Location of Thermostat on the 3.1L and 3.4L GM engines

If you've been wondering if it's possible to test the thermostat on your 3.1L or 3.4L GM equipped vehicle without removing it, the answer is YES.

As you may be already aware, the thermostat is located under the intake manifold's throttle body and it can be a total pain in the neck to replace it... so testing it to make sure it's BAD or not will save you time and money (not to mention a ton of frustration).

In this article, I'll show you how to test it without removing it from the vehicle, not only that, you'll be able to say that it is or it is not causing the Overheating Condition your GM vehicle is experiencing.

To help you navigate this article, here are its main points:

  1. Important Suggestions And Tips
  2. Symptoms Of A BAD Thermostat
  3. TEST 1: Thermostat Operation
  4. TEST 2: Fan Operation
  5. Thermostat Basics
  6. Other Tips And Suggestions
  7. More Test Articles

En Español You can find this tutorial in Spanish here: Cómo Probar El Termostato (3.1L, 3.4L GM) (at: autotecnico-online.com).

Important Suggestions And Tips

TIP 1: In this article, I'll show you an indirect way to test the thermostat with the engine at operating temperature. There's no need to remove it to test it.

This ‘indirect way’ is accomplished by using a scan tool with 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: Under no circumstances should you open the radiator cap while the engine is running and/or while it's at its normal operating temperature (whether the engine is running or not). Your safety is your responsibility, so be careful and use common sense.

TIP 3: It's very important that you start your troubleshooting with a completely cold engine.

Symptoms Of A BAD Thermostat

The thermostat in your engine will fail in one of two ways. It will either stay stuck in its open position or will stay stuck in its closed position.

CASE 1: If the thermostat gets stuck open, you'll notice some of the following symptoms:

  1. Really 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.

Yes, the thermostat has a direct impact on the amount of fuel that's injected into it by the PCM. So, if the thermostat is stuck open (or if you have removed it), your vehicle will use more gasoline.

CASE 2: If the thermostat gets stuck closed, you'll notice some of the following symptoms:

  1. The engine will overheat.
  2. Cooling fans come, but the engine does not cool 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 come on.

Alright, let's get testing...