History of the Hummer H2 can be traced all the way back to 1979, when AM General began designing the M998 High Mobility Multi-Purpose Wheeled Vehicle -- the Humvee. The Humvee went on to be built as a military-only vehicle until 1992, when AM General released different versions of the vehicle for civilian use, naming it the Hummer. The name didn't carry the H1 or H2 badge until after GM acquired ownership of the brand. In 2006, I was lucky enough to have a short term lease on an H2. In the six months I drove it, its only failure was the thermostat -- this is what I did to fix it.
To release the radiator pressure, I turned the radiator cap about a quarter turn, until I heard a hissing noise. Once the noise went away, I removed the radiator cap. After deciding I didn't need to jack up the front end, thanks to how high my H2 sat, I drained the coolant from the drain cock on the passenger side, lower corner of the radiator. I had to turn the drain cock counterclockwise to open it and clockwise to close it. Now that I had the radiator drained, I had to find where the thermostat was on the engine. I loosened the clamps from the air intake tube, and removed the tube from the engine bay. I could then look down, just above the air-conditioning compressor, and see the thermostat housing. I loosened the clamp for the radiator hose, then pulled the hose off of the housing. I removed the two bolts and pulled the housing and thermostat away from the engine. I had to peel the gasket off of the engine.
Before I installed the new thermostat, I wanted to make sure I wasn't going to have any leaks, so I cleaned the mating surface on the engine and the thermostat housing. I installed the thermostat and new gasket with the spring pointing into the engine, then tightened the housing bolts to 18 foot-pounds. I installed the upper radiator hose and tightened the clamp, then I did the same for the air intake tube. I filled the radiator with almost two gallons of Dexcool engine coolant. I set my heater controls to maximum, started the engine and let it warm up. When the engine got to operating temperature I noticed the coolant level in the radiator dropping, so I continued to add more until the level held steady. When I felt the warm air from the vents I knew I was done. I let the engine cool, and installed the radiator cap.
