I've had to do this a couple times on my 6 cyl '99 Malibu.
1) Disconnect your negative battery cable and push it off to the side so that it doesn't spring back and accidentally touch the battery terminal.
2) Take the cap off the coolant tank against the passenger side wall and drain the radiator. The radiator petcock is on the bottom front of the driver's side. You should be able to get to it from the front of the car. It's a white plastic plug. Use a 1/4" square drive to turn it. It should pop out toward you as you turn it, but it shouldn't come all the way out of the radiator.
3) The thermostat sits on the driver's side of the engine underneath the air intake, so remove the air cleaner assembly and move the throttle cables out of the way. Go ahead and disconnect the sensors on the assembly and move them out of the way as well.
4) The thermostat housing is connected to the big black upper hose that goes from the radiator to the engine. It should also have a short pipe that takes coolant back to the heater core. Those hoses will need to be disconnected as well if you want to put the thermostat housing on your bench to clean it. (I can't remember if there's anything else in the way. I've only removed it when I replaced the intake manifold gasket and later when I replaced the heads, so everything was out of the way when I got to putting a new thermostat in.)
5) Make sure you use a new gasket. The thermostat should have a small rubber ring in the box or perhaps it might already be on the thermostat itself. Look at your old thermostat to see how it fits.
5a) EDIT: Make sure to clean all the parts where metal meets metal. This includes the inside mating surfaces of the thermostat housing and the part of the engine block where the housing mates to it.
6) When you get it back together, you'll need a gallon to a gallon and a half of 50/50 antifreeze. The coolant that comes with these cars is the orange DexCool, so that's what I stick with.
7) When the engine gets back up to operating temperature, gently loosen the bleed screw to let the air out of the system. You should hear hissing and then see coolant bubbling out of the bleed screw hole when the air is out of the system. Put the bleed screw back in.
(There is a black pipe, about 1/2" in diameter, that runs from left to right just above the spark plugs on the front of the engine. At the far left of this pipe is a small screw sticking straight up. It needs a small socket, maybe 7 or 8 mm. Be very careful! It's not a big screw, and you don't want to snap it off! Very little torque is needed to tighten it.)
When I went to the auto parts store to get a new thermostat, they had two of them rated at different temperatures. I'm not sure why there would be two of them, but I opted for the lower temperature one, I think it was 180. The other was 195, I believe. According to my temp guage, the engine usually runs around 180, so I thought that was the right choice.
Monday, February 18th, 2008 AT 9:13 AM