TEST 2: Electric Fan Motor Operation
This section will help you to further confirm what's causing your vehicle to overheat by checking the operation of the fan motor (or motors).
The radiator fan motor is programmed to run (and cool the engine down) when the engine coolant temperature reaches a temp of 225 °F.
The cool thing is that you and I can confirm that the radiator cooling fan does OR does not come on at the programmed temperature with a scan tool in Live Data mode.
This is how we'll check the operation of the radiator fan motor in this test section.
NOTE: Before you start, you need to make sure that the A.C. is turned off.
Alright, this is what you'll need to do:
Restart the engine in your car and continue checking the coolant temperature your scan tool is registering (in Live Data mode).
As the engine runs, the coolant temperature (your scan tool is reporting) will reach 225° F (107° C). When it does reach this is the temperature, at least one of the two radiator fan motors should start (if equipped with two).
If the fan motor has not turned on by 227° F (108° C), then turn off the engine (any temperature above 230° F and the engine is considered to be overheating).
If the fans did come on, then they should turn off when the coolant temperature reaches 200° F.
Let's interpret the result of your test:
CASE 1: The fan (or fans) did NOT come on at 225° F - 227° F. This confirms that the overheating problem, your vehicle is experiencing, is due to a fan motor issue.
Now, it's beyond the scope of this article to further test the fan motor(s), but at least now you know that you do have a fan motor issue. The next steps are for you to test the fan motor, fan motor relay and fan motor fuses.
CASE 2: The fan (or fans) did come on at 225° F - 227° F, but the vehicle continues to overheat. If you have also confirmed that the thermostat is good (in TEST 1), then the most likely cause of the overheating issue is a blown head gasket.
You can find three specific tests, to verify a blown head gasket in this article:3.8L Blown Head Gasket Test.
CASE 3: The fan (or fans) did come on at 225° F - 227° F and the temperature DID go down. This confirms that the fan(s) and thermostat are OK. No further testing is needed.
Engine Thermostat Basics
In case you're wondering why the engine has one and needs one, well this section might shed some light on the subject (don't worry, I won't go into minute technical mumbo jumbo).
In a nutshell, the thermostat's job is to help regulate the engine's temperature. The PCM needs to have the engine within a certain temperature range to effectively control the emissions that the engine produces and to help you get the maximum amount of gas mileage possible. This temperature range is between 190° to 225° Fahrenheit (87° to 107° Celsius).
The reason for this is that the cooler the engine is, the more gasoline it needs to keep running smoothly (and unfortunately, at these temps, it pollutes more). As the engine heats up and reaches the optimal temperature range described above, the engine needs less and less fuel to maintain it's optimal performance and of course pollutes less.
In a nutshell, here's how they both keep the engine from overheating and running optimally:
The thermostat is the one tasked with keeping the engine from going below 190° Fahrenheit.
The fan motor (or fan motors) are tasked with keeping the engine below 227° Fahrenheit.
When you start your vehicle, the thermostat is closed, thereby keeping the coolant from circulating to the radiator. This allows the engine to warm up faster.
As the coolant circulates in the engine only, its temperature increases and when it reaches 190° F, the thermostat opens.
With the thermostat now open, the coolant can now circulate between the radiator and the engine.
Even tho' the thermostat has opened, the temperature of the coolant will continue to increase. When the coolant's temperature reaches 227° F, the fan or fans come on.
The fans will bring down the temperature of the coolant (inside the radiator) which is circulating to the engine (due to the open thermostat). Once the temperature decreases down to around 200° F, the PCM then turns off the fan or fans.
If the coolant's temperature is brought down below 190° F, the thermostat will close. This helps keep the engine's temperature within the desired temperature range.
With the fans off, the coolant now begins to absorb more heat and the process I've described above repeats itself continually the whole time the engine is running.
The cool thing is that you can observe these temperature changes with your scan tool in Live Data mode and this is the way I'm gonna' show you how to test the thermostat.