The thermostat in your 1999 GMC Jimmy is a cooling system device that controls the flow of antifreeze entering the engine to lower its temperature. When the engine temperature increases, the thermostat opens automatically and allows antifreeze to enter the engine block. A poorly functioning thermostat can fail to open when necessary, allowing the engine to overheat and causing major engine damage. Replacing this fairly inexpensive part in your truck can prevent you from having to perform extensive repairs caused by overheating.
Park the truck on a level surface and allow the engine to cool. Put on your safety glasses, raise the hood, and take off the radiator cap to release the pressure in the radiator hose.
Disconnect both battery cables for added safety. Place a clean bucket directly under the antifreeze release drain located on the bottom of the radiator.
Slide underneath the front end of the truck on your back and loosen the drain bolt using a socket wrench. Drain enough antifreeze to clear out the upper radiator hose that attaches to the thermostat cover, and then tighten the bolt. Check to see if it is clear by squeezing the hose and feeling for any remaining water. Repeat this step until the upper radiator hose is empty.
Remove the carburettor air cleaner and filter unit by unscrewing the centre cap using your hand.
Disconnect the upper radiator hose where it attaches to the thermostat cover and tube. It is secured in place with a spring hose clamp. Squeeze the clamp using channel-lock pliers and slide it off the cover. Keep the opposite end that connects to the radiator attached during this procedure.
Remove the three mounting bolts that secure the cover to the block using a socket wrench. Gently tap the cover using a hammer to break the seal. Remove the cover and set it aside.
Stuff a small clean rag in the thermostat opening to keep debris from entering the engine block. Using a paint scraper, scrape off the old gasket and blow away the loose debris then remove the towel.
Apply a small dab of gasket adhesive to the replacement gasket using your finger. Place the gasket onto the block at the thermostat opening. Align the mounting holes correctly so that the thermostat and cover will seat properly.
Test the new replacement thermostat before you install it. Fill a 3- to 4-inch deep bowl with very hot water. Grab the thermostat upright using channel-lock pliers and hold it in the hot water, covering the entire unit. The thermostat should open immediately, indicating that it is functioning properly.
Insert the thermostat upright with the spring side inserted into the engine. Carefully replace the housing cover and attached tube. Do not move the gasket and secure it on with the mounting bolts.
Replace the carburettor air cleaner unit, then reconnect the upper radiator hose to the thermostat cover and secure it in place with the spring hose clamp. Inspect the hose for damage or cracking and replace it if necessary. Reconnect both battery cables.
Start the engine and pour the drained antifreeze back into the radiator. Let the engine idle for several minutes and inspect the thermostat and hose connection for leaks. Add additional antifreeze if necessary and then replace the radiator fill cap.
Always wear safety glasses when repairing an engine.