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.
Problems for specific Mercury Grand Marquis years:
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Repairpal verified 2003 Mercury Grand Marquis problems
Wear on the rear axle shafts near the bearings can create excessive play and lead to gear oil leaking past the seals. Gear oil will leak onto the brake backing plate, brakes, and wheels.