(Weird) P0301 Check Engine Light Issue

Okay, so I pulled the car out of the garage to give it a thorough washing today. When I was done, I started it to move it back into the garage and it struggled to idle smoothly, eventually throwing a check engine light. I pulled the car into the garage, turned it off quickly, and took out my OBDII reader which showed one code: P0301 - misfire in the 1st cylinder. I know the problem could be more serious but my first thought was that it could just be the coil. I replaced the spark plugs relatively recently but can't remember ever changing out the coils. Btw, it's a 2004 C4S Cab with 48,000 miles on it. I went inside pissed, ate some dinner, and then it hit me. The same thing happened a year ago, the last time I gave it a thorough wash. It hasn't gotten much use this year so I've mostly just used a waterless wash and detailer to remove some of the dust it's picked up. Anyway, when it happened last time, I put it back on the battery tender, went inside, again pissed, and researched the issue. When I went outside the next day, I started it again to start troubleshooting -- no more check engine light. The problem was gone. Gone! Cars don't fix themselves but I could not replicate the issue. It's run smoothly the entire year...until today. It was in for routine maintenance late last fall and the tech didn't find any problems. I even had it out for a 2 hr drive yesterday (~100 miles) without any issues. So, out of morbid curiosity, I just went outside a few minutes ago (about 5hrs after I parked it) and started it again...just to see...because there's no way it could really be the same thing, right? Again, no problem. Smooth idle. I cleared the light, checked it again, restarted it a few times to see if I could do something to cause the light to recur. Nothing. No new code, no light, no rough idle. Any thoughts on what could be going on? A cracked coil? Could something be getting wet and fouling it up? Seems odd. I'm open to any suggestions. -Eric

Most likely a cracked ignition coil. If your coils are as old as the car (original), then it�d be wise to change them all out at your earliest convenience. I had a similar issue, a temporary P0304 with noticeable vibration that stopped after I restarted my car, the morning after I drove in the rain. I drove in the rain some more afterwards, but didn�t notice any issues. I replaced all my coils about 6 months later to find that they were all cracked.

Yeah, it's a common wear item. Usually it was triggered on my car from a car wash or rain storm. Looking at my records, since 2002, one or more can go at any time. For the first 10 years I would just replace the cracked ones. Over the last 12 years I replaced all of them twice as preventative maintenence. Never had a problem since I went to that routine. Always good to have a durametric in your possession to run a diagnostic as a JIC.

After a bath I always take my '04 C4s cab on a nice run.... at least to rub off any parasitic rust molecules on the rotors

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Thanks, guys. It�s definitely useful feedback and I�m glad to hear it could just be a cracked coil. I�ve owned the car for about 14yrs but I can�t think of any time I�ve replaced them in the past 5-10yrs probably. Then again, I only average about 2-3k miles a year these days, at most, especially now that I�m working from home (after Covid). I used to commute with it. On most cars, I know it�s pretty straightforward maintenance. I haven�t really worked on this car�s engine before. How difficult of a job is it without a lift? Thanks � good advice under other circumstances. There was no way I was driving it with the way it was running though. I wasn�t sure I�d be able to pull it safely into the garage let alone drive it down the road. -Eric

I think it�s a pain in the butt to remove the ignition coils, but that�s only because a couple of them are blocked by the mufflers. You�ll have to fit your hand into tight spaces and have a variety of tools that are the proper length/height to remove the coils without removing the mufflers . It is made much easier if you remove the mufflers, but I opted not to do that. Check out this TSB about the ignition coils. https://www.pca.org/sites/default/fi...tion_Coils.pdf If you purchase new ignition coils, you�ll likely receive the updated 997 versions. These are taller, so you�ll have to purchase some washers/spacers for the heat shields to accommodate. You can purchase the bolts too, but I�ve read of others repurposing the bolts from the old coils instead. Once the coils are off, you can change the spark plugs. That�s a much simpler job. If you haven�t changed your plugs recently, I�d do that too. Otherwise, if the coils aren�t constantly acting up, I would wait until you change the spark plugs to address the coils. It�ll give you some time to collect the parts and tools necessary. For the new coils, you�ll need an E-Torx to tighten the bolts. I used a regular E-Torx wrench to tighten them down, but it was tedious. In hindsight, I would buy a ratcheting E-Torx wrench to speed up the process. Something like this: https://www.homedepot.com/p/GEARWREN...224D/204616985 edit: an E-torx wrench is only necessary if you don�t remove the mufflers. There isn�t much room for a socket & ratchet unless they�re very low-profile.

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Thanks so much for the thorough write up! Plugs are pretty new, unfortunately(?). It would be a waste to swap them out any time soon. I�ve got a couple of other projects I�d like to tackle before getting into the car that much. I�m used to working in tight spaces. I�ve got a Saab Sonett that I do all of the work on. It�s still not fun. lol I might end up waiting, like you suggested, and pray it can wait�or just pay the Porsche tax and have it done.

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I reused the old coil bolts for the heat shields without issue when I replaced my coils with the 997 ones. When I did my plugs and coils I removed the mufflers, which can be a bit of a pain if they've never been removed before. One tip I have is to pull on the wires to make sure they're completely clicked in. That will save you from having to go back in after you've buttoned everything up. (Ask me how I know). If you're not comfortable doing the work yourself it's probably not too expensive to have them replaced by an independent Porsche specialist.