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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.

Thanks, DanBaub.
 

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The coil/plug boots (rubber) get old. They crack and the spark intermittently shorts into the block. Without enough amperage to fire, the plug does not spark. Change out the coil boot....BMW makes a silicone upgrade replacement part that will last longer than an original.
 

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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.

Thanks, DanBaub.
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 :)
 

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One more thought

I can't argue with the above advice. I have a 95 M3 (track car). It developed a random misfire at idle. A sage old mechanic told me a trick to help isolate what was going on. I injected a little propane into the air intake while it was idling and the miss went away. It tells me that its a lean idle miss. Turned out to be the MAS.
 
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