If you’ve driven recently, wait until the engine cools completely before starting. Otherwise, the test may be inaccurate. Don’t open the radiator cap immediately after running your vehicle since it could be extremely hot and cause burns.
If you notice coolant flowing into the radiator right when you start your vehicle, then the thermostat is stuck in the open position. Don’t run your vehicle in an enclosed space since it will create harmful fumes.
If you can’t tell if the coolant in the radiator is flowing, shine a flashlight inside so you can see it better.
The temperature that your engine overheats will vary depending on the make and model of your vehicle, though most engines overheat above 220 °F (104 °C).
If you’re worried about your vehicle possibly moving, put blocks in front of each tire to keep it in place.
You can buy an infrared thermometer from your local hardware store or online.
The temperature for the radiator hose should be lower than the temperature of the engine. If they are the same, then the thermostat is stuck in the open position. Take the temperature of the lower radiator hose as well. If there’s an imbalance between them, it’s probably a sign that your car’s thermostat is closed.
Watch the engine temperature gauge on your vehicle’s dashboard to see if it enters the red danger zone, which means the engine is too hot and could overheat. If you notice your vehicle overheating, turn off the engine right away to avoid causing further damage.
If the hose is the same temperature as the engine but it’s still overheating, then there may be another problem with your vehicle instead of the thermostat.
Vehicle coolant can be toxic, so keep it away from children or pets. Don’t drain the coolant unless your vehicle and engine are completely cool or else it could be extremely hot and cause burns. If your radiator doesn’t have a drain plug, use a screwdriver to loosen the clamp around the lower radiator hose before pulling it out of place. Then, drain the coolant from the lower hose into the bucket.
Place a shop cloth or rag in the end of the hose to prevent dirt or debris from getting inside it.
If it’s difficult to grab the thermostat by hand, use a pair of needle-nose pliers or a screwdriver to pry it out. The bolts for your thermostat housing may be different lengths. Make note of where each bolt went inside the housing so you can reattach it correctly later.
Don’t let the thermostat rest on the bottom of the pot since it could affect the temperature on the reading. You can also hang the thermostat on the side of the pot with a piece of string or metal clip.
You can get replacement thermostats from an auto supply store or online. Make sure the thermostat matches the model vehicle you have.
