C2-996

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.

P9C

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.

Fishah

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.

Fishah

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!

Fishah

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!

C2-996

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

Eurocarguy911

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,

Fishah

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.

bantamben

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

Fishah

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.

bantamben

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

Fishah

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.

Hal

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

Beru 14 FGR-6 KQU
https://www.ebay.com/itm/111857259506

NGK BKR6EKUB (7969)

FYI, the Porsche branded plug was $24/ea.

Hope this helps.

wildbilly32

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.

Fishah

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.