The throttle position sensor (TPS) may fail. Common symptoms of a defective TPS can be rough idle, hard starting, hesitation on acceleration, poor fuel economy, and stalling.
Problems for specific Mercury Grand Marquis years:
Car problem reports
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Most reported 1990 Mercury Grand Marquis problems
If you experience difficulty starting the engine when cold, stalling at idle, or hesitations during acceleration, it may be the idle air bypass valve. This valve helps the vehicle idle steady when the accelerator pedal is not pressed.
It is not uncommon for the intake manifold and/or valve cover gaskets to develop a coolant or oil leak. Leakage from the intake manifold may result in coolant in the engine oil. Coolant mixing with the engine oil severally impacts the lubrication ability of the oil. Internal engine may result if this condition is not repaired promptly.