Ford Mercury Villager 1997 Owner's Manual page 335

Cleaning the Engine
A clean engine is more efficient because a buildup of grease and
dirt acts as an insulator, keeping the engine warmer than
normal.
Extreme care must be used if a power washer is used to clean
the engine. The high pressure fluid could penetrate sealed
parts and assemblies causing damage or malfunctions.
In order to avoid possible cracking of the engine block, do not
spray a hot engine with cold water.
The alternator, distributor and air intake must be covered.
Covering these components will help prevent water
damage.
Never wash or rinse the engine while it is running. Water
getting into the engine may cause internal damage.
Maintenance and Care
331