Every time when starting a cold engine (not driven for six hours or so) for the past couple of weeks, I have a rough idle for about 30 seconds, like the engine is misfiring. Then, by itself, the rough idle goes away, and no issues until next cold start.

After a couple of weeks of this behavior, one morning the check engine light went on with the rough idle. Had mechanic check the codes, which suggested #2 cylinder misfiring. The codes are 16684 (P0300) Misfire Recognized, and 16686 (P0302) Misfire Detected Cylinder 2, both recorded as intermittent. #2 plug looks fine, plug wire looks fine, and no visual problem with the coils or connections. Cleaned connections and cleared codes. Next morning, rough idle and check light comes on again. Still, everything seems fine after the 30 seconds of rough idle.

Anyone else experience this, or have good ideas for diagnosing? It is tough to "catch it in the act" since it happens just once with a cold start and then has to get "cold" again to repeat.

'99.5 A6 2.8, 102K miles, full 90K maintenance done at 90K miles.

 

My 3.0 does it as well, although not for 30 seconds, just about the first 5 or 6 revolutions. I wonder if we have the same problem.

 

If you are running 91 octane or premium fuel, try using 89 octane or midgrade fuel. I had the same problem with my '00(didn't get the engine light) and the service adivsor at the "stealership" said switch to midgrade. My car has been running fine since then.

 

i was also told by a good indi mech to use the mid grade gas.

 

Might wanna check the O2 Sensor. Computer should pick it up if its bad, but you never know. I think you are looking for something that is temperature dependent. Could be some sensor that control fuel when the engine is cold too.

 

Tried different gas over a few tankfuls. No apparent difference. Same behavior.

 

Maybe, but it happens pretty fast. Somewhere between 10 and 25 seconds after a cold start, then no problem at all. It would have to be something that warms up very quickly.

 

I'm wondering if it's injector drip down, causing a small pool of gas on the piston(s), which misfires. A mechanic could run a compression test to see if it's a leaky inector.

 

Would you smell gas in the car?

 

As I understand it, the O2 sensor is electrically heated for just that reason. It needs to be hot to measure the excess O2/fuel on startup. After startup, the exhaust temp keeps it hot.

I am awaiting the repair CD, not sure if it has diagnotics info, but I will look when I get it. Hopefully early next week.