Tools Needed:
- Flat blade screwdriver
- 10 mm socket wrench
- 15 mm socket wrench
- Pliers
- Funnel
- New thermostat (Dorman 902-810)
- Coolant (Prestone AF1100 or equivalent)
Step 1: Safety First
- Let your engine cool down completely before starting.
- Park your Grand Prix on a level surface and engage the parking brake.
Step 2: Locate Coolant Drain
- Crawl under the car and look for the coolant drain petcock on the bottom of the radiator.
It's usually a plastic plug with a square head.
Step 3: Drain Coolant
- Place a drain pan under the petcock and turn it counterclockwise to open it.
- Let the coolant drain into the pan. This may take a few minutes.
Step 4: Remove Thermostat Housing Cover
- Go to the engine bay and locate the thermostat housing.
- It's on the driver's side of the engine, at the front of the intake manifold.
- There are two 10 mm bolts that hold the cover in place. Use a 10 mm socket wrench to remove them.
- Gently pry off the thermostat housing cover. Be careful not to spill any remaining coolant.
Step 5: Take Out the Thermostat
- You will see the old thermostat inside the housing.
- It is held in place by a single 15 mm bolt.
- Use a 15 mm socket wrench to remove the bolt.
- Lift out the old thermostat.
Step 6: Transfer Gaskets
- There will be two gaskets on the old thermostat.
- One is a small rubber gasket, and the other is a larger, paper gasket.
- Carefully transfer the gaskets to the new thermostat in the same order they were on the old one.
Step 7: Install the Thermostat
- Place the new thermostat into the housing, with the gaskets in place.
- Replace the 15 mm bolt and tighten it securely.
Step 8: Replace Housing Cover
- Place the thermostat housing cover back on and replace the two 10 mm bolts.
- Tighten the bolts securely.
Step 9: Refill the Coolant
- Remove the radiator cap and pour in new coolant until the level reaches the "FULL" mark in the overflow reservoir.
- Replace the radiator cap.
Step 10: Start the Engine
- Start your Grand Prix and let it run for a few minutes.
- Check for any leaks around the thermostat housing. If you see any leaks, tighten the bolts until they stop.
- Let the engine reach its normal operating temperature.
- Check the coolant level again and top it off if necessary.
Step 11: Check for Leaks
- Keep an eye on your coolant level for the next few days to make sure there are no leaks. If you notice the level dropping, check for leaks around the thermostat housing and radiator cap.
That's it! You have successfully replaced the thermostat in your 1999 Pontiac Grand Prix GT.
