A rough idle is most likely the cause of needing a tune-up or a vacuum leak. My concern is the 3.1L are known for having intake gasket problems. Check around the intake for any evidence of coolant leaks. Some people will actually spray starting fluid around it with the engine running and if there is a leak, the engine RPM's will increase. My only concern is the danger of starting a fire if the starting fluid hits an exhaust manifold or other item that is really hot. If I had to guess, it would be an intake gasket problem.
As far as the knock sensor, they are designed to advance the ignition timing to the max point right before you get a ping or knock from the engine. That is the point the engine will probide the most performance and fuel efficiency. I questions if it would cause a rough idle.
Let me know what you find with the intake gasket. Most common point of leak would be the passanger side front corner of the engine.
Wednesday, January 28th, 2009 AT 10:01 AM