How to Replace a Bonneville Thermostat

The Bonneville was a long-standing model in General Motor's Pontiac lineup. It was discontinued in 2005. The Bonneville thermostat is a bit more difficult to access than other models because the air intake assembly must be removed. Most Pontiac Bonnevilles were equipped with a 3.8L V-6 engine. Some special models in the late '90s had a supercharger, and Pontiac offered the Bonneville with an optional 4.6L V8 in 2004. These engines are drastically different than the 3.8L. These instructions will help you replace the thermostat in your Bonneville's 3.8L engine, not the 4.6L or the supercharged engine.

Instructions

1

Let the engine cool off for 30 to 40 minutes before getting started. Engine coolant can reach 200 degrees F and cause severe burns if proper precautions aren't taken. Unhook the negative battery cable with a pair of pliers.

2

Pull off the plastic engine cover on top of the engine bay. Place a drain pan underneath the radiator and remove the radiator drain plug. Let the coolant drain for about 30 minutes.

3

Remove the air intake assembly while the coolant is draining. Loosen the clamps from the intake hose and detach the clips from the air box.

4

Remove the air filter. Remove the air assembly mounting bolts and set the assembly aside. It must be removed to access the thermostat.

5

Make sure the engine coolant is drained before moving on. Remove the hose clamps from the radiator hose where it attaches to the thermostat housing. Pull the hose away.

6

Remove the single bolt from the thermostat housing. This bolt is difficult to access, even with the air assembly removed. For best results, use a socket extension to help you access it.

7

Pull away the thermostat housing and set it aside. Lift the old thermostat out of its mount. Remove the old gasket.

8

Position the new thermostat and gasket into its mount. Replace the thermostat housing, and tighten the bolt to 20-foot-pounds of torque.

9

Reattach the radiator hose and tighten the clamps. Reassemble the air assembly, starting with the base, and add the filter, cover and air intake hose. Make sure the air assembly hoses are properly attached.

10

Replace the radiator drain plug and replenish the engine coolant. Replace the plastic engine cover and reconnect the negative battery cable. Start the engine and inspect underneath for leaks. Closely monitor the engine temperature gauge.