OK, this is what you signed up for, the actual thermostat test, but before you start the test on your pick up (van or SUV), remember two very important things:
1.) You need to start the test with a completely cold engine. This is not only for safety reasons, but for the sake of an accurate test result too.
2.) Don't open the radiator cap for any reason once the engine starts to warm up.
OK, I'll stop yakking and this is what you need to do:
Check that the radiator is completely full by removing the radiator cap and checking to see if it's topped off.
Once you're done checking and/or topping of the radiator, re-install the radiator cap.
Check the temperature of the upper radiator hose with your hand.
The upper radiator hose should be cold to the touch. By cold, I mean at ambient temperature.
Next, hook up your scan tool to the vehicle and get to its Live Data mode. Scroll down to the PID for the coolant temperature Actron CP9580 Scan Tool).
On most scan tool this PID will simply say: Coolant °F or Coolant °C (see the photo in the image viewer above).
Start the engine and let it warm up, while you observe the coolant temperature on your scan tool.
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 coolant temperature sensor should report a temp reading between 95 to 105° F.
The coolant temperature will eventually climb to 150° Fahrenheit (65° C) on your scan tool. When it does, check the temperature of the upper radiator hose by hand one more time by lightly touching it.
If all is OK, the hose should still be cold (ambient temperature). In other words, it should be the same temperature you noticed in Step 2... if so, continue to step 6.
If the hose is hot, this immediately tells you that the thermostat is either missing or stuck open. You can stop here and replace the thermostat.
Continue monitoring the coolant temperature on your scan tool. When the coolant temperature reaches 190° Fahrenheit (87° C), lightly touch the upper radiator hose once again.
At 190° F (87° C), the upper radiator hose should be warm to hot to the touch.
If the upper radiator hose still feels cold at this point, 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, no and ifs or buts.
Alright, whether the upper radiator hose is hot or not... turn the engine off and let's interpret your test 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.
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.
“Remember, this hobby is supposed to be your therapy, not the reason you need therapy.”