I have a lot of experience wrenching on BMWs and have now learned more than I probably want to about MINIs (but clearly not enough). I've got one now that truly has me stumped and I'm looking to the community for some advice.

The subject is a 2009 Clubman non-turbo (N12 engine) with a 6MT and around 115K miles. It came to me with a leaking thermostat housing, a fault code for the pre-cat O2 sensor, and the dreaded random multiple misfire. I replaced the t-stat and O2 sensor along with new coils/plug. That took care of the coolant leak and O2 sensor code, but the misfire is still there. It was very noticeable while I was running the engine to bleed the coolant and according to my scan tool, #3 is the worst offender. There are occasional misfires on #1 and sometimes #2, but #3 is by far the worst.

What's weird is that the misfire goes away when driving. When I watch the data stream with the engine idling, it's very obvious that #3 is misfiring, with only occasional misfires from #1 and #2. But as soon as I rev it up or start driving, all misfire monitors drop to 0 and stay there until I stop and allow the engine to idle - then they come right back.

I've seen plenty of brand new parts fail, so I tried to avoid falling into the trap of thinking that just because a part is new, it can't be the problem. I have another 2009 N12 MINI that's running fine, so I tried swapping a lot of parts to see if it would make any difference. It didn't.

This is what I've tried so far:

I suspected that I may have a lazy injector, so I swapped the entire set along with the fuel rail. It made to difference.

Then I thought that maybe, somehow, I got a defective plug/coil. So I swapped those from the other car. It made no difference. While I was at it, I unplugged the coil for #3 with the engine running and it ran even worse. So that seems to suggest that the plug for #3 is getting power and sparking.

Clogged PCV? Swapped the valve cover.

Low fuel pressure due to a weak pump or clogged filter? I tested the fuel pressure and it's 54 psi. Swapped the fuel pump anyway.

I thought maybe it could be a problem with the ECU. I had it tested and there was a problem with the O2 sensor circuitry, but nothing related to fuel delivery or ignition. Nevertheless, I bought a new ECU and had it coded for the car.

Low compression due to head gasket issue or burned valve?
#1 = 175
#2 = 162
#3 = 162
#4 = 178

Maybe #2 and #3 are a bit low? But only 16 psi difference between the highest and lowest. Is that enough of a difference to worry about? And again, it runs perfectly fine at higher engine speeds.

I really do not want to pull the head off this engine (possibly for no reason), but I'm at my wit's end with this car and have run out of ideas. Is anyone seeing anything here that I missed, or interpreting the data in a different way that offers a clue about what's going on?

 

Hhmmm. Maybe an intake problem? (Since you covered anything else). Intake manifold leak, carbon build up in the head, TB / idle air restrictions?
How did the plugs look? Any sign of rich or lean miss?

 

I figured it out. Somebody had done some work on the engine top end and installed the Valvetronic springs incorrectly. They had also stripped the threads for the screws that secure the springs. I put Time-Serts in the holes and correctly installed the springs. That seems to have taken care of it. So it wasn't necessarily a problem with the engine that had developed over time. It was the result of shoddy previous repair work.