Your vehicle's thermostat opens to let the coolant flow through the radiator and cool the engine. However, if the thermostat sticks in the closed position, the coolant will not make it through the radiator, causing the engine to overheat. To quickly detect if the thermostat is stuck, you need to watch the coolant flow through the radiator. For a more accurate test, you should measure the temperatures of the engine and the hose leading to the radiators to see if they are similar. If you want a method to check the thermostat directly, you must remove it from your vehicle and heat it in a pot of water to see if it opens.
Method 1 of 3: Observe the Coolant Flow
Step 1. Remove the radiator cap
Open the hood of your vehicle and support it so that it does not fall during the test. You should locate the radiator, which looks like a narrow silver or black box and sits at the front of the vehicle directly behind the grill. Find the circular metal cap on the left or right side of the radiator and turn it counterclockwise to loosen it.
- If you've driven recently, you should wait until the engine cools completely before starting. Otherwise, the test may be inaccurate.
- Do not open the radiator cap immediately after turning off the vehicle, as it could be extremely hot and cause burns.
Step 2. Start the engine and let it idle for 10-20 minutes
Leave the vehicle parked and start it so the engine starts. From a cold start, your vehicle's thermostat will remain closed and you won't notice the coolant flowing into the radiator. Let the vehicle run for about 10-20 minutes so it can reach the optimum operating temperature.
- If you notice that the coolant is flowing into the radiator right after starting the vehicle, then the thermostat is stuck in the open position.
- Avoid starting the vehicle in a confined space, as it will produce noxious fumes.
Step 3. Look at the radiator to see if the coolant is flowing through it
After 10 to 20 minutes, keep your head at least 1 to 2 feet (30 to 60 cm) away and look at the radiator where you removed the cap to check the coolant. As the engine warms up, the thermostat should open and allow the coolant to flow from the radiator to the engine. If you see the coolant flowing through the radiator, then the thermostat has opened and is working properly. If not, it is most likely blocked.
If you can't tell if the coolant in the radiator is flowing, you can turn on a flashlight to get a better view
Step 4. Check if the engine temperature gauge is in the red danger zone
While your vehicle is still running, you should check the engine temperature gauge on your dashboard to see if it is rising. If the engine temperature is in the red section of the gauge and the coolant is not flowing to the radiator, then the thermostat is likely the cause of the problem.
Engine overheating temperature can vary by vehicle make and model, although most engines overheat above 104 ° C (220 ° F)
If the engine is overheating but coolant is still flowing through the radiator, then your vehicle is likely overheating for a different reason (for example, a faulty water pump or a leaky cooling system).
Method 2 of 3: Measure Engine and Hose Temperatures
Step 1. Start the vehicle to let the engine idle
Take your vehicle outside so the exhaust can escape. Park the vehicle or activate the parking brake before starting the engine. Let the engine run throughout the test so the coolant can warm up inside.
If you are concerned about your vehicle moving, you can place blocks in front of each tire to keep it in place
Step 2. Check the engine temperature with an infrared thermometer
Take an initial temperature reading as soon as you start the vehicle. Locate the thick black hose that runs from the side of the radiator to the top or side of the main engine block. Point the infrared thermometer where the hose connects to the motor and pull the trigger to measure the temperature. You should wait until the number on the thermometer stabilizes before recording the temperature.
You can buy an infrared thermometer at your local hardware store or online
If you don't have an infrared thermometer, you can also lightly touch the area with your hand to check how hot it is, but this will not be as accurate. Avoid keeping your fingers on the motor for too long, as you could burn yourself.
Step 3. Take the temperature of the upper radiator hose
Point the thermometer at the black radiator hose connected to the engine so that it is about 3 to 4 inches (8 to 10 cm) away from where you first recorded your measurement. Squeeze the trigger on the thermometer to measure the temperature of the hose. Write down the measurement you detected so you don't forget it later.
The temperature of the radiator hose must be lower than that of the engine. If they are the same, then the thermostat is stuck in the open position
Step 4. Measure the temperatures again after 10-15 minutes
Let the engine continue to run for at least 10-15 minutes so the engine can warm up to the optimum operating temperature. Record the measurements from the same places where you took them earlier and record the results so that you can compare them.
Look at the engine temperature gauge on your vehicle's dashboard to see if it is in the red danger zone, which means the engine is too hot and could overheat. If you notice that the vehicle is overheating, you should immediately shut down the engine to avoid causing further damage
Step 5. Check if the measurements are within 18 ° C (10 ° F) of each other
Observe the temperature of the engine and top hose after the vehicle has run for 10 to 15 minutes. If the temperatures are within 10 ° F (18 ° C) of each other, then the thermostat is working properly. However, if the hose is still substantially cooler than the engine, then the coolant is not flowing through it and the thermostat is closed.
If the hose is the same temperature as the engine, but is still overheating, then there is likely another problem with the vehicle instead of the thermostat
Method 3 of 3: Test the thermostat in hot water
Step 1. Drain the coolant from the radiator
Crawl under the front of the vehicle to position yourself under the radiator. Find the drain plug (the white or black plastic wing nut) near the bottom or side corner of the radiator tank. Place a bucket under the drain plug to prevent coolant from spilling out when draining the radiator. Unscrew the drain plug and let the coolant flow into the bucket until the radiator is empty. Screw the drain plug onto the tank once you're done.
- Vehicle coolant can be toxic, so keep it away from children or pets.
- Avoid draining the coolant unless the vehicle and engine are completely cool. Otherwise, it could be extremely hot and burn you.
- If your radiator does not have a drain plug, you can use a screwdriver to loosen the clamp around the lower radiator hose before removing it from its place. Then drain the coolant from the lower hose into the bucket.
Step 2. Disconnect the upper radiator hose from the engine
The thick black hose runs from the side of the radiator and connects to the top or side of the engine block with a pipe clamp. Use a screwdriver to loosen the nut on the tube clamp, then remove the hose from the motor. Set the end of the hose aside for now so you can access the thermostat housing on the engine.
Put a cloth or rag at the end of the hose to keep dirt or debris from getting in
Step 3. Remove the thermostat after disconnecting the thermostat frame from the engine
The thermostat frame is the block-shaped metal part connected to the engine block to which the hose was connected. Use a socket wrench to loosen the bolts that hold the frame in place and set them aside so you don't lose them. Pull out the engine frame to expose the thermostat, which looks like a small metal cylinder with a spring and 2 metal rings around it. Take the end of the thermostat and pull it out.
- If you find it difficult to hold the thermostat in your hand, you should use needle-nose pliers or a screwdriver to pry it out.
- The bolts on the thermostat frame can be of different lengths. Make a note of where you placed each screw inside the frame so that you can reattach them correctly later.
Step 4. Submerge the thermostat in a pot of water to prevent it from touching the bottom
Use a pot deep enough to completely submerge the thermostat in the water. Fill the pot with tap water and place it on a stove. Hold the top of the thermostat with tweezers so that it is under the water.
- Don't let the thermostat sit at the bottom of the pot as it could affect the temperature reading.
- You can also hang the thermostat on the side of the pot with a piece of string or a metal clip.
Step 5. Heat the water until it reaches approximately 90 to 104 ° C (195 to 220 ° F)
Turn the stove on to high heat and place a thermometer in the pot of water so it can record the temperature. Continue heating the water until it reaches 195 to 220 ° F (90 to 104 ° C), which is the temperature where the engines work best. Turn off the heat once the thermometer registers a reading within range.
Look at the thermostat to see if it is at the indicated operating temperature. If so, the thermostat should open once it reaches that temperature.
Step 6. Remove the thermostat from the pot to check if it is open
Use tweezers to remove the thermostat from the water and place it on absorbent paper to dry it. As the thermostat heats up, the spring should compress and the center ring should separate from the outer ring to leave a space in the middle. If the spring is compressing and you detect a gap, then the thermostat has opened and is working fine. If the thermostat does not have a gap between the circular hardware, then it is stuck and needs to be replaced.
You can find replacement thermostats at an auto supply store or online. Make sure the thermostat matches your vehicle model
- Do not try to remove the radiator cap or drain the coolant immediately after turning off your vehicle, as it will be extremely hot and could cause severe burns.
- Avoid driving your vehicle if it overheats frequently, as it could cause permanent engine damage.