How To Test The Thermostat (GM 3.1L, 3.4L)

TEST 2: Fan Operation

If you have already confirmed that the thermostat is indeed working fine, and yet your car or mini-van is still overheating, this section might help.

Since you still have your scan tool connected (to the vehicle), you can check to see if the radiator fan (or fans) are turning On at the right coolant temperature.

The testing steps I'm introducing here are pretty much a continuation of the previous test.

Alright, this is what you'll need to do:

1. 1

   With your scan tool still connected from the previous test (Don't have a scan tool? Need a scan tool? Check out my recommendation: Actron CP9580 Scan Tool).
   
   Let your 3.1L or 3.4L car or mini-van's engine continue running while you keep your eyes on the coolant temperature sensor's reading on your scan tool.

2. 2

   When the coolant temperature sensor shows you that it has reached 225° F (107° C), you should see the fan or fans start to turn on.

3. 3

   The maximum temperature that the engine should reach is 227° F (108° C).
   
   At this temperature, the fan (or fans) should be turning at full blast and you should see the temperature decreasing on your scan tool's display.

4. 4

   If the fans do not come on at 227° F, immediately turn off the engine.
   
   This let's you know that you have a radiator fan issue. Read the interpretations below for some testing suggestions.

5. 5

   If the fan (or fans) did come on, then you should see the temp sensor's reading decreasing to about 200° F (93° C). At this temperature point, the fan (or fans) should turn off and the cycle will repeat.
   
   If this did not happen, read the test interpretations below for further testing tips.

Let's interpret the result of your test:

CASE 1: The fan (or fans) did NOT come on at 225° F - 227° F. This confirms that the overheating problem is due to malfunctioning fan motors.

Possible malfunctioning components could be: a bad fan motor, a bad fan motor relay, or/and a blown fan motor fuse. Although, testing these components is beyond the scope of this article, you now at least have a solid lead on what's causing your vehicle to overheat.

CASE 2: The fan (or fans) did come on at 225° F - 227° F, but the vehicle continues to overheat. If you have also confirmed that the thermostat is good, then the most likely cause of the overheating issue is a blown head gasket.

You can find three specific tests, to verify a blown head gasket in this article: How To Do A Blown Head Gasket Test (GM 3.1L, 3.4L).

CASE 3: The fan (or fans) did come on at 225° F - 227° F and the temperature DID go down. This confirms that the fans and thermostat are OK. No further testing is needed.

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. 1

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

2. 2

   The fan motor (or fan motors) are tasked with keeping the engine below 227° 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. When the coolant's temperature reaches 227° F, the fan or fans come on.

7. 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. 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

   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 3.1L, 3.4L equipped GM car 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:

1. GM, in all its wisdom, uses plastic intake manifold gaskets on the 3.1L and 3.4L engines that are super 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.
2. So, any time you pop the hood on your car, always check the ends of the intake manifold to see if they are coolant.
3. 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.
4. 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.
5. If you don't or can't flush the anti-freeze, at least do a radiator drain and fill.
6. 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.
7. 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.
8. 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 3.1L, 3.4L ‘how to’ tutorials that may help you troubleshoot the issues on your GM car or mini-van. You can find the complete list at: GM 3.1L, 3.4L Index Of Articles.

Here's a sample of the GM 3.1L, 3.4L V6 articles you'll find:

• How To Troubleshoot Misfire Codes (GM 3.1L, 3.4L)
• How To Do A Fuel Injector Resistance Test (GM 3.1L, 3.4L)
• How To Test The Fuel Pump No Start Tests (GM 3.1L, 3.4L)
• How To Test The Coil Packs (at: easyautodiagnostics.com).
• How To Test The Ignition Module (at: easyautodiagnostics.com).