asneiman

I purchased a 2011 C4S a few months back, it had 44,500 miles on it. Knowing the spark plugs haven't been changed, I had a local Porsche Independent change them for me. Since then, I've noticed a nasty hard idle on startup when the car has been sitting overnight. It bounces between 600 RPM - 1400 RPM, and sounds like the motor might die once or twice. Once the choke turns off it's much smoother.

I took it back to the shop, and the checked and there weren't any errors in the computer. They also didn't witness this, because the car wasn't cold.

Does anyone have any suggestions of what this could be? If a plug wasn't gapped correctly, wouldn't there be missfires and codes logged?

Jaws1

Sounds like a throttle body issue. You can start by cleaning the TB and MAF. Spray them down and wipe out the TB. Many just replace the TB because there's an inexpensive Bosch unit available which is the same as the Porsche part.

snaphappy

I don’t like that this started after you got new plugs. Hopefully it’s just coincidence. PS - a new throttle body is like $120 and takes 10 minutes to install, super easy DIY

Ironman88

Coil pack(s)?? One or more damaged during the plug change?

Petza914

I'm more familiar with the 997.1 than the .2 bit of it only happens on cold start and then goes away when the car warms up and then it drives fine,I'd be looking at a sensor that's part of the cold start protocol. On the .1, the intake air temp sensor is integrated into the MAF, there is also temp sensor info supplied to the DME by the coolant temp sensor, and I believe on the . 2 there's a secondary air pump to make the cats heat up quickly unlike on the .1 where the DME enriches the mixture to perform that function.

If then car also runs rough or the idle hunts even after it's warmed up, then it's something else like a throttle body, plug, coil pack, etc.

swingwing

There's no secondary air pump on the 997.2 engine. This from page 57 of the 2009 Technik Introduction:
"In combination with the special DFI catalytic-converter heating phase, it is also so efficient that the secondary air injection that was previously used can be omitted for the new models."

Petza914

Thanks. I knew the .2 did it differently than the .1, just wasn't clear on the details.

swingwing

There's no choke on these engines. On cold start the DME adds extra fuel pulses to heat the cats up quicker. This cycle lasts about 30 seconds. The spark plugs come pregapped so there's no adjustment. If you had misfires you'll get a CEL and codes. I wouldn't throw any parts at it. Cleaning the throttle body and MAF sensor are fairly easy, but make sure you use specific cleaning product on the MAF sensor. You'll see it on the shelf at auto parts store. While you're in there, consider replacing the air filters if necessary. Use top-tier fuel and an injector cleaner additive.

asneiman

The car is back at the shop. This way they can hook it up to their system in the morning during a cold start.

I used an app last night to see if there were any codes, and I have a:

P0302 "Pending fault present during this driving cycle" OBDII Cylinder 2 - misfire detected.

I haven't seen a CEL yet, but figure something is going on with the plug, or coil for cylinder 2.

millertimefl

This is probably a sign that the HPFP is starting to fail. Same thing happened to my 2010 C4S. It was pretty random but one day it wouldn't even stay running.
If you end up changing it have them also do the water pump (just pump not housing...less $$) at the same time...they are next to each other and coolant needs to be drained for both jobs.

swingwing

I don't think HPFP is the culprit here. If it were failing the engine goes into limp mode with corresponding CEL and codes. OP says it's only on cold start he's seeing an issue. For what it's worth my '09 C2 is at 44k miles with no issues. She spins up nicely as a matter of fact.

Vincent713

My 2009 C2 cold starts a little rough at first and the RPM jumps between 9-11k for less than a minute then idles smooth from 8-9k. Runs fine and no CEL. I did recently clean the throttle body and replace the O ring inside.