Bought this car about a month ago, runs fine. Only at idle will it surge in revs. We've cleaned the throttle body, changed the air filter, attempted to reset the idle controller, and checked the spark plugs. We took it to O'Reilly's to have it checked for any stored CEL codes and it told us an O2 sensor was bad and the engine is running rich. The reader, for some reason, didn't say which sensor was bad, but we replaced the one the mechanic recommended. It didn't change anything. There was also an incident where the revs struggled for about 30 seconds, I lost power, and the car shut off. It was a one-off but it has me paranoid. I know people probably aren't active much on this forum, so this is a last ditch effort. Thanks for any help.

 

Have you checked for vacuum leaks?

 

Not yet. But could that be the issue if the surges are random and short? It only surges about 100-300 rpms. There is no rhythm to it at all.