Misfires and weird idle

[danili16]

Hi all,

I have a 2012 M3 6MT with ~57k. Since I bought it back in 9/2022, it’s had one issue, with another developing recently.

1) When warmed up and at idle, especially after driving for a while, the RPMs will suddenly drop and then bounce up/down for ~10-15s before either stabilizing or stalling (restart is fine). This doesn’t happen or worsen after refueling. No other patterns of note.

2) Back in October, I got a horrible miss followed by an engine malfunction/SES. Codes were for bank 2 O2 sensors, and misfires were for all bank 2 cylinders. The misfire cleared on the next restart, but now comes back intermittently after a hot start and lingers for a minute or two, then suddenly clears. Interestingly, it hasn’t re-triggered the SES/EM lights.

For further context, when I got the car, it had high-flow cats/aftermarket O2 sensors/tune. Carvana ended up paying a *ton* to replace both cats, all four O2s and re-flash the car. The O2s have all been replaced a second time under warranty. I’ve also since replaced the spark plugs and moved coils around, both to no avail.

Only codes that ever come back now are misfires.

Anyone ever experience something similar? Either issue related, you think?

Appreciate any insight!

[RocketyMan]

Get ISTA and run the ionic misfire detection. You could have a coil going out and moving them around is just going to continue them misfiring unless you know which ones you moved where and are tracking what you are doing.

P.S. You made sure to use the stock spark plugs, ya? This engine platform can only take one type of spark plug. Also, all the coils are OEM, ya?

Please report back your findings.

[danili16]

Thanks for the response!

* Plugs and coils are OE
* Misfire hasnÂ’t followed the coils; itÂ’s static on the same bank every time

Thinking IÂ’ll get it to a specialist of course, just hoping I might be able to gain some insight here if anyone else has been through it. The dealer tried pretty diligently a few times since I bought it in mid-2022, but canÂ’t replicate it and hasnÂ’t been able to diagnose it yet.

Other thought is to just drive it until it completely dies or wonÂ’t come out of engine malfunction I guess.

[RocketyMan]

Originally Posted by danili16

Thanks for the response!

* Plugs and coils are OE
* Misfire hasnÂ’t followed the coils; itÂ’s static on the same bank every time

Thinking IÂ’ll get it to a specialist of course, just hoping I might be able to gain some insight here if anyone else has been through it. The dealer tried pretty diligently a few times since I bought it in mid-2022, but canÂ’t replicate it and hasnÂ’t been able to diagnose it yet.

Other thought is to just drive it until it completely dies or wonÂ’t come out of engine malfunction I guess.

I don't think that's the best option.

It'd be nice if you got some good readings of typical things during the issue. Like O2 readings/voltages...fuel trims LT/ST...fuel pressure...
I bet there's a common issue here that just needs to be found. Some common ideas are the idle air control valve, fuel vent (maybe), SAI valves....

This is challenging to diag over the internet without substantial information/data.