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Discussion Starter · #1 ·
A few weeks ago, car developed a sudden misfire mid-day out of the blue. Started off with a bit of rough running, then CEL, and then finally limp mode w/4500 RPM limiter.

Scanning codes gave me cylinder 10 misfire, both codes, plus the usual limp mode codes, and a bank 2 O2 sensor code. After resetting, the misfire codes came back. I didn’t have any real driving to do with the car so I didn’t address it right away, was still able to drive it around town for errands, but kept re-scanning and it was consistently the #10 misfire, occasionally #7 as well, only received the O2 sensor code one other time.

One other thing to note is that it was starting up fine for a few seconds then seemed to “settle” into a misfire.

After running some tests in INPA, seemed like everything was fine expect for the quality of combustion in #10, was blowing out the scale constantly. Nothing with #7 incidentally. I figured the spark plugs were 40k miles old and the coils I believe were original, so ordered new ones and replaced them. This seemingly only moved the issue to primarily #7, and still getting the occasional O2 sensor code.

I am pretty stumped. Almost the entire VANOS system is new. Throttle actuators are new, idle actuators are still fairly fresh, and both are fine given the INPA tests. Plugs and coils are now new. Swapped ionic modules and that did not change anything. All vacuum lines seem fine, wiring I saw seemed fine. Last thought is O2 sensors, but not sure if that would cause a persistent misfire in one cylinder?

One other thing - #10 plug seems to be a bit more “wet” than all others yesterday. Made sense, as it was consistently the cylinder throwing the error. Last night, 3x in a row, it is now #7. I have yet to monitor combustion in INPA but will be doing that soon.

TL;DR - can O2 sensors cause single cylinder misfires? If not, what could that hasn’t yet been replaced on this car? Hitting the limits of my knowledge here.
 

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Discussion Starter · #3 ·
I have heard of this happening, but I have serious doubts about that in this case given that it pulls cleanly and smoothly to redline until limp mode kicks in, especially now that I've changed coils and plugs. It also does not at all smell like it's running rich. that along with the fact that seems to have changed cylinders makes me doubt that it's injectors, although I have thought about it. Not sure about the fuel filter, I am the 4th owner of this car so I will assume it hasn't been done. Definitely on the list. It's also been about a few hundred miles of this at this point, I would think a stuck open injector would cause immediate severe drivability issues and hydrolock shortly after. Will be driving it shortly to run some tests on INPA and report back.
 

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I'd start with fuel injectors. Check the fuel pressure on the rails, then start testing injectors.
 

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Discussion Starter · #6 ·
When was the last time you cleaned the MAFs? I had a similar issue and using this helped quite a bit. CRC Mass Air Flow Sensor Cleaner, 11 Wt Oz
Good call, I never have, have to imagine they never have been. Will definitely give that a try.

Just had it out for about 30 minutes, actually managed to drive a few miles without getting any errors, drove perfectly, then eventually kicked to limp mode. Error has definitely moved to #7 from #10 now. Also - monitoring combustion in INPA shows that #10 seems to be running fine, whereas #7 is now showing issues with the combustion quality integral...in the opposite direction that #10 was, incidentally. Very strange. Spark length was not too far off from the other cylinders, unlike #10 was, but still not in line.

I will probably have to have a shop test the fuel system, I just do not have the tools to do that myself right now, will have to stick to the low-hanging fruit and go from there. I still find it strange that the problem would occur suddenly, and that #10 would magically clear itself up and that #7 would suddenly start having issues. I have no experience with fuel issues in this car, but I have to imagine a broader fueling issue would have crept up a bit over time, instead of suddenly occurring one day? And if it was an injector, can't see how one would "fix" itself and another one would suddenly "break" at the same time.
 

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They don't really break, they get clogged, or stuck open. Sometimes they can get unclogged, and unstuck, but most of the time the damage is done by then.
 
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Discussion Starter · #8 ·
They don't really break, they get clogged, or stuck open. Sometimes they can get unclogged, and unstuck, but most of the time the damage is done by then.
Sure, bad word choice on my part, but I still am a bit suspicious of a fuel injector that clogs up, causes issues for weeks, then clears itself out and then simultaneously another injector develops the same issue.

I suppose what I'm really interested in is if issues w/MAF sensors or O2 sensors can cause single-cylinder misfires? That doesn't quite sit right with me either but little about this makes sense in light of everything else that's been done. Will hopefully have a chance to clean the MAFs this week and report back.

One other thing - I know the charcoal canister can cause issues...I know mine has to be replaced because refueling, especially when hot, can cause surging idle and has even caused the car to stall. Wondering if maybe it's bad enough now to cause this issue...seems far-fetched but possible based on the fact that it's caused a CEL previously.
 

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One other thing - I know the charcoal canister can cause issues...I know mine has to be replaced because refueling, especially when hot, can cause surging idle and has even caused the car to stall. Wondering if maybe it's bad enough now to cause this issue...seems far-fetched but possible based on the fact that it's caused a CEL previously.
Open the gas cap after a quick run. If you hear air ingression from tank vacuum most likely the charcoal canister is clogged and you'll get a leak detection pump code (US cars only) . There are also 2 tank vent valves under the plenum that should be replaced periodically.
 

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What about swapping the ionic module? #7 and #10 are in the same bank. I think its worth a try. I've been through something similar drama as well but my E60 came out OK fortunately. When in doubt, do a leakdown test - it will tell you if its rings, headgasket, or valve issue. Goodluck
 

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is it possible that you have a wiring issue to the coils on that side? That would be something I would check, other than the leakdown/injector tests also suggested.

My intuition is that if it were a MAF/O2 issue, you would find more random misfires and emissions related codes rather than persistent cylinders.
 

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Discussion Starter · #13 ·
@NoVA8675309 Yeah I’ve had that issue for a while now, just haven’t gotten around to doing the work of replacing the canister. Where are those vent valves located? I think I know what you’re referring to but can’t quite picture them.

@specialized2010 Did that right away, no difference at all. Will probably have to do leak down/compression test as well since I know broken valve springs can happen. Would he quite a bummer if that’s the case, half the engine is new in this car at this point.

@Garfunkgrin I agree for sure, doesn’t seem to be a good answer here. I also have a hard time believing that one injector suddenly fixed itself and simultaneously another failed. Seems improbable.

Besides cleaning MAFs (since that’s easy) - will try some injector cleaner (even though that seems to never work in cases like this) and then fuel filter will probably be next.
 

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Discussion Starter · #15 ·
Update here - fuel filter is on its way, although was delayed. In the meantime, I decided to just do the easiest thing possible and run a bottle of the best Techron product they make through a tank of about 12 gallons of gas. Did that yesterday, upon startup limp mode was gone and did not return during about 60-70 miles of driving. Didn't push it though, drove very conservatively. Today I reset all codes, and took it out for a short drive. Drove it about as hard as I could on the street, no emissions errors or limp mode. No codes at all. Seems like perhaps I just had a dirty fuel system. Feels better than ever right now. Will definitely still be replacing the fuel filter, charcoal canister, fuel cap and possibly those fuel tank breather valves when I get a chance. With 117.5k miles, all these things are just normal wear & tear items at this point. Will still probably run another bottle of Techron through it after this tank, and possibly some Sea Foam as well.
 
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