Have been on the E36 M3 message board for a few years, no response on this question, so perhaps someone hear has heard of this and has an answer. I also have a track instructor who has a supercharged M3 experiencing the same problem I have; my car BTW is not supercharged. Neither of us has found a solution or cause for this problem, both 97 cars.

And the problem is intermittent rough idle and then a CEL, and the only codes I ever get are misfire, cylinder 5, or 6, or 4, or all three, but NEVER 1,or2, or3. Only the rear bank. Once or twice I have gotten a cat below threshold code on the cat coming from the 456 bank of cylinders, but not sure if this is a cause or the result of the misfires.

I have gone thru all the probable causes, crank position sensor, fuel injectors, o2 sensors, switched coils to see if it was a coil, installed new boots on the coils, switched knock sensors, all to no avail. Also, installed some new vacuum hoses, idle control valve.

The baffling part is that it is intermittent, usually starts after I have been driving, could be 5 minutes, could be three weeks, doesn't matter if the car is hot or cold or run hard or just cruising, ANd it is only cylinder 4,5, 6.

Any help or ideas appreciated. Perhaps it IS a cat problem?

Phil

From of all of your diagnosis that you have already done, and that you have seen that faul for the cat for bank 2, sounds like the cat is causing your misfire.

So, you think it IS the cats? My only question of the cats being the problem is that I didn't get a cat code for several months following the misfire codes, so thought that perhaps the misfire was possibly rich in fuel and causing destructiion of the cat matrix.

I will say that for the past few years the cats or one of them was rattling a little, and then it stopped rattling for a short while, and then started again, so perhaps this IS the problem?

Thanx for the input.

Phil