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 fuel pump may fail causing engine to stall and not restart.
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.
Fuel injectors can short-circuit electrically causing a rougher than normal idle and reduced performance.
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.
The engine vacuum hose to the MAP sensor may crack resulting in a vacuum leak causing the engine to run rough and/or stall.