Geeforce

Hi all,
I recently purchased a 2005 c2 with 117k miles on it. Car was well documented and serviced at the dealer every 6k miles for its entire life. On original IMS but the oil/filter and lab analysis came back clean of any debris or metals. There are no strange noises present either.
The spark plugs and some coils were replaced 57 k miles ago. The AOS was replaced 15 k miles ago.

I notice that the engine is not smooth at idle, kind of lumpy. It shakes every 1.5 seconds and I feel the car surge slightly under throttle in the same timing. I connected the durametric and it shows no faults or codes.
I have no cels.

I tried cleaning the MAF and also replaced the sagging motor mounts.

Any ideas on how to diagnose the problem?

JW911

That's a tough one without any codes. That is going to be the go-to place for any technician to check. How about changing those spark plugs? With 57K miles on them they are about due. If the coils were bad you would be getting a code. But not a bad idea to replace those now too.

USMC_DS1

My guess would be worn coils/plugs or a borderline oxygen sensor. FWIW... spraying some water under the car directly at the engine area where the coils are located. Use this as a troubleshooting method to eliminate cracked coils. BTW, I would not recommend this other than for troubleshoot. If you have cracked coils the CEL codes will likely light up afterwards. One of our members posted a similar issue recently... "engine starts running very rough" and it was related to bad coils. https://rennlist.com/forums/997-foru...ine-light.html

Also recheck the MAF connector since you serviced that recently. GL and let us know what you find.

Geeforce

Thanks for the advice. I see in the service history that one or two coils were replaced when the owner complained of a CEL. This means that there are 4 coils that were never replaced on a 117K mile car.
I'll replace the plugs and all coils and see what happens.
Anyone know the best place to purchase these? Pelican prices seem reasonable.
Thanks

USMC_DS1

I used pelican for both the plugs and coils.

Spiffyjiff

agree - plugs, coils, are the best bet without any other evidence/codes.

and can't the OP disconnect the MAF to see if the idle smooths out? if no change to rough idle, at least this will narrow down the culprits.

PelicanParts.com

Geeforce, if you choose to do the plugs and remaining coils you might want to check out our DIY tech article to get you through the process. You can review the info here:

Replacing Spark Plugs and Coils on Your 997

Let us know if you have any questions.

Mark/Pelican Parts

Geeforce

Update**
the CEL came on and along with it are fault codes finally so now I have a hint as to whats causing the rough idle.

P1090 HFM electrical fault
P0300 Misfiring checksum error
P0301 misfire cyl 1
P0306 misfire cyl6
P0302 misfire cyl2
P0305misfire cyl5
P0303 misfire cyl3

Odd that cyl 4 is not misfiring

I am assuming the main P1090 code is whats causing the cylinders to misfire.
Anyone know what the culprit is here?

Thanks

Spiffyjiff

the usual - MAF. might be a loose (ground/harness) wire or you may need to clean it.

P1090
Hot-film mass air flow meter – above/below limit value

Diagnostic conditions
Supply voltage 10 V ... 16 V
Engine start

Possible fault causes
-Open circuit, short circuit to B+ or short circuit to ground in sensor line to hot-film mass air flow meter
-Open circuit in power supply line (12V), ground supply line or sensor power supply line (5V) to the hot-film mass air flow meter
-Hot-film mass air flow meter faulty
-DME control module faulty

Function description

Diagnosis of plausibility of MAF sensor signal
-The measured value supplied by the MAF sensor is compared to a value from a map. This map is determined by engine speed, throttle valve position, ambient pressure and intake air temperature.
-The mass air flow upstream of the throttle valve is identified as implausible if the measured value deviates too greatly from the map.
-If the switching cups stick on a small stroke on one or both banks (e.g., oil duct blocked), the MAF sensor can also be detected as faulty. This is because the expected increase in the air-flow rate does not occur.
-The monitoring of electrical faults (open circuit, short circuit) takes place in a separate diagnosis.