How To Test The Thermostat (1997-2010 4.6L, 5.4L Ford F150, F250, F350)

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 thermostat is the one tasked with keeping the engine from going below 190° Fahrenheit.
• The fan and fan clutch are tasked with keeping the engine below 225° Fahrenheit.

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. When you start the engine, the thermostat is closed, preventing engine coolant from flowing to the radiator. This helps the engine warm up quickly.
2. As the coolant circulates solely within the engine, its temperature rises. Once it hits 190° F, the thermostat opens.
3. Once the thermostat opens, coolant starts circulating between the radiator and the engine.
4. Despite the coolant now circulating between the engine and radiator, its temperature will continue to rise.
5. The rotating fan and fan clutch now step into the picture, as they ensure the coolant temperature doesn't exceed 227° F.
   • The fan, fan clutch, and the thermostat now work together to maintain the engine temperature between 190° and 225° F.
6. If the coolant temperature drops below 190° F, the thermostat closes preventing coolant flow to the radiator (to keep the engine within the specified range) and the cycle repeats itself.

The cool thing is that you can observe these temperature changes with your scan tool in Live Data mode and this is the way this tutorial shows you how to test the engine coolant thermostat.

Other Tips And Suggestions

Regular maintenance of the cooling system in your 4.6L or 5.4L V8 equipped Ford pickup can save you a lot of money in the long run. While maintenance does require time and money, ignoring a small coolant leak or continuing to drive the vehicle with an overheating engine can lead to a major financial headache.

Unattended overheating and coolant leaks (even small ones) usually result in a blown head gasket problem, so it's important to address these issues right away.

The following are personal recommendations:

• Ford uses plastic intake manifold gaskets on the 4.6L and 5.4L V8 engines, which are notorious for deteriorating, breaking, and leaking coolant. While this typically doesn't cause overheating, it prevents the cooling system from pressurizing correctly. This can create internal hot spots in the engine, eventually leading to blown head gaskets.
  • Whenever you pop the hood, always check the ends of the intake manifold for any signs of coolant.
• If you notice cracks in the radiator's plastic tanks, replace or repair the radiator as soon as possible. These cracks can also prevent proper pressurization and, over time, may cause head gasket issues.
• Flush the coolant at least every three years. From personal experience, I don't believe the claim that coolant can last 100,000 miles, and I suggest you don't either. If a full flush isn't possible, at least do a radiator drain and refill.
• Replace the thermostat as part of regular maintenance, similar to replacing a fuel filter. While it doesn't need to be replaced yearly, doing it every three years is a good practice.
• Replace the upper and lower radiator hoses if they're bulging at the clamps or the rubber feels too spongy.
• If your vehicle overheats while driving, pull over to a safe location and have it towed to your home or a trusted repair shop.

More 4.6L, 5.4L Ford F150, F250, And F350 Tutorials

There are quite a few 4.6L, 5.4L V8 F-Series pickup 'how to' tutorials and wiring diagrams you can find here:

• 4.6L, 5.4L Ford F150, F250, And F350 Index of Articles.

E-Series Vans: You can find all of the available tutorials and wiring diagrams for the 4.6L and 5.4L V8 E-Series vans in this index:

• 4.6L, 5.4L Ford E150, E250, And E350 Index of Articles (at: troubleshootmyvehicle.com).

Crown Vic/Grand Marquis: You can find all of the available tutorials and wiring diagrams for the 4.6L V8 Ford Crown Vic and Mercury Grand Marquis in this index:

• 4.6L Crown Victoria And Grand Marquis Index of Articles (at: troubleshootmyvehicle.com).