How to Troubleshoot a Faulty Thermostat

Video tutorial on how to determine if you have a faulty thermostat. A thermostat is the main controller of the engine cooling circuit, this is what is used to regulate the engines temperature. On a warm-up cycle, the thermostat will remain closed so the coolant does not circuit through the radiator to allow the engine to warm up faster. Once the engine and coolant reach a certain temperature, the thermostat opens up, allows the coolant to flow through the radiator, which in turn cools the coolant, that cooler coolant is pumped through the engine lowering it’s temperature.

Tools/Supplies Needed:

• new thermostat
• coolant
• seal
• tool
• infrared thermometer

A faulty thermostat will either become stuck open, stuck closed, or perhaps something in between where your temperature fluctuates large amounts. When stuck open, the engine will take a significant amount of time to warm up, especially more noticeable in cooler climate which is the currently problem with this vehicle. If the thermostat were to stick closed, then the engine would overheat and if left for a longer period of time, cause extensive damage. If it were something in between, then you’ll notice the engine not maintaining a constant temperature, running hot or cold.

As an example, the vehicle shown in the video was just running about 15 minutes ago and it is a typical winter temperature today. It did not reach normal operating temperature after being driven for about an hour. Starting the vehicle, you can see the gauge was at a certain temperature and then dropped slightly. This is from the thermostat being stuck open, the cooler coolant within the radiator was pumped through the engine, pasting by the temperature sensor, lower the gauge reading and engine temperature. It will increase as the vehicle isn’t being driven and an excessive amount of air isn’t being pushed through the radiator.

Next we can use an infrared thermometer to check the surface temperature reading before and after the thermostat. In this case the thermostat house is hidden well, so we can’t take both readings right at the housing. When using an infrared thermometer, it’s important to use similar types of surfaces as this can affect the accuracy between readings. Here I will be using the upper and lower radiator hoses, one is removing coolant from the engine and the other is pumping the cooler coolant back into the engine.

So at this top radiator hose, we are looking at about 55C or 131 Fahrenheit.

Next moving onto the lower radiator hose. Unfortunately I can’t get as close as compared to the top hose so you can see it on camera, so the temperature will be slightly lower as the sensor is measuring over a larger distance and the coolant has passed through the radiator. If you have an electric fan, it should be running as it’s controlled by a temperature sensor. You may have the tackle the issue of a mechanical fan as it’s promoting air movement when the engine is running.

This time around we’re looking at about 51C or 124 Fahrenheit. We have a slight variation between how the thermometer is angled.

If we were to continue to measure the temperature with the engine running from a cold start until it slowly warmed up, we would find the values to be the same and very close as the coolant is being pumped through the radiator.

If the thermostat were to be stuck close, making the engine run hot or overheat, then the value at the thermostat would be extremely high and the lower radiator hose would be extremely cold.

There are two types of radiator flow designs, one is down flow and the other is crossflow. Both designs will have the input for hot coolant at the top and the bottom will be the output, feeding cooler coolant to the engine.

As for a replacement overview, the coolant will need to be drained a few litres, but this will depend where the thermostat is located with regards to the engine’s cooling system design. But if you are draining the coolant, it is best to probably replace all the coolant as a preventative maintenance step. Ensure the new coolant does meet your vehicle’s requirements, along with what type of climate you live in.

Most radiators have a drain to remove the coolant, if it does not then you can remove the lower radiator hose.

Styles will vary, but in this situation the upper radiator hose on the thermostat housing needs to be remove, then the thermostat housing and finally you have access to the thermostat. A new thermostat will need to be purchased, along with a seal.

Once that new thermostat is installed, replenish the coolant system and bleed the system to remove any air. Bleeding procedures will vary between vehicles and this is needed as air can become trapped in the cooling system causing issues.

A proper functioning thermostat will reach it’s specified temperature which is normally around 175 Fahrenheit or 80C to 200 Fahrenheit or 93C and open within a few degrees of that spec.

Now as a demonstration once it has been replaced. Now the vehicle is able to warm up and maintain a constant temperature in a cold climate.

Using the infrared thermometer again, as the vehicle slowly increases in temperature, the upper and lower radiator hoses remain a colder temperature. You may notice a slight increase in the upper radiator hose past the thermostat as it may have a small bypass which allows the coolant to circulate a small amount. Upper radiator hose is about 32C or 90 fahrenheit and the lower is about 28C or 82F.

Until it reaches a certain temperature, it opens and you’ll see the upper radiator hose increase in temperature, the coolant will flow through the radiator cooling down and the temperature will be much cooler on the lower radiator hose. Again taking a reading we’re looking at about 43C or 109F on the upper hose and about 28C or 82F on the lower hose.

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