I just purchased a 2002 Seville STS with 51,000 miles. The next day the Service Engine Soon comes on. I read on here how to find the code and it is P0300 repeated misfire. After clearing the code it seems to come on only when driving 40 to 50 mph with very slight or no throttle pressure. If driving at highway speeds, light doesn't come on. I haven't noticed any misfire or hesitation. Car had just been detailed including engine bay which now looks brand new. It was only driven 10,000 miles in the last 4 years.

Appreciate any help on this.
Steve



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Detailing is a possible cause...normally will take a few days to correct itself. The low annual mileage can cause problems because of carbon build up. Going to parts store for free DTC scan could show additional code(s). I suggest to try W.O.T. see if it helps. P0300 is a generic misfire code, there are other misfire codes that isolate cylinder - on-board won't always bring them up. Post what you

 

The detail was done about a month ago. I pulled the EGR today and the pin was free and very little carbon. I made sure all electrical plugs were well attached. The parts store I went to "OReilly" only showed the P0300 code.I have WOT'd it several times (some just for fun) to see if would clear up. After codes are cleared light comes back on soon as you go 35 to 50 with light throttle.

 

P0300 means there is a misfire, but which cylinder(s)? Things you can check:
Spark plugs - for overall condition, if oem or not, carbon lines on white part of plug.
Coil towers for rust or corrosion - since they have to come off to check plugs.
Vacuum leak(s) - vacuum leak normally will set other codes, but a small leak could cause a misfire code.
post what you find.

 

Took the car to a large Chevy dealer today. I know the serv. mgr. and most of the techs there. They pluged in the tech II and it showed all 8 were misfiring at the same time. They needed to research more to figure out whats next.

 

Post what they tell you, all 8 is interesting.

 

as slowroll said, interesting indeed ...
I have to wonder if the "detailing" damaged the coil packs ...

let us know ...

 

Ok car is now fixed. I would never have believed it. After extensive checking of the entire electrical system, it was the serpentine belt! The belt had no cracks but they said it had some wobble. Evidently that was enough to give the sensor a false reading. The tech agreed with me that when it set a code of all 8 missing at the same time there was no miss felt. All i can say is it runs fine and no codes.

Thanks for your help. I wanted to make sure and share the results.
Steve

 

was the tensioner lose ?

this is a new one on me, for engine misfiring ..
maybe that is the clue: all 8 cylinders misfiring !

good thread, thanks for the final update.

 

Stealth, the tensioner was fine. I should add, the belt was a GM belt and most likely the original. This has most likely happened before with higher mileage cars. Maybe they replaced it as regular maintaince while trying to figure out a problem. Does GM have a tech line to ask or report an unusual problem and solution.

Steve