Nate Tempest

So, here's the story. After getting previous knock issues sorted out, car's been running great for several months. Then a couple weeks ago under boost I suddenly got a significant, continuous misfire. CEL came on, and code showed #6 misfire. Afterward it was occasionally misfiring even at idle, and misfire counter showed #6.

All six plugs and coils were replaced 11 months ago, and are nearly new. Still, this obviously pointed to a bad #6 plug or coil, so indy shop replaced those. The serious misfire (which happened every time at full boost, so was fully repeatable) went away, and car seemed to test drive fine, including under boost. The next day though, I had a chance to go for a run and found it was stuttering under boost, but usually only at higher load, like a 3rd gear pull. This is much less severe and less repeatable, but there's definitely something going on. The mechanic rode with me and agreed that it feels like a misfire. Problem is, there are now zero codes, and data logging with AP shows nothing out of the ordinary. (No significant knock, not fuel starvation, etc. Even ran the log by Sam, and he said it looks fine.)

So, what now? The mechanic is hesitant to do anything without a code to go on, which I can understand considering how hard it is to get at anything. Logic is telling me that the most likely culprit is the thing that just changed—the new #6 plug and coil he just put in. Also, he unfortunately used a different model of plug than the others that were in there, which seems to me like it could cause problems. I had Denso plug PK20PR P8 at the recommendation of Sam @bydesign, and he put in the stock Porsche plug from Bosche. (Although he did gap it the same—25 thousandths.) So, my inclination is to replace the #6 plug again (and coil while in there, although I think the problem is probably the plug), but to obviously use the same plug as the other cylinders this time. #6 is a bit easier to get to than the other plugs, being at the back, so that should "only" be a few hours labor.

Alternatively I could 1) replace all the plugs and coils, or 2) do nothing, drive the car more, and hope it eventually throws a code. (Although I've given it a decent thrashing already, felt plenty of this stuttering, and got nothing. So I don't know if more driving will make any difference.)

What would you do?

o7silverturbo

You need to figure out what is going on and not just throw parts at it.

If your indy has a Porsche level scanner should be able to drive the car with the scanner hooked up and see what cylinder is misfiring even tho there are not codes.

Once you know what cylinder it is, then I would start by moving the plug and coil to a different cylinder and see if it moves. Its more work to do it this way but then you are not throwing parts at it.

Nate Tempest

He's tried hooking up a scanner to it. Doesn't show any misfires, but we can both feel a misfire-like stutter that shouldn't be there.

I'm leaning toward changing out the plug to match the ones in the other cylinders since this problem started when that plug was changed, and it's known that mixing plug types can cause problems.

atcbi5

cstyles

Nate, sorry to hear you're still dealing with issues. You've been dealing with things like this for a while now. I imagine you just want to be able to enjoy your car stress free, hopefully that is coming sooner than later.

Are you tuned with the Cobb? If so, custom or OTS map? If I can offer some unsolicited advice, that may or may not have any positive effect on your situation, I would suggest selling the Cobb AP and using the funds to engage with Kevin from UMW. That's exactly what I did and am so glad I did it.

The real value from Kevin isn't his custom tune for your car (which is exceptional by the way, track verified) - it's his knowledge, advice and support you get once you've engaged him. Kevin isn't just a tuner, he is IMO the most knowledgeable expert on late model Porsche turbo platforms in general. He helped me diagnose and deal with some things remotely (albeit not big issues like yours), using his experience, knowledge and data logs I sent him. It might be worth your while considering the $ you gain from selling the Cobb will completely cover the costs to engage with Kevin. I would think he could offer you some support on tough issues like what you're dealing with currently.

Hope it all gets sorted for you and you get to start enjoying the driving season with no more issues. Cheers

Kevin

If you want send to please send me your logs kevin@ultimatemotorwerks.com

Do you have aftermarket motormounts installed? Is your car a 6 speed or Tippy

Kayaknfly

Nate Tempest

Yeah, the car was actually finally sorted out and running well for a few months (through the winter, unfortunately), then this. It does feel like it's been one thing after another almost since I got it though! Hopefully this is the last one.

I do have a Cobb with a custom tune from Sam @ By Design. I've been happy with his service, but there's only so much he can do remotely when the logs don't show an obvious problem. He has said that mixing plugs could certainly cause problems, so I'm going to have them switch out the plug they put in for the same type as the other cylinders. Won't be *too* bad since #6 is easier to reach (relatively speaking!) (And if that does solve the problem they'll do it under warranty.)

If that doesn't fix it I'll try taking some more logs; hopefully something shows up that didn't before.

Nate Tempest

Changing the plug appears to have done it! Car's running great now. So, don't mix spark plugs!

cstyles

Damn dude, don't jinx it!

Good luck man, hopefully it's smooth asphalt from here out