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:
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.
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:
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:
Alright, let's get testing...
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Kevin Costner in The Postman