TheMurse

First for some background:

2006 997.1 C4 MT, 114xxx miles, clean borescope in 2020 with PPI, none since. The car has the larger, non-replaceable IMS bearing.

I noticed an inconsistent, rough idle on cold start yesterday which continued until the cat warm-up phase and appeared to smooth completely after this first 90 seconds. In fact, the car's idle dropped to it's traditional 700 +/- RPM before I could grab the Foxwell scanner off the workbench. I was mindful to note there was no smoke on start-up and no soot on the tailpipes (which I'd just polished a week prior, making inspection easy). I took the car for a short drive and had no sluggishness or hesitation, no loss of power and kept her under 3000 RPM until reaching temperature, as per usual. Made a run up through the gears and into higher RPM and the engine was smooth, strong, and gave no sign of it's earlier hunting idle.

Just before pulling into the garage the CEL came on and the Foxwell came back out: P0300 Random/Multiple Cylinder Misfire. I cleared the code and drove the car once more later in the afternoon with no codes, no rough idle, no signs or symptoms of any kind.

Start-up this morning and the process repeated but thist time the CEL came on immediately before the motor dropped form 1000-1100 RPM down to it's normal 700 +/- RPM idle. The roughness at idle in that first 60 seconds or so was notable, the car sounded sluggish and the vibration in the cabin was odd. This time the P0300 code was accompanied by misfire codes for all cylinders with the exception of #5.

So my question for the RL brain trust is this: where to start? I've read/researched quite a lot and I'm not sure if I should attack plugs/coils first or look elsewhere like MAF sensor, vacuum system, etc...

Thanks in advance!

Graufuchs

When was the last time the plugs/coil-packs were serviced?

MAF Cleaned?

TheMurse

@Graufuchs Plugs/coils replaced at 60k miles by original owner, service carried out at the dealer that sold him the car, to my knowledge they've not been serviced since. MAF has not been cleaned in my ownership (3+ years).

CAVU

How long ago in time/miles was the last fill-up?

Any work/cleaning done in the engine compartment recently?

Graufuchs

Its time for plugs and coil packs with your mileage (Every 60K), I would also clean the MAF.

CRC MAF Cleaner -
Coil packs/Plugs - FCP Euro - HERE (Coupon Code - "FIVENOW" 5%OFF)

TheMurse

@CAVU I'm right now at about a quarter tank, last fill-up was 4/28.

Nothing done in the engine compartment since oil/filter change 2/11.

TheMurse

@Graufuchs Plugs/coils are on the to-do list as I approach 120k. I've read a number of threads on this issue where drivers have changed plugs/coils and CEL codes still pop up. Crankshaft position sensor, camshaft position sensors, variocam solenoids, AOS, there are a number of other potential culprits to chase down since the misfires are happening on both banks.

Graufuchs

Do the easiest thing first, clean the MAF. Then move from there.

Petza914

I do plugs every 30k and coils every 60k. You're definitely due for those. Bad crankshaft position sensor and the car wouldn't start or run. Bad camshaft position sensor and the misfires would be on the bank with the bad sensor. Since you're having them on multiple cylinders you need to look at things common to both banks. The MAF is a possibility as is a vacuum leak. Smoke test the intake. When was the AOS replaced? If never, at 114k miles, that should be on the list too.

TheMurse

Thanks, @Petza914 , MAF/vacuum is my inclination as well. Does the fact that the condition is only present during the enrichment cycle at start-up give any clue as to the underlying cause?

AOS and water pump are on my summer project list.

Petza914

It's a little odd. Often problems show up right after the cold start protocol completes and the car switches from open loop to closed loop and starts using all the sensors, like camshaft position, etc. Yours is doing the opposite so the baseline setttings it should be using without the additional input from from sensors are causing an issue. Since you say the idle is elevated, the coolant temp sensor is working so it knows you're doing a cold start. If you use a scan tool during the cold start, what do the MAF readings look like for intake air temperature and MAF values. Is the temp close to what you think the outside temp is and do the MAF readings look like they make sense as the idle is high (they'll be higher) and then do they go to a lower number as the idle comes down?

I'm still thinking vacuum leak or MAF. Have you installed anything like an IPD plenum. The boots that go from the T ends of the plenum to the intake manifold are very tight and often don't get put on quite right and come off. Have you cleaned your throttle body plate, especially at the edges where the plate closes against the housing. Manually push the plate open and clean with carb cleaner of MAF cleaner on a rag. Clean both sides of the plate and the inside of the housing especially right behind the inlet lip. How much oil do you see sitting in the intake pipe when you have the throttle plate open and shine a light in there - just a little is normal - if there's a pool of it, that's not as good and you might need an AOS.

TheMurse

@Petza914 I too thought it was odd that the rough idle isn't repeated under any other conditions and the engine runs strong without hiccups, excess oil consumption, or other symptoms.

I'm running an aftermarket cold air elbow with helmholtz delete but factory throttle body/intake plenum. I cleaned the throttle body plate when I installed the elbow in June of 2020 and I've not inspected it since. I'll pick up some MAF cleaner and apply it to both the MAF and the throttle plate as well as take a look at the intake pipe.

Graufuchs

I am running those Silicone Elbows on both cars also, they are fine since they maintain the same distance from the MAF to the T-body. A bit better airflow and some additional noise. Clean the MAF, hopefully its a quick fix for you!

NevilleNobody

Mine is a .2 but had the exact same issue and CEL. dealer diagnosed as oxygen sensors, turned out to be the MAF.

larrytrk

Gas cap check, or oil cap anyone?