How To Test The Thermostat (Ford 4.0L)

Engine Thermostat Basics

In case you're wondering why the engine has one and needs one, this section will 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 (190° to 225° Fahrenheit) to effectively control the air/fuel mixture and emissions that the engine produces. If the engine is able to stay within this temperature range, it pollutes less and gets the best gas mileage possible.

The reason to keep the engine within this temperature range is that the cooler the engine is, the more gasoline it needs to keep running smoothly (and unfortunately, at these colder 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. 1

   The thermostat is the one tasked with keeping the engine from going below 190° Fahrenheit.

2. 2

   The fan and fan clutch are tasked with keeping the engine below 225° Fahrenheit.

3. 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. 4

   As the coolant circulates in the engine only, its temperature increases and when it reaches 190° F, the thermostat opens.

5. 5

   With the thermostat now open, the coolant can now circulate between the radiator and the engine.

6. 6

   Even tho' the thermostat has opened, the temperature of the coolant will continue to increase. This is where the fan and fan clutch come into play, since they won't allow the coolant's temperature to go beyond 227° F.

7. 7

   The fan (which in this case is attached to a fan clutch) and the thermostat will work together to keep the engine's temperature between 190° and 225°F.

8. 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. 9

   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 4.0L equipped Ford vehicle will save you a ton of money. Yes, maintenance involves 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:

• On late model 4.0L engines, the intake manifold gaskets are very prone to breaking and leaking antifreeze. Although this issue does not usually lead to overheating problems, it doesn't let the cooling system pressurize like it's designed to, this lead to internal hot spots in the engine that eventually lead to blown head gaskets.
• So, any time you pop the hood on your car, always check the ends of the intake manifold to see if they are coolant.
• 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 once every two/three years. 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, then 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 Ford 4.0L 'how to test' tutorials that may help you troubleshoot the issues on your Ford pick up, van or SUV. You can find the complete list at: Ford 4.0L Index Of Articles.

Here's a sample of the Ford 4.0L articles you'll find:

• How To Test For A Blown Head Gasket (Ford 4.0L).
• How To Test The Fuel Injectors.
• How To Test Compression (Ford 4.0L).
• How To Test The Ignition Coil Pack (at: easyautodiagnostics.com).