Originally Posted by danbaub
Hey, I have a '98 M3 that I just swapped engines in. It now has a random misfire (Fault code PO300). The previous engine ran perfectly (Swapped due to unrepairable water port problems in block). I changed everything out from original motor and placed it on the new motor. I've been through and through the fuel rail, coils, plugs, etc. the problem's not there. Any ideas what would cause this? I'm new to Beemers.
Go back to basics:
1) check compression values
2) check for traces of coolant in your compression tester.
3) coolant pressure test. Is the pressure holding? yes? no?
If the above check out, then that is when you do suspect plugs, coils, fuel filter, etc...
TIP: your OBD2 scan tool should be able to tell you which cylinders are misfiring. If ALL are misfiring, suspect a part that is central to all cylinders