Testing the engine coolant thermostat on your V8-equipped Ford Crown Vic or Mercury Grand Marquis is quite straightforward. In this guide, I'll show you how to test it using a scan tool with Live Data capabilities.
By following these steps, you can quickly determine if the thermostat is working properly or if it's causing engine overheating or cooling issues.
Contents of this tutorial:
APPLIES TO: This tutorial applies to the following vehicles:
- 4.6L Ford Crown Victoria: 1995, 1997, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011.
- 4.6L Mercury Grand Marquis: 1995, 1997, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011.
Important Suggestions And Tips
Tip 1: To use this tutorial, you'll need a scan tool with Live Data capability to monitor the coolant temperature. This method is the quickest, easiest, and safest way to check if the thermostat is faulty.
You don't need a Ford factory scan tool or an expensive professional-grade scan tool. A generic scan tool will do great and I've written this tutorial with this tool in mind (Don't have a scan tool? Need a scan tool? Check out my recommendation: Actron CP9580 Scan Tool).
Tip 2: Never open the coolant expansion tank when the engine is warm or hot, regardless of whether it's running or not.
Opening the tank when the engine is hot can result in hot coolant spraying out under pressure, causing severe burns. Always prioritize safety.
Tip 3: Begin your thermostat test with a completely cold engine. One of the main reasons this is important is for the accuracy of your test result. The other is for safety reasons.
If the engine is hot, let it cool down fully before testing. You'll know the engine is cold enough when the upper radiator hose is cold to the touch.
Symptoms Of A Bad Thermostat
The thermostat in your engine's cooling system will fail in one of two ways:
- Stays Stuck Open: This type of failure allows constant coolant circulation between the engine and radiator.
- Stays Stuck Closed: This type of failure prevents coolant circulation between the engine and radiator.
When the thermostat is stuck open, you'll see:
- Bad gas mileage.
- The heater doesn't work (winter time).
- The temperature gauge consistently shows a low reading or no reading at all.
- Your Crown Vic (Grand Marquis) fails the state emissions test.
On the other hand, when the thermostat is stuck closed, you may notice:
- The engine overheats.
- The temperature gauge hits its maximum sweep.
- The temperature warning light illuminates to notify you of an engine meltdown.
Checking The Thermostat's Performance
Before we get started, I want to remind you of two very important things:
- Start your coolant thermostat test with a completely cold engine. This is important, not only for safety reasons, but for the sake of the accuracy of the thermostat test.
- Don't open the coolant expansion tank for any reason once the engine starts to warm up.
Alright, these are the test steps:
- 1
Check the coolant level in the coolant expansion tank.
Top off the coolant expansion tank with water or coolant if necessary. - 2
Connect your scan tool and open its Live Data feature.
- 3
Check the temperature of the upper radiator hose with your hand.
NOTE: If the engine is cold (at ambient temperature), this hose will be cold. If the hose isn't cold, then you'll need to let the engine cool down further. - 4
Check the temperature of the coolant on your scan tool.
On most scan tools this PID will simply say: Coolant °F or Coolant °C (see the photo in the image viewer above).
If you started out with an engine at ambient temperature, your scan tool should report a coolant 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. - 5
Start the engine and let it warm up.
- 6
At 150° Fahrenheit (65° C) on the scan tool, check the temperature of the upper radiator hose by lightly touching it with your hand.
What you're looking for is to make sure that the upper radiator hose is still cold. In other words, it should be the same temperature you noticed in Step 2, if so, continue to step 6.
If the hose is hot, this confirms beyond a shadow of a doubt, that the thermostat is either missing or stuck open. You can stop your testing here and replace the thermostat. - 7
Let the engine continue running.
- 8
At 190° Fahrenheit (87° C) on your scan tool, lightly touch the upper radiator hose once again.
At this temperature point, the upper radiator hose should be hot to the touch.
If the upper radiator hose still feels cold at this point, don't worry about it just yet, continue to the next step. - 9
At 200° F (93° C) on your scan tool, lightly touch the upper radiator hose once again.
The hose should now be hot -no ifs, ands, or buts.
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: This is the normal and expected result of a properly operating thermostat since it confirms that the thermostat was closed below 190° F and that it did indeed open after 190° F to let the now hot coolant circulate to the radiator.
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 not let any coolant circulate within the hose and the hose would be at ambient temperature. Since the thermostat is stuck open (or missing) the coolant starts to circulate immediately and the hose will feel warm to hot as soon as the engine has been running for a few minutes.
