The thermostat may allow the engine to run too cool, which will cause it to run rough at idle. Replacing the thermostat should fix this concern if the engine is running too cool.
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Repairpal verified 1991 Ford Probe problems
If the engine runs rough, stalls, hesitates on acceleration, is hard to start, or has generally poor performance, this can be caused by a cracked or broken air inlet tube. Loose clamps on the air tube can also cause this these problems. Tightening the air inlet tube, or replacing if broken, should correct the problem.