How to Replace the Thermostat on the 3.1 Grand Prixby Kevin Mclain
The cooling system inside of the Grand Prix 3.1-liter engines comes equipped with a spring-loaded thermostat that opens and closes according to the temperature of the antifreeze. The thermostat has a bottom spring that compresses and opens the top of the thermostat to allow the antifreeze to flow through the engine block. The main purpose of the thermostat is to prevent the engine from overheating by regulating the flow of the antifreeze through the engine. If the thermostat fails, it has a safety feature that will allow the thermostat to return to the open position.
Pull the Grand Prix with the 3.1-liter engine into a safe and level work area. Set the emergency brake and open the hood. Shut the motor off and wait two to three hours for the engine to cool down completely.
Slowly unscrew the radiator cap and set it to the side. Make sure that the cap is not hot and does not have pressure on it. Slide the drip pan underneath the radiator drain plug on the bottom of the radiator. Loosen the drain plug with the pliers and allow a couple of gallons of coolant to drain into the drain pan. Tighten the drain plug back down tight.
Locate the top radiator hose that connects to the top radiator port and the intake manifold. The other end of the radiator hose connects to the thermostat housing on top of the intake manifold. Unscrew the hose clamp from the end of the hose that is attached to the thermostat housing port with the pliers. Pull the clamp back onto the hose. Twist and pull the hose off the thermostat housing port.
Unscrew the two thermostat housing mounting bolts with a ratchet and a socket. Pull the housing off and set it to the side. Pull the thermostat out of the intake manifold and discard it into the drain pan. Scrape away any of the old gasket from the base of the intake manifold and the bottom of the thermostat housing. Wipe away the excess gasket, dirt and debris from both places with a clean rag.
Spread a thin line of the red high-temperature silicone around the bottom of the thermostat housing. Push the gasket onto the bottom of the housing so that the silicone will hold the gasket in place and act as a extra sealer. Make sure that the bolt holes are matched up on the gasket and the thermostat housing.
Set the thermostat housing back over the intake manifold and match the bolt holes up. Screw the two mounting bolts through the thermostat housing and tighten with the ratchet and socket.
Push the top radiator hose back over the thermostat housing port and line the hose clamp over the radiator hose and the port. Tighten the clamp with the screwdriver until the hose is securely tightened to the thermostat housing port.
Add the antifreeze to the radiator until it is full. Leave the cap off the radiator. Crank the motor and let the engine heat up until the thermostat opens. The thermostat generally opens around 180 degrees. When the thermostat opens, the antifreeze level will drop inside of the radiator. Keep adding more antifreeze to the radiator until the antifreeze stabilizes at the top of the radiator cap. Screw the cap on tight and turn the engine off.
- The thermostat replacement process requires the same process for the 1988-2007 3.1-liter engines.
- The red high-temperature silicone is specially designed to use for components that are around high heat. This silicone can be bought at most auto-parts stores.
- Use caution when working around a hot radiator and a hot engine.
- Dispose of antifreeze into a container and not onto the ground.
Items you will need
- Drain pan
- Flat-head screwdriver
- 1/2-inch drive ratchet
- 1/2-inch drive socket set (metric)
- Flat metal scraper
- Clean rags
- Red high-temperature silicone
- New thermostat with gasket
- "Pontiac Grand Prix Repair Manual Years 1988 to 2007"; John H. Haynes; 2009