I just replaced the head gaskets on my 96 chevy camaro with the 3.8 v6. During test drive the check eng light came on I checked the code and it was a po300 multiple misfires. I checked for vaccuum leaks and other problems and found nothing. I cleared the lite and drove the car again it ran perfect. However when I took it for its smog check as it sat idling to warm up, the eng started to idle rough and the the lite came on again the tech checked it and again it was a po300 code. Any clues????

I had the same prob with my 96 3.8. The read outs were diff every time I had it put on the machine. I took the plug wires off the coil packs, held the end next to the poles, and watched for a spark, if it sparked, that coil pack was good, one of my coil packs did not spark on any of the two poles, I changed it, and SHAZAM! I dont know why the head gaskets brought this prob to the front, but it sounds like the coil packs, but Im no GURU, just a shade tree mec. also, EGR will cause this, MAF sensor will as well. PCV, FUEL Filter, and as you said, vac leaks... if none of those, beat on your computer mod, left side under hood up under the window, not too hard, if the idle changes with your beats, its the prob, if not, test the FUEL PUMP, and if thats the prob, GOOD LUCK! HAVE FUN!

A bad distributor