Tracking down two instances of very brief multiple misfires at startup
 

Hey all,

I have a 2005 C2S with 67k miles; bought it in January with 63k miles. Since buying this car I have performed the 12 year and 60k services, including replacement of the plugs with Bosch OE plugs, replacing the air filter, and have also replaced the air/oil separator which I believed to be faulty.

Since buying the car and performing the services (within the first month of ownership), the MIL has come on twice after startup for random multiple misfire and specific misfire codes (all cylinders except #2 the first instance, and all cylinders but #2 and #3 the second instance). A symptom of sluggish acceleration immediately following cold start has happened on both days where the codes have come on, and maybe the day or two prior to the codes coming on. As soon as the car has been driving for a minute (or less), the symptom disappears and the car runs absolutely fine. When the sluggish acceleration briefly occurs it does not feel like an engine misfire in the way I am used to feeling it (for those of you know what a misfiring cylinder feels like); it seems like the amount of throttle input at the pedal is not equal to the amount of the throttle plate being opened, if that makes sense.

These symptoms have been isolated to only these two times in the last four months; the week following the service, and today, or roughly 3 months in between. Could this be a sensor or something that could be causing this brief intermittent random misfire? The rest of the time the car has been absolutely fine. Fuel mileage is normal, engine runs great, have done a lapping day and two autocross events in addition to my normal 12mi/day commute and no hiccups beyond these two. I just don't know where to start looking!

When you first start the car, do you get the correct elevated idle of 1,100-1,200 rpm or does it idle at the regular 700-800 rpm when first started?

You running the stock airbox or aftermarket intake and/or filters?

Correct elevated idle that eventually settles to the 7-800rpm range. Stock airbox and filter.

1. Have you looked closely at the ignition coils? You did not mention if they have ever been replaced. They are made from glass filled polyester resin and are subject to aging from hydrolysis. The housings can eventually crack.

2. Possibly the MAF sensor?

I cross-posted this on a 997 FB page, and most of the suggestions were to replace the ignition coils. Thought I'd be cheap and skip them but I guess I can't be cheap anymore. It makes sense that the car would misfire when its damp out; not unlike a car with bad wires. I'll order some up and report back once they're installed.

So its been over a month now and the issue has not cropped back up; I'm just posting this to keep a record. I am still likely going to replace the coils soon but some other issues are taking precedence.