How To Test The Thermostat (1995-2011 4.6L Crown Victoria, Grand Marquis)

Engine Thermostat Basics

If you're curious about why the engine has a thermostat and what it does, this section should clarify things (don't worry, I won't get too technical).

The PCM (Powertrain Control Module) needs the engine to stay within a specific temperature range to effectively manage emissions and maximize gas mileage. This ideal range is between 190° to 225° Fahrenheit (87° to 107° Celsius). To make this possible, the engine uses the coolant thermostat and the radiator fan motor.

To be a bit more specific:

• The thermostat's role is to prevent the engine from dropping below 190° Fahrenheit.
• On the other end, the electric fan motor works to keep the engine temperature from exceeding 225° Fahrenheit.

The reason that the engine needs to be within this temperature range is simple: a cooler engine requires more gasoline to run smoothly, which unfortunately increases pollution. However, as the engine warms up and reaches the optimal temperature range, it needs less fuel to perform efficiently and produces fewer emissions.

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. When the PCM sees the ECT sensor reporting a temperature of around 220° F, it commands the radiator fan ON.
6. The engaged fan now pulls more air across the radiator, cooling the coolant within it.
7. Since the thermostat is open, the cooler coolant in the radiator enters the engine to cool it down.
8. 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 Ford Crown Vic or Mercury Grand Marquis will save you a lot of money and headaches 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 Crown Vic And Grand Marquis Tutorials

There are quite a few 4.6L Ford Crown Victoria (Mercury Grand Marquis) 'how to' tutorials and wiring diagrams that you can check out here:

• 4.6L Crown Victoria And Grand Marquis 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.

F-Series Pickups: You can find all of the available tutorials and wiring diagrams for the 4.6L And 5.4L V8 F-Series pickups in this index:

• 4.6L, 5.4L Ford F150, F250, And F350 Index of Articles (at: easyautodiagnostics.com.com).