How To Test The Thermostat (GM 4.3L, 5.0L, 5.7L)

When the thermostat, in your GM 4.3L, 5.0L or 5.7L pick up, SUV, or van fails, it does one of two things: it stays stuck open or stuck closed.

If it stays stuck closed, your pick up (van or SUV) will overheat. If it stays stuck open (or if it's missing), the engine will not warm up enough and the heater will not work (if its winter time).

Well, in this article, I'm gonna' show you just how easy it is to test the thermostat without taking it out from the vehicle.

Important Suggestions And Tips

TIP 1: The way I'm gonna show you how to test the thermostat is by monitoring the temperature of the coolant with a scan tool that has Live Data capability.

You don't need the GM factory scan tool or an expensive professional technician level scan tool. A generic scan tool will do great and I've written this article with this tool in mind (Don't have a scan tool? Need a scan tool? Check out my recommendation: Actron CP9580 Scan Tool).

TIP 2: Never open and/or remove the radiator cap with a warmed up or hot engine. The Anti-freeze can suddenly shoot out and cause you severe burns. Your safety is your responsibility... be careful, use common sense and think safety all of the time.

TIP 3: The key to testing the thermostat is to start the test with a completely cold engine.

In other words, the engine has to be at ambient temperature to get an accurate test result. If the engine in your vehicle is hot or has been running for an extended amount of time... let it cool down completely. This is important for safety reasons and for the accuracy of the thermostat test.

Symptoms Of A BAD Thermostat

As I mentioned at the beginning of the article, when a thermostat goes BAD... it fails by either staying stuck open or closed.

Here are some more specifics:

CASE 1: If the thermostat is stuck open, you'll see the following symptoms:

  1. BAD gas mileage.
  2. If it's winter time, the heater will not work.
  3. If your vehicle is equipped with a temperature gauge (and not just a temperature light), it will always show you a low reading or none at all.
  4. Won't pass the State Emission's Test.

That's right, a missing or stuck open thermostat will cost you more at the pump since an engine that operates below its normal operating temperature will need/consume more fuel.

CASE 2: If the thermostat is stuck closed, you'll see the following symptoms:

  1. The engine will overheat.
  2. The fan clutch isn't able to cool the engine down.
  3. If your vehicle is equipped with a temperature gauge, it will max out.
  4. If your vehicle is equipped with a temperature light, it will shine nice and bright to let you know that an engine meltdown is occurring.

OK, let's get testing...