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.
2004 Mercury Grand Marquis Problem Reports
RepairPal Verified 2004 Mercury Grand Marquis Problem Reports
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.