How To Test The Thermostat (Ford 3.0L, 3.8L)

By: Abraham Torres-Arredondo
Created: 14 August 2011
Updated: 12 December 2022
Articl Id: 77

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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:

1
The thermostat is the one tasked with keeping the engine from going below 190° Fahrenheit.
2
The fan motor (or fan motors) are tasked with keeping the engine below 227° Fahrenheit.
3
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.
4
As the coolant circulates in the engine only, its temperature increases and when it reaches 190° F, the thermostat opens.
5
With the thermostat now open, the coolant can now circulate between the radiator and the engine.
6
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.
7
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.
8
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.
9
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.

Other Tips And Suggestions

Keeping on top of the cooling system, on your 3.0L, 3.8L equipped Ford car, mini-van or pick up will save you a ton of money. Yes, maintenance does involve spending time and money, but if you let that little coolant leak go, or continue driving the vehicle even tho' it's overheating, the end result will be a major financial headache.

Overheating and coolant leak issues usually lead to blown head gaskets if they are not attended to immediately.

The following are personal recommendations:

If the radiator plastic tanks crack, replace the radiator or fix it as soon as possible. These cracks will also prevent the cooling system from pressurizing and over time and many many miles, you'll have a blown head gasket issue.
Flush the anti-freeze at least twice a year. From personal experience, I don't buy into the hype that anti-freeze can last 100,000 miles and I suggest you don't either.
If you don't or can't flush the anti-freeze, at least do a radiator drain and fill.
Replace the thermostat as a maintenance issue, just like you would replace a fuel filter. You don't have to replace it every year, but if you plan to keep your car for a while, replace it every two years.
Replace the radiator upper and lower hoses if they are bulging at the hose clamps or the rubber the hose is made of is too spongy.
If your vehicle overheats while you're driving, pull over into a safe spot and have it towed home or to your trusted automotive repair shop.

More Test Articles

I've written quite a few 3.0L, 3.8L 'how to' tutorials that may help you troubleshoot the issues on your Ford car, pick up or SUV. You can find the complete list at: Ford 3.0L, 3.8L Index Of Articles.

Here's a sample of the Ford 3.0L, 3.8L V6 articles you'll find: