plug change so chances are fairly high that the #8 coil moved too.

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post #6 of 20 Old 9th February 2019, 04:18 AM
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Quote:
Originally Posted by M3Fan View Post
Thanks, this is super helpful and informative. If an injector is in fact leaning out (on either #2 or the opposite cylinder) is there harm in running the engine for hundreds of miles through multiple tanks of gas to get these cleaners/lubes through the system? Am I at risk of burning a valve here by continuing to run it or would the engine computer continue to intervene before damage is done? Is it smart enough to throw a misfire on a "kinda lean" condition on one cylinder? I don't know if I'm making sense here but I'm worried about running the car more, I guess.
Your code is for a misfire, but without turning off the fuel. Can you even feel it? Does it even set a light? If yes does the light turn off on the next start? It may not be going lean it might be rich. Depending, it is more common for the injector to not close which causes rich. But there is no real way to know. You could try watching live data but that is what the DME does and if it sees a fluctuation in the O2 readings it will shut the fuel off to that cylinder.
Since the DME can't see anything in your readings I doubt you or I could either. The DME sees all, we only get to see a smoothed effect, do to reporting to the scanner being a tad slow.
Don't really have an answer to the question but it is not a dangerous misfire or the DME would shut off the fuel.
That said my advice would be gentle, then do a hard pull to see what is up. Gentle, at a 1/4 tank gone do a hard pull to see if things are different, repeat every 1/4 tank. But be gentle in between, just not so gentle you can't feel any difference.
Is that vague enough for you? Lol Sorry no real answer there is a risk when things are not right, but I have no way to know. You will have to judge for yourself and live with your decision. You could just run out and buy 8 new injectors, that is what I would do if I found a problem with one 15+ year old injector. I would not send them to be cleaned just replace.
But you still need to see if it might be injectors, what is causing your miss is still up for debate.
Hope this vague non committal answer helps in some way.
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post #7 of 20 Old 9th February 2019, 04:46 AM Thread Starter
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The C5 is misfire Cyl #2 w/fuel shutoff. It violently cuts power and throws a CEL, then the car runs on reduced power until restart.

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post #8 of 20 Old 9th February 2019, 06:10 AM
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So it is. The list I was using had no mention of fuel cut off, guess I will throw that list out and use my old list. Such a pain to bring up excel every time I need to see a code.
Ok my bad that changes things. Don't test and use fuel throttle, except for once maybe twice. Not sure I would even do that. If it only happens occasionally and not every time you give it the gas that will change things too. If it happens every time you hit the floor I might be inclined to just pull the injectors and have them tested. Once they have them then you are likely stuck getting them cleaned and reinstalling them. The cleaning and rebuilding I have had zero success with nor any faith in anymore.
Guess I will go out on a limb like it was my car and not some one elses car. If it was my car I would run the techron, empty the tank start the next tank and do one pull. Then I would do two tanks of Lucas or other upper head lube and do another pull. If there was no change I would worry but I would likely still think injector.
The thing you want to look at is how long you are full throttle before the misfire, if that changes that would be a strong indicator it is injectors.
If I had that sign I would likely just buy new injectors and switch. Even if I was wrong the injectors are old and a refresh would just be maintenance. If there was no change then I would park the car and send out the injectors for testing and if one was bad I would at min replace it even if it could be cleaned, but clean the rest.
I hate this but.... I don't ever suggest blindly throwing parts on, but that is what I would do, bugger the cost buy 8 new injectors just because they are old, if that does not help it is money you did not need to spend but it is good maintenance and you will save the money on sending them out and the cleaner and lube.
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post #9 of 20 Old 9th February 2019, 01:32 PM
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There is one thing I learned about the S62, it's throw out conventional logic .... As sailor said the DME is pretty bad at detecting the exact cylinder that's misfiring at higher rpms. If it were me... I'd do the following the steps some you have done.

1)Swap coils
2)Check and compare all plugs
3)Run a vanos test this is key because if it's slow to adjust or won't fully adjust you'll get misfires, you get multiples most of the time but don't write that off as gospel.
4)Check compression

if all that checks out then move on to fueling, pump, filter, regulator and injectors.

Also what are your long and short fuel trims?

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post #10 of 20 Old 9th February 2019, 02:44 PM Thread Starter
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No idea on the trims. I have INPA so I can dig into that stuff when I get time to take another crack at this which will be in a week or so. This is all helpful input. I drove home from work yesterday (30 miles) only using about medium throttle at most and never tripped the CEL. The times when it happens are usually at highway speed, 5th gear pull, maybe a couple of seconds into the pull. It's happened maybe 3x total in say the last 150 miles. Never had it off the line when the car has a major mechanical advantage in lower gears. FWIW It seems like the car is down on power a bit in general but that could just be seat-of-the-pants feel, I think we all suspect that from time to time but it does seem a little off.

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