I have a 94 lumina with a 3.1... picked it up for 400 bucks.... Engine has erratic idle throughout rpm range, sometimes runs strong, sometimes runs weak. If left to idle for five minutes, service light comes on. If I turn the key and drive it for four hours, the service light never comes on. The car has died and will turn over but no fire for 10+ minutes--- come back to it, starts right up. The sevice code is for rich mixture.