Engine burns some oil (no external leaks) - probably half quart every 500 miles(?), w/ no visible smoke from exhaust. Got fault code of P0304, "Cylinder 4 misfire." Pulled the plug and it was the worst looking plug I've EVER seen (genuine volvo) w/ all three prongs 80-90% gone and center electrode 90+% gone. It's possible that these were original -- has 41,000 miles. I got it at about 37k, no service history. I reset CEL, replaced all plugs w/ Bosch Super Plus copper; #4 was clearly the worst. Don't think that it was "placebo effect," but it seemed to run amazingly better, real acceleration for the 1st time.
About 2 days later, same code reappeared.
Car idles rough ONLY w/ cold engine, but doesn't miss as far as I can tell.
Should I switch the coil pack between 4 and another cylinder then get codes re-read? What else should I do if that doesn't help?
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