CEL after sitting; P0300, P0301, PO305

Hey guys! I did search a bit here but I'm feeling slightly lost. Any thoughts would be appreciated. Took the car for a spirited drive and filled it up with gas about four days ago. Parked it in the garage; no issues. Started it up yesterday, backed it out of the garage, and let it idle for a minute while I ran inside to grab something. When I came back, the CEL was on solid. Code says random misfire cylinder 1 and 5. I drove the car around and it didn't clear but no symptoms. I cleared the codes and drove for another hour and it didn't come back. Car is a 2004 AE with 111k miles. Any idea what could cause two cylinders to misfire randomly after being cold? I could not reproduce it on a hot engine. Plus are reasonably new from the previous owner. I did do a fuel treatment that I bought from LN when I changed the oil about 500 miles ago. Thanks everyone!!

Was that a one time incident or is it misfiring during every cold start? ps. In this case 1 and 5 and relevant because they are consecutive cylinders in firing order so they are not necessarily just random so it's probable - although not absolutely - that something happened just before or during that time when 5 and then 1 was firing.

You say plugs are newish, but what about coils? They crack and can be a primary cause of random misfires.

If it�s misfire codes the first thing I�d recommend is coils. I chased a whole bunch of other stuff because my coils only had about 15k mi. on them... big mistake - wasted lots of time (almost all summer last year) and a fair amount of $$ (thank god I did the work myself)... Do yourself a favor and change them out!

Since it's a cold engine misfiring how'bout intake manifold leak or maybe a minor vacuum leak?

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Could be - mine didn�t misfire til hot...Vacuum leaks shouldn�t be too difficult to track down. You can first try with ether/starting fluid - let car idle and spray around suspected areas of leak, if rpms increase you�ve found your leak. If it�s deeper toward the front of the engine you may have to smoke test...

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Thanks for the replies everyone! It did not happen the following day when I did a cold start. So far just a one time incident. What type of event could cause what you're describing? Something fuel related? A voltage drop? The A/C was running if that makes a different. Just checked my records. Plugs and Coils are less than 2k miles old. P.O used a Beru kit: https://www.fcpeuro.com/products/por...ru-996ignclkt1 Do we have any experience with Beru coils? Just wondering what the odds that two new coils on the same bank randomly misfired in succession. Would there be any other symptoms associated with this? I wish I was as confident as @dporto lol

Beru are standard OEM replacement coils. It�s certainly possible that two out of six are defective - but unlikely... I�d start with the vacuum leak route

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Ok, fun. Thanks! I appreciate your input.

Come to think of it though... If it idles without stumbling, the chances of a vacuum leak are slim to none...

Don't start throwing parts at it hoping it will fix it.

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If it doesn't repeat, other than the smoke test, it will be hard to diagnose, and personally I would have a hard time digging into swapping coils until it repeats. But here goes my thoughts: If you can, you can do a smoke test for intake/vacuum leaks. It's a bit of a pain, but you could also swap 1,5 coils with others (marking the suspect ones), and drive for a while to see if the problem repeats and follows them. I might pull and double check that the plugs look okay while you're there. Coil 5 will be the harder of the two, and so in for a penny in for a pound. Also, check the engine ground point! There is only one behind the rear passenger wheel just above the coil heat shield. It's easy to access, and could be worth removing, cleaning the contact points, and reinstalling. Many people have chased intermittent coil issues for a long time, only to discover a bad engine ground. Next would be a fuel issue I think. Others would have to weigh in on trouble troubleshooting that.

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I feel attacked.

I bought a 997 from a good friend. He had had it serviced at a very experienced shop with the owner having been of the crew for a factory 962. That shop put new plugs in the car and told my friend he would never have to change them. A few years later in my ownership, your CEL’s started popping up. First thought was coil packs. Replaced them. CEL’s came back. Finally pulled the non-standard plugs and replaced with Porsche approved plugs. CEL’s went away and never returned. YMMV

Thanks again guys. The issue did happen again tonight. I heard it stumble once, just briefly, then recovered. Again it was a totally cold engine, idling for about 30 seconds after start when it happened. Again gas tank completely full. Once it stumbles it recovers and idles fine. Codes say Cyl 1 & 5 again. I'll definitely check the ground and swapping coils seems like the next best idea. Could this be related to outside temperature? I live in the desert but both times this happened it was cooler weather than we've been having all summer. Like, could a battery weakened by the cold cause this? - I'll be honest chasing vacuum leaks sounds scary to me.

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Definitely agree with those above to be sure there aren�t any vacuum leaks. If you�re concerned with the charging system at all- try to first get the battery on a tender over the evening, and if you see any weird voltage spikes or drops you could have a voltage regulator issue (attached to back of alternator) it�s no an uncommon item as these cars rack up some mileage/age. I�d absolutely check all ground points and vacuum leaks first however.

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Did both of these incidents happen after you had filled the gas tank - next morning etc? Ok, this is far fetched theory and idea but "how full"? Did you fill it up past the first "click" to the point that it "overflows" or just until the first click from the handle? As far as I know cars should newer be filled "completely" full so that you continue to fill until the it leaks from the filling hole because the tank needs a little bit empty air so that the EVAP system works properly.

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Got it. So check for bad ground, potential voltage drops, and vacuum leaks. Also inspect plugs and swap coils. 'm heading out of town so hopefully can dig into it next week. The tank was full, but not overfull. I don't fill past the click, and it was about ten miles from the gas station to my house. I did mention the full tank just in case anyone thought that might be related though.

Hi everyone! Just wanted to add a couple data points in case this triggers a light bulb for anyone. I confirmed that any time the car does a cold start, it stumbles and then revs to compensate. Once the revs come back down, the car runs great. No issues with hot starts or running hot. Codes show CYL 1, 3 and 5 all missed. The car stumbled HARD and almost wants to die if I remove the oil cap while running. Is this a clue? A sign of something else? The car has a new K&N filter from the previous owner. I just removed that and replaced with stock. I have not yet had time to do the other tests that we discussed. I do have a Durametric now though. Not sure if that will be handy in diagnosis here? Thanks again all!

Just an update: - I took the car to a reputable local shop last Tuesday to sort this out for me. - They decided the spark plugs are to blame, even though they were new. Swapped them for me and they've driven it for a week with no issues. I collect the car on Thursday as they're gonna do a Window Regular while they have it. Fingers crossed we are solved!

Great news! Keep an eye on everything but glad to hear it was a simple problem to resolve

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This is encouraging. I have a very similar problem, misfire on cylinder 1. I've swapped coil packs between one and two. However, the Beru coil packs have 6K miles on them. The spark plugs are brand new with less than 200 miles on it. The plan is to swap those out this weekend. We'll see. Best,

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Thanks. Best of luck with your issue as well. He did say that his initial thought was vacuum leak until he saw that the misfires were on both banks at which point he ruled it out almost immediately.

What plugs did you have and what did you switch too. I am having a similar issue misfires at idle only. I too have new plugs I think the ngk R plug one with blue writing

Funny that you just replied to this. I was coming here to do the same. Sadly my light came back today. Same codes. Plugs weren't the problem. I did test the battery and it was meh.... So I replaced that. We'll see.... Otherwise it'll go back to the shop.

Actually the plugs I have are the green writing with the four prongs

Hi everyone. The replacement battery did not solve the problem. Still getting a miss on 1, 5 and sometimes 3. The car actually starts, runs and drives fantastic. No notable stumble at all... But still get the CEL. I still kind think cold starts are relevant as the light almost always appears if I start the car cold and leave it idling in my driveway. Only once has it appeared on a hot engine. I took it for a One Hour drive last night, with plenty of runs to redline, with the Durametric connected. During the entire trip I got one misfire on cyl 5, and one misfire on cyl 1. Both times in the 3-5k RPM range just cruising. I ruled out: - Plugs - Physically damaged coils or bad connection on coils (maybe internal damage on brand new coils?) - Voltage problems The shop I work with (Stuttgart SW in Phoenix) was very quick to rule out vacuum leaks since the misses were on both banks, and the car idles fine. I did not however do a smoke test. Engine ground looked pretty good. Very clean on one side, just a bit oily on the other. No corrosion. I've now gone through multiple tanks of gas, so we can rule that out too. Notes: - I owned the car for about a month before this happened. - I added a bottle of Driven fuel additive just before this happened. So what do you guys think? I trust this shop, but it sucks being without my car for days and days at a time and even the shop seems a bit lost. Do you think this could be a fuel/injector problem? Thanks again guys.

Don't want to be repetitive, but as I said earlier in the thread. I had the exact same issue. Changed to a Porsche approved spark plug and problems went away. I no longer have a 996, thus no access to an owners manual. I would start by looking in it for what is listed as Approved. Short of that, I went to a large, well respected vendor. I looked up plugs for the 996 (normally aspirated). In the title for a Porsche branded plug, they also listed three alternatives. Here is what I found: Bosch FGR-6-KQE Amazon Amazon Beru 14 FGR-6 KQU https://www.ebay.com/itm/111857259506 NGK BKR6EKUB (7969) Amazon Amazon FYI, the Porsche branded plug was $24/ea. Hope this helps.

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Don't wish to break bad on a shop I know nothing about but they seem to be "guessing" at the problem and costing you unnecessary $$. I.E.: Changing 2000 mile spark plugs out. Good luck with the problem.

Hi Hal, thanks for chiming in. The car did have Beru plugs in it that were only 2k miles old when this problem started. As a precaution, they were removed and replaced and unfortunately the problem did not change.

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Agreed. It's very frustrating. If I take the car back to them, we'll have a conversation about it. The shop is well respected as being the top (or one of) Porsche indy shops in Phoenix and the owner has been nothing but gracious. I do think they're trying but perhaps overwhelmed. In their defense, they road-tested the car for five days after swapping the plugs and the CEL did not return for them. The owner even took the car home with him one night to try to replicate a dead-cold start... No problems. They thought it was sorted... Until the light came back on at my house two days later.

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With all due respect, have you tried a Porsche approved plug? There is more to it than just a brand. Was the Beru a 14 FGR-6 KQU?

Hal, I appreciate your persistence. I double checked my notes. The plugs that were installed initially were actually Bosch FGR5KQE-0. Purchased from FCP by the P.O: https://www.fcpeuro.com/products/por...kt#description Pelican Parts does list this as the correct Bosch plug for my car? This does appear slightly different than your note above though. As far as the plugs that were used in replacement, I don't know the exact model. I can find out. I certainly am not willing to rule out anything but... Just thinking about probabilities here; even if the plugs were to blame, what are the odds that the exact same three cylinders have issues on two completely different sets of plugs? I'd love to be proven wrong, genuinely curious how this might work. Also just one additional note, this car does have an X51 package so I'm not sure how that might affect a plug recommendation.

X51 is a great motor, but I don't think it is a direct factor. It seems to me that you have an impedance issue. The system is expecting X and it is getting Y. That is why you need to research the correct, Porsche Approved plug for your motor. Not just brand, but exact model of the plug. Do you have the correct Owners Manual? The correct plug should be listed there. Otherwise, somewhere, someplace is a listing for the Porsche approved plug. Before you throw more money at other problems, make sure it is not something as simple as the plugs.

Hal, thank you. I do have the owner manual and unfortunately nothing is listed for plugs. The websites I've checked show the Bosch plug that I had/have, but I will continue to research. I do have some additional data points from further testing with my Durametric - On a dead cold start, I couldn't get my Durametric hooked up fast enough but when I finally did it captured the following misses: Cyl 1 - 10x Cyl 2 - 1x Cyl 3 - 1x Cyl 4 - 1x Cyl 5 - 8x Cyl 6 - 1x - Once the car is warmed up driving around, all six cylinders are about 99% perfect. However, Durametric will show random misfires on all six, maybe once every 8-10 minutes for each. Perhaps this is normal operation? ---- So as we discussed, Plugs (replaced twice) and Coils (Beru) are new and while I'm certainly not willing to completely rule them out, wondering what else might cause this? - Can a Vacuum leak cause misses on cylinders on opposite banks when cold only, but no further symptoms? - Does the fact that this happens badly to two cylinders when cold, but all six cylinders slightly, point to anything? - Doesn't this rule out any grounding issues since it's clearly temperature related?

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You must be doing something wrong.😜 Hope some of the forum folks with experience chime in with suggestions for you. It has got to be frustrating.

Suggest checking and cleaning your harness round connectors. Use some CRC followed by dielectric grease. Disconnect the battery neg cable first, to be ECU safe. Probably not the problem, but easy, quick and free to do. Also, as mentioned, the engine ground wire behind the pass wheel. On another vehicle i had a gremlin that turned out to be a corroded ground strap that looked fine but it wasn't.

These engines are not that sensitive when it comes to spark plugs. I use regular copper core Champions and don't have any misfire issues. They won't last as long as the exotic tipped ones but since I do all my repairs, I don't care. For lack of better terms, a plug is a plug as long as it's in the same heat range and same physical dimensions.

reviving an older thread here, but wonder if you got it figured out? My 03 turbo has a P0300 with misfire Cyl 6 and 3 P0306 and P0303. The plugs and coils were replaced about 3k miles ago/4 years. I do hear what could be the misfire, whenever the car has been under load and I put in in neutral and it idles. A little "vroom vroom" from the rear right side of the car. But cyl 3 and 6 is on both sides of the car right?