The EGR valve position (EVP) sensor can fail on this vehicle. The EVP sensor measures the EGR valve position and transmits the date to the engine control computer. If it fails, drivability and fuel economy can suffer, the Check Engine Light will commonly illuminate.
Problems for specific Mercury Grand Marquis years:
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Newest reported 1994 Mercury Grand Marquis problems
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.
I changed the thermostat and installed it correctly but usually my Grand Marquis puts out hardly any heat at all.