How to Fix a Leaking Thermostat Gasket

Set the shifter in park, for an automatic. Set the shifter in neutral, for a manual transmission. Apply the emergency brake. Raise the hood and disconnect the negative battery cable with a socket and wrench. Place a drain pan under the radiator petcock drain valve. Remove the radiator cap. Use your fingers to turn the drain petcock valve clockwise to drain the coolant. Use pliers if if the petcock drain valve resists finger-turning.

Locate your thermostat housing by following the upper radiator hose to the engine block. Remove the engine plenum cover, if so equipped. Push away any wire harness that impedes clearance to the thermostat mounting bolts. Remove the heat sensor wire from the thermostat, if so equipped. Use a slot screwdriver to loosen the upper radiator hose clamp. Pull the hose off the thermostat housing neck and wedge it out of the way.

Spray the thermostat mounting bolts with penetrating oil and let it soak for 15 minutes. Use a socket, extension and wrench to loosen and remove the two (or three) thermostat housing mounting bolts. Pry the thermostat housing off the engine block with a screwdriver. Push a rag into the engine block opening. Use a gasket scraper to remove all residual gasket material from the engine block. Clean the engine mounting surface with carburetor cleaner and a rag.

Place the thermostat housing upside down in a bench vice. Scrape all the old gasket material off with a gasket scraper. Clean the housing inside and out with carburetor cleaner and a rag. Place a flat file over the thermostat mounting surface and gently file down the seat edges until bare metal shows. Use circular motions with the file.

Take the thermostat housing back to the vehicle. Place a new thermostat gasket on the engine block. Place the thermostat housing over the block and align the screw holes. Apply some anti-seize lubricant to the mounting bolt threads. Screw the mounting bolts in by hand, then tighten them firmly with a socket, extension and wrench.

Wipe the hose neck down on the thermostat. Push the upper radiator hose onto the neck and tighten the hose clamp with a screwdriver. Replace the temperature sensor wire, if so equipped. Place the engine plenum back into position, if you have removed one. Use a socket or screwdriver to tighten the plenum bolts or screws.

Turn the petcock valve counterclockwise to shut the radiator drain valve. Refill the radiator with new or your reclaimed coolant. Replace the radiator cap. Reconnect the negative battery cable with a socket and wrench. Start the engine and check for leaks.