TEST 1: Thermostat Operation
OK, this is what you signed up for: the thermostat test. As mentioned in the previous page, you'll need a scan tool that can read Live Data.
Just a friendly reminder, do not open the radiator cap if the engine has been running for an extended amount of time and/or if the engine is hot or you run the risk of getting scalded by the hot coolant.
OK, this is what you need to do to get this show on the road:
- 1
Start with a completely cold engine (by cold, I mean at ambient temperature).
If the engine is completely cold, remove the radiator cap and top off the radiator with coolant if necessary. Once you've topped off the radiator, re-install the radiator cap. - 2
With the cold engine, touch the upper radiator hose with the palm of your hand and notice the temperature. The hose should be cold (ambient temperature). If the upper radiator hose is hot, then you need to let the engine cool down completely and start the test then.
It's important to check the temperature of the upper radiator hose now, since you'll compare this temperature to the temperature of the hose after the engine has reached a certain temperature (in a few test steps down the road). - 3
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).
- 4
Start the engine in your car (with the hood open) and while you keep your eyes on the coolant temperature sensor's PID values, let the engine warm up.
If you started out, as recommended with a cold engine, then the coolant temp sensor should report (on your scan tool) a 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. - 5
When the coolant temp sensor shows 150° Fahrenheit (65° C), lightly touch the upper radiator hose.
The hose should still be cold (ambient temperature). If the hose is still cold, continue to step 6.
If the hose is hot, then this confirms that the thermostat is stuck open or missing. You can stop here, since there's no further need to go on to the next steps. Replace the thermostat. - 6
When the coolant temp sensor shows 190° Fahrenheit (87° C), lightly touch the upper radiator hose.
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. - 7
Let the engine run till the scan tool reports a temperature of 200° F (93° C) and lightly touch the upper radiator hose again.
If the thermostat is functioning correctly, the hose will be hot.
Let's interpret the result of your test:
CASE 1: The radiator hose got hot at 190° F and stayed hot at 200° F. This confirms that the thermostat is functioning correctly.
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.
