About 10 months ago, my STS (with N* 4.6L engine) started throwing Cam Position sensor codes and was running terribly. Had to order a sensor and before it came in… broke a timing chain. This proved to be costly as it took out sprockets, bent valves, ruined a head, etc. After repair, it ran well for about 6 months. Would throw an occasional catalyst messages, then started throwing cam position sensor codes again. After the engine gets warm will start running extremely rough like it’s way out of time. When it starts running rough, I can shut it off and restart and it will run smooth again until 2000 RPM is hit then begins to run rough again.
I did find out that GM extended the catalytic converter for a VIN range that my car is in from 8yr/80,000 mi to 12yr/200,000 mi. Took it into the dealer, unfortunately it wasn’t throwing the catalytic converter codes at the time. Dealer says that they want to tear into the engine to look at a check valve between the block and the head (for $4000). Before it starts running rough, it runs incredibly. I’m at a total loss on what the problem could be. I’ve replaced cam position sensors. Don’t feel like spending $4000 so the dealer can tear into the engine when they say they don’t know if they’ll find anything. Waiting for the car to throw the catalytic converter code so they will replace them for free ($2500 if it wasn’t under warranty.) Normally will throw that code when tank is around 1/4 full… I know, that sounds ridiculous.
In the meantime, has anyone run into something like this? Could a problem like this be caused by the catalytic converters? Any other suggestions?