3rd cylinder misfire after heavy rain...
 

Hey all,

I've been dealing with this problem for a while.

It's bizarre, but after every moderate to heavy rain (and once on a real foggy morn with melty snow), I get a blinking check engine light that throws a p0304 code, 3rd cylinder misfire I believe, and the car runs like garbage. It back fires with moderate acceleration.

I've tried replacing a cracked ICM wireing harness (courtesy of billboost37), and a new fuel sender and pump, as the old one was rusting out and starting to malfunction.

I suppose it could be electrical, but could be water getting into the fuel?

I thought it was a plug/wire problem, but I don't think that's the case anymore.
https://www.gmforum.com/1992-1999-91/re-occuring-plug-wire-failure-285665/

Any suggestions?

The way you describe it.

I'd open the hood and start the car. Now wet the coil for 3/6 Wet the wire next if that doesn't do anythign, then went right at the cylinder.

A moist issue is typically coil/wire/plug.

Although I did take one of my cars outta commission showing a friend that you can clean the motor. I didn't know the crank sensor was cracked at the plug and it allowed water in.

Sounds like moisture getting into an electrical connection. If you tried misting the area in the dark and did not see any arcing, I would suggest disconnecting, cleaning and reconnecting with dielectric grease. Include spark plug wires at both ends (plug and coil), injector connectors, ICM, etc.

P0304 is a misfire on cylinder 4, not 3. That means you also want to make sure the #4 spark plug wire is not contacting the O2 sensor or O2 sensor wire.

I chased this issue on my 97 Bonneville for months but never any codes.
I decided to just change the coils and when I pulled off one of the coils the 2 connectors from the ICM were green with corrosion. Cleaned them up and installed new coils and the issue was gone.

Is it possible that the 4th cyl wire and O2 sensor wire to arc even if insulated?
I'm gonna have to get a tub of that dialectric grease, I'm thinking of disconnecting everything and lubing them all liberally.

I had the coils and plate all apart, cleaned them up and put them back together. There was no real corrosion though, just what looked like a spot of rust in one spot that I cleaned up.

Is it possible for the coils or plate to "go bad"? Worth replacing without them being corroded or failure to start? I was told that the coils wither work or they don't.

Mine will throw a O2 code whenever the cylinder 6 wire gets even close to the O2 wire. This is even with new wires. Real sensitive. I never saw anything like it.

Take a spray bottle with water and mist the plug wire on #3, as well as the others while your at it. Then turn off the lights. Look for the light show.

I only mention the O2 wire based on proximity to the cylinder that is misfiring. Takes a minute to check.

Based on the issue occurring during damp conditions, something is cracked (and letting moisture in) or one or more of your water tight electical connectors isn't water tight. Going at it with a tub of dielectric grease is an inexpensive way for a possible fix.

Maybe a cracked ceramic insulator on the spark plug.