TEST 1: Thermostat Operation
I know you're itching to get started and get testing, but let me remind you of two very important things:
1.) It's critical, for the accuracy of the test (and your safety), that you start out with a cold engine
2.) As the engine warms up, do not open the radiator cap for any reason.
OK, this is what you need to do to get this show on the road:
With engine completely cold (by cold I mean at ambient temperature).
Check that the radiator in your car (or mini-van) is completely full, by removing the radiator cap and checking the radiator and not the coolant Overflow bottle.
Once you've checked that the radiator is indeed full of coolant, re-install the radiator cap.
Now, touch the upper radiator hose with your hand and make a mental note of how cool/cold it is. The hose should be at ambient temperature. If the upper radiator hose is Hot, then you need to let the engine cool down completely and start then.
Don't skip this step since you need to make sure that the upper radiator hose is cold (at ambient temperature) before cranking and starting the engine.
Connect your scan tool to the vehicle and get to its Live Data mode. Scroll down to the PID for the coolant temp sensor (Don't have a scan tool? Need a scan tool? Check out my recommendation: Actron CP9580 Scan Tool).
On most scan tool this PID will simply say: Coolant °F or Coolant °C.
OK, now start the engine but leave the hood open (the hood must remain open for the entire test). Your job now is to keep your eyes glued on your scan tool's display and check the temperature reading of the coolant.
If you started out with an engine at ambient temperature, then your scan tool should report a coolant temperature sensor temperature that should be within 10 degrees of ambient temperature. For example, If it's 95° F outside, then the engine coolant temperature (ECT) sensor should report a temp reading between 95 to 105° F.
As the engine starts to warm up, the coolant temperature will eventually show 150° Fahrenheit (65° C) on your scan tool. When it does, lightly touch the upper radiator hose to check its temperature.
The hose should still be cold (ambient temperature). In other words, it should be the same temperature you noticed before you started the engine on your car or mini-van, if so, continue to step 6.
If the hose is hot, then now you know that the thermostat is stuck open or missing (if the thermostat were not stuck open or missing, the hose would still feel cold). You don't have to continue to the next round of test steps. Replace the thermostat.
As the engine continues to run and gain more temperature and the coolant temperature reaches 190° Fahrenheit (87° C) on your scan tool, lightly touch the upper radiator hose once again.
You should feel a definite warming up of the upper radiator hose (compared to its temperature at the beginning of the test).
If the hose does not feel like it's heating up, don't worry about it just yet, continue to the next step.
Let the engine run till the scan tool reports a temperature of 200° F (93° C) and lightly touch the upper radiator hose again.
The hose should now be hot.
Alright, whether the upper radiator hose is hot or not... turn the engine off and let's interpret your test results.
Interpreting The Results
CASE 1: The radiator hose got hot at 190° F and stayed hot at 200° F. This confirms that the thermostat is functioning correctly.
Here's why: If the hose got hot at 190° F, then the thermostat opened up and let the coolant (which has been absorbing the engine's temperature) circulate to the radiator.
CASE 2: The radiator hose DID NOT get hot at 190° F nor at 200° F. This confirms that the thermostat is bad and is stuck closed. Replace the thermostat.
If you'd like to buy the factory original AC Delco thermostat (at a savings), take a look at the section: Where To Buy The Thermostat And Save.
CASE 3: The radiator hose got hot below 150° F. This confirms that the thermostat is bad and is stuck open. Replace the thermostat.
Here's why: If the thermostat were working correctly, it would not let any coolant circulate within the hose and the hose would be at ambient temperature. Since the thermostat is stuck open (or missing) the coolant starts to circulate immediately and the hose will feel warm to hot as soon as the engine has been running for a few minutes.