The engine may overheat at idle when the AC is on. A new, upgraded fan motor and resistor may be required to correct this concern.
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Repairpal verified 2004 Mercury Grand Marquis problems
The composite (plastic) intake manifold may crack near the thermostat housing and cause a coolant leak. Ford released an updated manifold that was reinforced to prevent a recurrence. No recall was issued for this problem but Ford did extend the warranty to seven years on some models from the date of purchase.