I get a P301 code after a heavy rain. Once it drys out, no code.
All other times no codes

sanity check, bad coil pack on cyl 1?

How hard is it to change it out. Everything looks a bit tight on the 996. I have done them on my Boxster and it was an easy DIY

 

Definitely sounds like you have an aged coil pack that has developed some cracks and is not watertight. I would suspect they should all be looked at if they are the same age.

 

Going through this right now. Turns out my coolant res was leaking down onto the coil causing a misfire in cylinder 1. Pulled out the coil and swapped it out with another. Yup was definitely the coil. You could see small hair line cracks. I didn't change it but my brother did. He did it in about 45 minutes and that was with the car on the ground with no jack and the car has been lowered. A little tight but doable. With a jack it's a 30 minute job.

 

Had this just a month ago. I was getting misfires on three cylinders though. It had gone from freezing temperatures to a lot of rain really quickly. Started the car up and right away it felt like it was running on at least four or less cylinders. After I **** my pants and calmed down I figured out it was probably coils and plugs. The coil packs were not super hard to get to, but enough of a pain in the butt. I decided on removing the mufflers for easier access to the coils and plugs. At least for me trying to get my hands in there getting the exhaust out of the way was a huge help.

 

Yup, definitely sounds like a cracked coil pack or possibly a cracked spark plug.

Some of the coils and plugs are pretty easy to get to, a couple of them are a real bugger especially if you have big hands. IMHO, it's faster in the long run to pull the mufflers rather than trying to work around them. Plus have plenty of different size extensions and flex joints, and a ratcheting bit driver for loosening the coil pack screws. Lots of threads on how to do the job.

 

by the way... this is what I mean by ratcheting bit driver. Very helpful for undoing some of the coil packs, especially on the later cars with Torx instead of hex bolts.

 

Thanks for all the advise. Confirms what I thought
Scared the crap out of me the first time it happened. Flashing CEL and very rough idle

I have the same HF ratcheting driver

 

Swap the coil out with another cylinder and see if the problem follows the coil and go from there. This way you'll know if it's a coil or a wiring or connector issue.

 

If it was a constant or frequent codes, I would, but since it has happened only twice and both times after unusually heavy rain, swapping won't tell me anything.
San Diego does not get a lot of rain. This winter is unusually wet

Besides, once I have it out, its just easier to replace it than swap things and eventually replace it any way

 

Do the two little O-ring seals on the plug well tube, and clean the hole as well. Only adds a few bucks, and you're already in there. May have water ingress into the well.