sbbarnett

2005 997 C2S, 63K miles. Recently purchased. Drove for several days no problems
Yesterday got CEL on cold start and OBDII with Random Multiple Cylinder Misfire Detected (P0300), and P0301, 2,3,4,5,6 for misfire in each cylinder. Reset it and cleaned MAF. Next start was fine, but that was warm. This morning cold start, same CEL and codes again, idle felt a little rough.

Suggestions on things to check?

shyamvenky

Did you have the ignition coils checked during a PPI or after your misfires? Cracked coils are notorious for this..

sbbarnett

Yes - coils were checked, and deemed good.

shyamvenky

What about the spark plugs? On my old 997 I got rid of the random misfires with new coils.. but my Tech said plugs and coils could be culprits (along with the MAF).

inspectX

@sbbarnett: Yes, spark plugs could be the culprit. A few P-cars I've owned in the past threw the same "random misfire" codes that you're describing. A major service (spark plugs, filters, all fluids replaced) did the trick.

sober_owl

probably coils anyways.. same thing happen when i bought mines..coils where cracked

JABowders

Coil Packs and Spark Plugs ...

Petza914

With misfits on all cylinders, I don't think its coils. If the plugs are all really old, that's possible but to me, also unlikely. Could be a bad MAF or a vacuum leak somewhere before the pkenum since its happening on both banks. First thing g to try is to unplug the MAF at the connector. This will put the car into limp mode forcing it to stop metering air and use middle of the road values for the fuel trims. If thus improves the way the car is running, then its either the MAF or a vacuum leak. If it doesn't change the way its running, then it's probably plugs and coils. A mess likely possibility is s failed coolant temperature sensor. The DME uses input from this as part of its fuel regulation and since you said it didn't happen until the next cold start, could be this, however, the primary temp sensor is integrated into the MAF.

Stock intake, stock exhaust?

sbbarnett

eddiek997

Im having exactly the same problem. Engine is rough when choke backing off to normal idle, CEL with code P0300.

New plugs and coils have not resolved. No vacuum leaks present. Installing new MAF today and will report if that resolves.

Note, i've been living with this for a year or two now and only had to deal with it because DMV inspection would fail due to certain flags not being in ready state.

semicycler

As other have said coil packs are notorious for cracking and causing misfires. But all cylinders at once hints at a bad crankshaft position sensor. Just one bank could be a camshaft position sensor on that side. Both cam position sensors going at once is possible but less likely.

Petza914

First off, we should stop talking about a "choke". The elevated idle when the car first starts is a enrichment mode to the fuel injection managed by the DME - these cars don't have a choke. That first start enrichment with the elevated idle is determined by the temperature sensor integrated into the MAF. As semicycler explains, for a misfire condition that effects all cylinders, you have to look at things common to both banks of the motor, like a vacuum leak on the downstream side of the MAF, the MAF itself, crankshaft position sensor, coolant temperature sensor (often overlooked but also reports to the DME), bad TPS (Throttle Position Sensor), and similar. Bad coil packs or plugs will cause misfires on only the cylinder(s) where they're bad, but extremely unlikely to go bad on all 6 at the same time. Camshaft Postion sensor - there's one for each bank so if you're getting misfires on 1, 2, & 3 but not 4, 5, or 6 or vice versa then this is the most likely cause. Also, probably not the issue in your case.

You can test the MAF, by unplugging it (leave it installed) and driving the car. This will put it into limp mode where instead of it metering air and adjusting the DME for it, it just uses, safe, middle-of-the-road values that it knows will allow the car to run. If it runs better with the MAF disconnected, then it's probably the MAF, if no difference, then it's probably something else in my first list.

Also, the cold-start protocol ignores input from certain sensors. When my Bank 2 Cam Position Sensor went bad, the car would run fine when first started for 2 minutes until it completed the enriched warm-up cycle, then it would start misfiring on 4, 5, & 6. I had to move it around the garage bays a few times and this scenario was completely repeatable - fine until moved then when the idle dropped, started misfiring. Hooking up my Durametric I was pretty confident in the diagnosis, because the CPS for Bank 2 wasn't providing any feedback to the DME - the #s were all 0s. Replacing it resolved my issue.



To diagnose this properly, you're going to need a way to track live values, like accelerator pedal position vs throttle plate position, MAF readings, Camshaft Deviation for both banks, crankshaft position, coolant temperature, intake air temperature, etc.

jad4ca

Thanks for the insight on this Pete. You saved me. I had the same issue on my 2009 C2S. I changed the spark plugs back in April (preemptively) and it ran beautifully for a few months and then the car started stalling out and had a really rough idle and misfires a few weeks ago. The codes confirmed the misfires and noted that they were on cylinders 1,2 and 3. I cleaned the air filters and the MAF to no avail. I took the rear bumper and mufflers off again and checked the spark plugs and replaced all the coils. Still no dice.

After reading your post, I ordered up VarioCam sensors for both banks and replaced them last night. All CEL's are gone now and the car is running great. A side benefit is that, whereas it used to hesitate/act like it was going to stall off the line, it is now much smoother. Very happy now and I'm sure I saved $$ by performing the work myself. Thank you!

Petza914

Great news. Glad you diagnosed and corrected the issue and are back in business. 👍

shyamvenky

Once again, Petza to the rescue! Rennlist is the best!