If a noise from the rear end is heard when going around turns or after driving at highway speed for an extended period, this could be caused by a failing deferential clutch kit. An updated differential clutch kit available.
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Repairpal verified 1994 Ford Aerostar problems
Transmission problems are common in this model, often as a result of a failed front pump. The pump loses its ability to create hydraulic pressure, which an automatic transmission needs to function. A sign of front pump failure can be a whine type noise.
If you experience poor idle quality and fuel economy (and the Check Engine Light illuminates) after the exhaust system or motor has been removed and replaced, it could be that the oxygen sensor wires are crossed at the rear of the motor. Wiring schematics should be checked to ensure that the correct harness plug is connected to the corresponding sensor.
The AC system may be performing poorly even if it seems to be functioning properly. This could be caused by the AC air blend door being out of adjustment.
Excessive cycling of the AC compressor clutch when the system is fully charged could be caused by sludge that has built up on the engine throttle body. Throttle body should be cleaned periodically unless there is a sticker on it stating no cleaning is allowed. Some throttle bodies have a special coating that should not be cleaned.
An engine knocking noise coming from the lower end during acceleration is common and could be caused by worn or damaged engine bearings.