Misfiring in the rain over 2000 RPM

[blitzkrieg59]

89' 325i. It's been misfiring in wet weather for a while anywhere above 2,000 RPM (generalization) under load. It does it intermittently through the revs after that. Now It's getting to the point that it misfires intermittently in all weather under judicious application of throttle. Where do I start to look to fix this?

[Gregs///M]

Key areas of inspection should be the distributor cap, CPS sensor and AFM sensor. If you do not have a brand new or refurbished CPS or AFM, You will have problems.

[Conrad311]

keep it simple, start with plus, ignition wires, then like ^^^ said, distributor cap/ rotor are likely suspects. While aged electronics do have isses, in my professional experience, it's almost always tune up items. especially in wet + under load condition. Crank sensor failing sometimes has a long crank/ hard start condition so if you've noticed that as well, maybe start there but don't change that without checking ignition components.

Run it in a dark garage (vented!) or at night and spray it with a water bottle and see if you can reproduce the misfire and see sparks grounding our on the valve cover or anywhere else.

[redlightpete]

+1 on the tune up items.

[Swendog]

I had the exact same problem- driving in the rain. I thought the worst and just like Conrad311 said- start simple. I have 235K on original engine and will be doing a valve job this winter so was holding off doing anything. Turns out the plugs were fouled- some REALLY bad likely due to my leaking head gasket! Change the plugs and voila! No more misfire.

[ForcedFirebird]

It's common for the CPS wire to get chaffed by the water pump pulley. Often people do the timing belt and don't put back the conduit that protects that from happening.

[packratbimmer]

...and the CPS wire heads straight north out of the input - so water runs down into it when it gets wet. Got stuck once with that failure. Use a BMW part when you replace.