Misfiring Codes After Test Drive

[MellowD]

I'm selling my car and just had a potential buyer come test drive my car (N54 335i with JB4 and some other mods and 105k miles). The car had just hit the minimum warmup temp and the guy was going kind of hard on it... not WOT but still more than I would usually do before the car is 100% warm. This triggered a flashing Service Engine Soon light and caused the car to go into limp mode. I checked the JB4 codes and it showed the following errors.


29CD Misfirings, cylinder 1
29CF Misfirings, cylinder 3
29CE Misfirings, cylinder 2
2D60 Fuel mass, monitoring
30BA DME digital motor electronics, internal failure
0000 Unknown Fault


I've looked up the individual codes and the first 3 say they can spark plugs, injectors, etc... but does anyone have any idea what the cause could be for this combination? I've searched the forums and someone with a similar issue said it could be a MOSFET or DME issue. I'm still not sure what that means but hopefully someone here can help before I start dropping a bunch of money.


Any ideas?

[RocketSurgeon]

It's almost certainly a MOSFET problem. It's like a big transistor that puts power to the coils. They are a common failure point on all N54 cars built before 03/2008. Shops will try to sell you a new DME for $1000 or more. You can buy the MOSFETs for under $2 each and have all six replaced at an electronics shop for under $100. Don't attempt to solder them yourself unless you have experience with Hot Air Rework and SMD soldering.
You can open the DME and check the MOSFETs with a digital multimeter.

[jclausen]

I would mark and move coil packs clear faults roadtest again if the misfire follows the move it's coil packs, you could look at spark plugs to see if one was black indicating to much fuel, but I would move coils. Start with the basics, spark plugs are changed every 45k

[MellowD]

UPDATE: I had someone replace the mosfets and I just put the ECU back in the car. All of the codes are gone, except the 29CD Misfirings, cylinder 1 is still there. Also it sounds like there's a slight knocking sound coming from the engine (which I didn't hear before) and the car is still running rough... but it could be something else since my engine bay is still somewhat disassembled. Any idea what I can do to fix this?

[Old525i]

I have a persistent cylinder 6 misfire when the engine is rev'd above 3000. Maybe the same problem?

[MellowD]

I have a persistent cylinder 6 misfire when the engine is rev'd above 3000. Maybe the same problem?

My car is running rough and the code shows up even when idling. My guess is I have a bad coil or injector... I don't think the spark plugs are the issue since they were replaced not too long ago.