Subaru issued a recall (WWG-90) because the mass air flow sensor has a manufacturing defect.
Problems for specific Subaru Impreza years:
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Repairpal verified 1999 Subaru Impreza problems
The leak may develop from the oil pump seal. Our technicians prefer to use thread locker when replacing the seal to prevent the bolts from becoming loose again.
A noise in the clutch called "clutch chatter" is common if the car is driven without allowing it to warm up.