How to Change the Thermostat on a 1994 Ford Aerostar

by James Werning

The thermostat is a valve in the engine cooling system. The thermostat stays closed at low temperatures to help the engine heat up quickly, and it opens at high temperatures to allow the coolant to circulate and cool the engine. When broken, the thermostat either stays closed (causing overheating) or stays open (causing the engine to warm up slowly so that the compartment heating system does not get up to temperature). If your radiator fluid is boiling over on warm days, the first thing to check or replace is the thermostat. This is one of the easier jobs you can do on your 1994 Ford Aerostar.

Park your 1994 Ford Aerostar in an area that will allow you to contain and clean up any radiator fluid that leaks onto the ground. Allow the engine to cool down completely--doing this job on a hot engine could cause serious injury.

Locate the radiator hose that comes out of the top, rear edge of the radiator. Follow that hose back toward the engine. Note how the far end of that hose is clamped to the thermostat housing, a metal piece that is bolted to the engine.

Remove the two mounting bolts that hold the thermostat housing to the engine, using a socket wrench. Pull the housing away from the engine, allowing radiator fluid to spill to the ground. Prop the rubber hose and housing out of the way.

Pull the old thermostat (with gasket attached) out of the round, shallow depression in the engine which is just big enough to hold the thermostat. Using a screwdriver or razor blade, remove any bits of torn gasket from the edge of that depression, as well as from the matching parts of the thermostat housing that you pulled aside. Clean those surfaces with a rag, making sure that no gasket bits remain.

Place the new thermostat into the depression in the engine, with the spring facing toward the engine. Place the new gasket on top of the thermostat. Place the thermostat housing on top of the gasket and thermostat. Replace the two bolts, tightening securely with a socket wrench.

Remove the radiator cap. Fill the radiator to the top with radiator fluid and water (usually a 50/50 mix; read the radiator fluid instructions). Replace the radiator cap. Run the engine to operating temperature while checking for leaks around the thermostat housing.

If leaks are found, cool the engine down, remove the thermostat housing and reinspect for old gasket bits that you can remove. Reassemble and repeat the previous step.


  • check If the air intake for the air filter makes it difficult for you to reach the thermostat housing, loosen its clamps with a screwdriver and remove the intake. Reinstall it when you are done replacing the thermostat.
  • check Your old thermostat can be tested, although it's better to replace it since it's an inexpensive part and easy to replace. To test it, place the thermostat and a food thermometer in a pan of water and heat it on the stove. The thermostat will open at about 200 degrees Fahrenheit. If it doesn't open, it is bad.

Items you will need

About the Author

James Werning has authored books and articles on various websites. His scripts have aired for more than 15 years on radio stations across North America. He is a small business owner and a world traveler with a master's degree in communications from the University of Edinburgh, Scotland.

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