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seabass20329 seabass20329 is offline
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Question Pretty Unique Misfire - 12-01-2017, 06:35 AM

I just registered here and this is my first post, but I've been searching around and reading plenty on this and other bmw forums for a few months now, specifically reading up on misfires.

I know misfires are very well documented, and many people have given a clear step-by-step guide to diagnosing and repairing them, but the way my car has been behaving seems really strange to me and I haven't found anyone report the same specific symptoms.

2008 535i with 80k
Bought it about two months ago.
Car misfires under heavy load.

This is where it gets weird though. If I back off the throttle as soon as the misfire occurs, the car behaves normally, doesn't throw a code, and continues to run smoothly unless I put it under heavy load again. BUT if I'm not quick enough to back off the throttle and the car misfires 2 or 3 times, it throws a code (always just a random misfire code) and becomes a dead miss, which will not go away until I pull over and restart the car. After restarting, the cel is still there but the engine operates perfectly smoothly, no hesitation or anything suggesting a failing component. I don't understand how I can have a misfire that only presents itself under heavy load, but then once that misfire occurs becomes a dead miss, misfiring at idle, wot, and everywhere in between, in any gear, UNTIL I restart the car, at which point everything is fine.

I'm new to newer (post e36) BMW's, is this a safety feature or something? Like the ecu cutting fuel or spark to that cylinder to avoid damage once it detects a misfire? I've searched plenty but I can't find anyone describing this situation.

So far all I've done is clean the intake valves (carb cleaner and bore brushes), which did fix the problem for all of 3 days, after which the problem came back in the exact same manner. Again, this makes me think it's something the ecu is doing. Incorrectly compensating for something?

I'm going to pull plugs and look at injectors this weekend to start diagnosing this the normal way, but I wanted to see if anyone here had any info specific to my weird misfire conditions.

FWIW I'm an averagely competent mechanic. Not a genius, but not an idiot either
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ms335i ms335i is offline
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Default 12-01-2017, 07:17 AM

Okay is it solely cylinder 6?
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seabass20329 seabass20329 is offline
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Default 12-01-2017, 07:27 AM

Not sure, the only code it throws is a generic "random misfire", though I haven't had the codes read in a few weeks admittedly.

Going forward my plan is drive the car hard to create the dead miss situation, then pull into the shop and start unplugging coils one at a time until I find the cylinder that doesn't change when I unplug it. At least then I'll know what cyl it is
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seabass20329 seabass20329 is offline
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Default 12-01-2017, 07:42 AM

Forgot to mention, car is completely stock
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ms335i ms335i is offline
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Default 12-01-2017, 07:43 AM

I had this issue and it only happened on cylinder 6 at high rpm. The car would hit just about normal boost, just slightly off but there was a boost leak. The car would counter that by killing fueling so hpfp would drop like a rock at 5900rpm and cylinder 6 would shut off until I stopped and reset. fixing the boost leak fixed it.
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ms335i ms335i is offline
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Default 12-01-2017, 07:44 AM

I did not have any 30ff no boost leak codes nothing weird, even boost was barely off.
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seabass20329 seabass20329 is offline
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Default 12-01-2017, 08:02 AM

Oh nice, I wouldn't have thought to check that. So your ecu cut fueling because your afr's were rich due to the boost leak. Wouldn't that effect all cylinders though and not just cyl 6? Either way, I'll add checking for a boost leak to my "to do" list. I can get mine to misfire at any rpm, but I could definitely see the problem being boost related since engine load is the only thing I find that definitely causes it to misfire
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ianc ianc is offline
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Default 12-01-2017, 09:04 PM

Ignition coils. They heat up under load and cause a misfire which goes away after you back off for a bit. Replace them if they're old...

ianc
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rhm135 rhm135 is offline
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Default 12-02-2017, 05:39 AM

This is nothing unique. The DME cuts fuel to the cylinder after so many misfires, hence the dead cylinder if you hold throttle too long. It sounds like you have a bad ignition coil.


2012 335is DCT - JB4 - Fuel-it! - MHD - DocRace 6466
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akambar akambar is offline
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Default 12-14-2017, 04:35 AM

I had a similar issue with my car and it ended up being a bad HPFP that tested within the range of acceptability according to BMW, but according to the independent shop i eventually got the car fixed at was not pushing enough fuel and caused the misfire. I was also getting random cylinder shutdown codes.
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killdro killdro is offline
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Default 12-14-2017, 01:52 PM

I have this same issue and I have narrowed it down to the injector. To the point where I pulled the injector and noticed that the seal was ****ed. I cleaned off around the seal and it went away until I can order a new seal.
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