P0300 multiple random misfire

Recently got this code again P0300. It is probably the 3 time I have had it in the last few years. The previous times it happened after work on the way home so I let the car sit overnight and it was fine the next day. This time it seems to be acting up a little more so I decided to try and fix it this time. Seems to really only happen after heavy rains.

What I have done so far:
Replaced PCV valve
Cleaned EGR valve
Changed and gapped new plugs to spec ( the back left and middle plug has some oil but doubt this will cause a misfire)
Cleaned MAF
Checked for vacuum leaks but couldnt find any (sometimes get small evap leak code P0442)
FPR and Crank Sensor were changed last year
OHM checked fuel injectors
Got some seafoam to try out
Knocked on Cat and didnt hear rattling.

I want to test the coil packs by doing an OHM test. Can anyone provide detailed step by step instructions how to do this? I also read it could be a bad O2 sensor but wouldnt that throw its own code? Or how can I test the O2 sensor myself? Any other ideas? The code hasn't come back yet (been 1 day) but a few times I picked it up as a pending code and one time the service light started flashing for a few seconds and went out so I am assuming the problem hasn't been fixed yet. 2000 intrigue 3.5L

Does this happen shortly after you get gas?

Does your trunk leak?

Both of those are addressing a possible burst of water in your fuel...I think Ive had this code once, and only once, cleared it and never came back.

An 02 sensor can get sluggish long before it ever throws a code, but that usually manifests itself in bad gas mileage...under moderate to heavy engine load the PCM ignores 02 sensor input if Im not mistaken.

Just seems stange it only does it after a rain, and in my case I think I thought about it long enough and realized I had just gotten gas.

Thanks for the reply. I have never noticed the trunk getting wet at all but will double check and post back if it is wet. I made sure to check for holes around the trunk when I was checking stuff out. As for the gas, I had gotten some the previous day (only about 5 gallons after the gas light came on) and probably drove around at least 3 hours before the problem started happening so I don't think it would have been bad gas.

I was thinking maybe since I do get the small leak code every once in a while that maybe some water gets into that??? Could that cause the misfire if that were happening?

Random misfires on random cylinders -

Air - Bad Air filter, MAF,IAT, upstream H02 sensors, dirty throttle body, or unmetered air leaks(vacuum leaks).
Fuel - Bad fuel, Bad fuel filter, or fuel pressure
Ignition Bad Cam, or crank sensors.
Exhaust - Plugged catalytic converters


Tests -
1) Fuel pressure test is for checking for bad fuel pressure.
2) Smoke Test is for checking for unmetered air (vacuum leaks) leaks.
3) Converter back pressure test, can cause random misfires & choke an engine out.

Faulty MAF can cause your symptoms. Cleaning helps, but sometimes it is beyond that. Below is an example where it was beyond cleaning, and needed replacement.

In that case, random misfire was occurring, and performance above 4000rpm was greatly reduced.

You can test the difference in driving/performance by unhooking the MAF, and going for test drive. You'll get another CEL code, but at least you can see if the MAF is sending bad signals creating performance changes before and after.

**of course, may be another issue, just thought I'd share this aspect!

https://lh4.googleusercontent.com/_Kkixd42mCfk/TXOhgTSPmTI/AAAAAAAAdWU/5Qt_bGo55kE/s800/IMG_2803.JPG