I’ve been fighting an intermittent misfire issue on my S600 for quite a while now. It started as a misfire code P0309 (“Misfire damages TWC”) as well as a few assorted other cylinders from time to time. I was also seeing occasional random misfires at idle, mostly on the left bank of cylinders. I changed the spark plugs and insulators, which made no difference. I replaced both coil packs with rebuilt parts from V12ICPack.com, Clark in California. Annoyingly, the errors persisted. I replaced the VT, the 180V/24V DC-DC converter that supplies the coil packs. No difference. By now it was OK, as long as I never floored it — anything over about 5000 RPM would throw a misfire code and shut down one or more cylinders.
I was talking to Clark via email — he doubted it was the coil pack. I disagreed, but was open to other ideas. I looked at the possibility of a bad head gasket, vacuum leak, bad throttle body, anything — but nothing made sense.
Then one night I was on my way home. The car had been running just fine that evening, I even had heat after replacing the valve for the second time. I drove about 12 miles in city traffic, the last 4 or so on the high-speed expressway at 55-65 MPH. No issues. When I got off at the ramp, though, everything went straight to hell. Very rough idle, no power, check engine light on — obviously the engine had shut down multiple cylinders. It took three restarts to get it to come back. The scanner showed misfires on all the left bank cylinders. That happened several more times over the next couple of days, always at idle… but even a very brief push of the throttle would also cause the same sort of multiple cylinder shutdown. It was very obviously badly broken. I am not proud of the words I used during this time.
To shorten a very long and painful story, I pulled the left coil pack and VT, and shipped them both off to Clark. At this point either it got fixed, or the car was going to become a really big gray BBQ grill. While the coil pack was out I got a slightly bent wheel repaired and a tire replaced (thanks, Omaha potholes!). Clark eventually did find an intermittent bad part and fixed it. I got the coil pack back, put it in, and went for a drive… seemed OK. Then the next day I again got a slow start, cloud of white smoke, rough idle, and misfire codes — this time on most of the right side cylinders!! The damn thing was possessed by evil spirits.
As it turns out, it seems that the last round of trouble may have just been a tank of really bad gas. Maybe combined with fuel and spark adaptations being really screwed up, I don’t know. All I know is, the bad idle performance started immediately after I filled the tank with gas from Sam’s Club, and disappeared half a tank later. Since then I’ve driven it for a week in city traffic, on the highway, and we even drove it to KC and back for a raid on Ikea. Zero trouble. Idle, full throttle, long cruises, city driving… I finally, at long last, have my car back.