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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Most reported 1998 Mercury Grand Marquis problems
The alternator may get too hot, causing premature bearing failure.