TEST 1: Thermostat Operation
OK, here's the actual test but before you start, I just want to remind you of two very important things:
One, start with a completely cold engine. By cold I specifically mean that the engine should be at ambient temperature. Even a slightly warmed up engine will skew the results of your test.
Two, don't open the radiator cap once the engine has warmed up, there's simply no need to. More importantly, this is a safety precaution.
Check the level of the coolant inside the radiator (not the coolant reservoir).
Once you have topped off the radiator (if needed), re-install the radiator cap. Also, leave the hood open for the length of the entire test.
Touch the upper radiator hose with your hand to check its temperature.
The hose should be cold (ambient temperature) to the touch. If the hose feels warm to hot, the engine has not cooled enough to accurately perform the test.
Check the temperature of the coolant with your scan tool (see the photo in the image viewer).
If you haven't already, connect your scan tool to the vehicle and get to Live Data mode. Scroll down to the PID for the coolant temperature (Don't have a scan tool? Need a scan tool? Check out my recommendation: Actron CP9580 Scan Tool).
Crank and start the engine in your car. From this point forward, you'll need to keep an eye on the coolant temperature your scan tool is registering.
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.
When your scan tool indicates that the coolant temperature has reached 150° Fahrenheit (65° C), 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 as before you started the engine, if so, continue to step 6.
If the hose is hot, this tells you that the thermostat has been removed or is stuck open and that coolant is circulating to the radiator as soon as the engine starts. You can stop at this test step and replace the thermostat.
When your scan tool indicates that the coolant temperature has reached 190° Fahrenheit (87° C), lightly touch the upper radiator hose once again.
The hose should start to feel hot or should definitely feel hot (compared to its temperature at the beginning of the test).
If the hose feels cold, don't worry about it just yet, continue to the next step.
When your scan tool reports a coolant temperature of 200° F (93° C), lightly touch the upper radiator hose again.
If the thermostat is functioning correctly (and has opened to let the coolant circulate to the radiator), the hose will be hot.
Alright, you're done. 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: the thermostat is stopping the flow of coolant to the radiator below 190° F, and this is the reason why the hose feels cool to the touch of the hand. Once the engine starts to run and warm up, the coolant absorbs the heat and when it eventually reaches a temperature of 190° F, the thermostat (which is in constatn contact with the coolant) opens up. When this happens, the hose feels hot.
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 prevent the coolant from passing into the upper radiator hose (and into the radiator) and the hose would be at ambient temperature. Since the thermostat is stuck open (or missing) the coolant starts to immediately pass from the engine to radiator (thru' it) and the hose will feel warm to hot as soon as the engine starts to run.
TEST 2: Electric Fan Motor Operation
If in the previous test you verified that the thermostat is indeed functioning correctly, but your vehicle still overheats, there's a good possibility that the fan motor may not be working.
If the radiator cooling fan comes on, at the indicated coolant temperature, then you know that the it's not the cause of the overheating problem your Ford vehicle is experiencing.
You can easily check to see if the radiator cooling fan is coming on at the correct temperature with your scan tool in live data mode, and in this section I'll show you how.
This is what you need to do:
Start up the engine and let it warm up some more.
Reconnect your scan tool to the car and observe the coolant temperature it's registering (Don't have a scan tool? Need a scan tool? Check out my recommendation: Actron CP9580 Scan Tool).
The coolant will eventually hit 225° F (107° C), as the engine runs and the coolant absorbs more of its heat.
At this temperature, the radiator fan motor should start.
The coolant may reach 227° F (108° C) but should not increase further.
And so, if all is OK, your scan tool should start reporting a drop in the coolant's temperature, as the fan continues to operate, till it drops to 200° F. At 200° F, the fan motor should stop.
If the fans do not come on at 227° F, immediately turn off the engine and let the engine cool down.
This let's you know that you have a radiator fan issue. Read the interpretations below for some testing suggestions.
Let's interpret the result of your test:
CASE 1: The fan did NOT come on at 225° F - 227° F. This confirms a malfunctioning fan motor or fan motor relay or Fuse.
Testing these components is beyond the scope of this article, but at least you have a solid lead on what's causing your vehicle to overheat.
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 or a clogged radiator.
CASE 3: The fan (or fans) did come on at 225° F - 227° F but the temperature DID go down. This confirms that the fan(s) and thermostat are OK and that your vehicle is not overheating. No further testing is needed.