This article comes about by the seemingly mass confusion of where the thermostat is located on 2002 and up Chevrolet Trailblazer, GMC Envoy and similar body styles with the 4.2 liter inline six cylinder engine. I’ve seen many times, in person and online, people claiming the thermostat is on the right (passenger) side of the engine, inside a small housing that looks very similar to the V6 engines housing. This is NOT the case at all, and people that do not know what they are talking about can be misleading the regular guy trying to fix his vehicle.
On earlier model vehicles, and even the S-series Blazer that was still out when the new body style Trailblazer came out, they had a V6 engine with the thermostat located right in the front, on top of the intake, just about dead center of the engine. When the inline six engine came out in the 2002 and newer Trailblazer, the thermostat was re-located to the left (drivers) side of the engine, behind and a bit below of the alternator. See pics below. The housing and hose that is now on the right side of the engine is just an inlet housing for the coolant. There is NO thermostat inside THAT housing on the Chevy Trailblazer.
Trailblazer Thermostat Pictures
As you can see in the provided pictures below, the thermostat on the Trailblazer 4.2L straight six cylinder engine is an integral part of the outlet housing below and behind the alternator, and is NOT serviced separately. You CANNOT just go to the parts store and get a thermostat in a box. This unit needs to be replaced as a whole. The repair can be made through accessing the bolts to the housing through the left front wheel well opening, or if you prefer, the belt, and alternator can be removed, but that is a bit more work.
I really hope this clears up any confusion you may have heard elsewhere. When it comes to a situation like this, you really need the correct information from someone who does this for a living!
Do i need to purge cooling system on 2004 GMC Envoy after draining cooling system?
After refilling the radiator, let the truck run for a while with the cap off. It should bleed out any air easily that way. You may have to top off once the thermostat opens, but probably not that much. Once it’s hot, and circulating, put the radiator cap back on and you’ll be good to go. This also works on the Chevy Trailblazer.