How To Test The Thermostat (2002-2009 4.2L Chevrolet TrailBlazer, GMC Envoy)

By: Abraham Torres-Arredondo
Created: 23 May 2022
Updated: 19 February 2024
Articl Id: 813

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Engine Thermostat Basics

The thermostat's job is to help regulate the engine's temperature for better emissions control and the best gas mileage.

The regulated engine temperature range is between 190° to 225° Fahrenheit (87° to 107° Celsius).

It's important to note that the cooler the engine is, the more gasoline it needs to keep running smoothly (and unfortunately, at these temps, it pollutes more).

As the engine heats up and reaches its optimal temperature range, it needs less fuel to maintain its optimal performance and emissions control.

Here's a brief explanation of how the thermostat helps regulate the engine's temperature:

When you start your vehicle:
- The thermostat is closed.
- The radiator fan clutch is disengaged.
As the engine coolant circulates in the engine only, its temperature increases, and when it reaches 190° F, the thermostat opens.
The hot coolant now circulates between the radiator and the engine with the thermostat open.
Even tho' the thermostat has opened, the temperature of the coolant will continue to increase.
The fan clutch senses the increase in the coolant temperature (in the radiator) and engages.
The engaged fan clutch now pulls more air across the radiator, cooling the coolant within it.
Since the thermostat is open, the cooler coolant in the radiator enters the engine to cool it down.
As the coolant's temperature decreases further, the fan clutch disengages, and the thermostat closes.
The coolant in the engine now begins to absorb more heat, and the process repeats itself continually the whole time the engine is running.

The cool thing is that you can observe these temperature changes with your scan tool in Live Data mode, which helps diagnose the thermostat as good or bad.