The upper intake manifold gasket and/or a PCV vacuum lines/hoses can develop vacuum leaks with age. This type of leak can result in a higher that normal or rough idle.
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Most reported 1990 Oldsmobile Cutlass Supreme problems
The high pressure power steering hose will commonly leak fluid; it should be replaced.
The rubber section of an automatic transmission cooler line may develop a fluid leak . In some cases the leaking section of hose can be replaced. In others, the complete cooler line must be replaced to correct this concern.
The fuel pump may fail causing engine to stall and not restart.
The engine temperature sensor wire for the engine temperature gauge, or warning light, may touch the rear exhaust manifold causing a short to ground causing erratic operation of the temperature gauge or warning light.
Fuel injectors can short-circuit electrically causing a rougher than normal idle and reduced performance.
The engine vacuum hose to the MAP sensor may crack resulting in a vacuum leak causing the engine to run rough and/or stall.