In this article, I'll show you how to test the thermostat, to see if it's stuck open or closed, without removing it from the engine.
As you might already be aware, when the thermostat fails... it does one of two things... either the engine overheats or stays so cool that the heater will not work (if it's winter time that is).
Thankfully, there is a way to test the thermostat to find out if it has failed or not, without having to replace it first.
Also, since all of the 3.8L V6 equipped vehicles that belong to the GM line (Chevy, Buick, Olds, Pontiac) use an electric fan motor to cool the engine down... I've included a simple test to see if the fans are coming on at the appropriate temperature.
To help you navigate this article, here are its main points:
TIP 1: The fastest, easiest and safest way to check that the thermostat is good or BAD is with a scan tool. This scan tool has to have Live Data capability... since a simple code reader will not do.
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: Testing the thermostat involves checking its operation with the engine at its normal operating temperature... and so you have to be careful. Do not, under any circumstances, open and/or remove the radiator cap with a hot engine.
Opening and/or removing the radiator cap with a warmed up or hot engine can cause you severe burns. Your safety is your responsibility... be careful, use common sense and think safety all of the time.
TIP 3: To successfully test the thermostat, it's very important that you start your troubleshooting with a completely cold engine. 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.
The thermostat in your 3.8L equipped GM car or minivan will fail in one of two ways: It will either stay stuck at its open position and let coolant circulate all of the time or it will stay stuck closed. When it stays stuck closed... your car or mini-van will overheat.
Here are some more specifics:
CASE 1: If the thermostat is stuck open, you'll see the following symptoms:
Most folks are usually surprised to find out that a stuck open or missing thermostat wreaks such havoc with gas mileage but it's true.
CASE 2: If the thermostat is stuck closed, you'll see the following symptoms:
OK, let's get testing...
“Some people see things that are and ask, Why? Some people dream of things that never were and ask, Why not? Some people have to go to work and don’t have time for all that.”