Have a 03 cts that is having rough idle. Pulled codes p0300 p0301 p0303 p0305. Put new plugs in and no change. Plugs were burning clean on both sides. Coil Pack?? It started when the drive belt broke. I noticed a vac line had been knocked loose at the bottom of the tb and fixed. Have reset ecm and starts up fine. After 10 seconds you can hear cylinders shutting down one at a time as the idle gets worse. Any Ideas?

 

Sounds like you need coil pack for odd cylinders - should be passenger side.

 

Thanks I will try that. I keep going through ideas in my head and come up with the same thing. I just dont want to spend 100 bucks and have it not fix it, but I suppose you cant test the things so...yeah. Its my brothers car and he cant afford to go to a shop, so i took it on.

 

Not really other looking for rust on coil towers. You may also check for oil pooling in spark plug holes from leaking valve cover gasket.