How to Determine If a Car's Engine Thermostat Is Stuck
What Does the Engine Thermostat Do?
If a car's engine is too hot, it can actually warp and become damaged. But, when it's cold, it doesn't run efficiently. So, it is important to keep the engine at the right temperature, which is where the thermostat comes in. An engine's thermostat regulates the temperature of the engine by controlling coolant flow.
The coolant does exactly what you imagine—it cools the engine down. The engine thermostat can open and close. When it's open, the coolant flows through, lowering the temperature of the engine. When it's closed, the coolant is blocked until the engine warms up. Engine manufacturers will install a thermostat that opens at the car's operating temperature (usually either 180 or 212°F, or 82 or 100°C). While the thermostat controls coolant flow, the radiator's job is to cool the fluid. The flow of coolant does not cycle through the radiator unless it is hot and needs to be cooled.
- When an engine is cold, the radiator fluid is cold, so the thermostat is in the closed position. In this position, coolant will only flow in the engine. It will not be able to leave the engine to cycle through the radiator to be cooled.
- When the engine starts up, and the coolant warms. When the coolant reaches a specific temperature, the thermostat opens. Each thermostat is gauged to open at a specific temperature. Once it opens, the coolant can circulate through to the radiator to be cooled.
The Other Radiator
There's another radiator-type component under the dashboard called the heater core. When the car is warmed up and the heater is turned on, a fan blows air over the heater core. The heat produced by the hot radiator fluid runs through the heater core and is dispersed into the cabin of the car, heating the car's interior.
How to Tell If Your Thermostat Stopped Working
When the thermostat gets stuck and stops working completely, there are several indicators you can check to determine the culprit. See the table below.
Thermostat Symptom Tests and Explanations
Cold engine test: Open hood and remove radiator cap. Start engine. Observe coolant action. It should not be moving.
When an engine is cold, the thermostat should be closed, thus bypassing the radiator.
Warm (not hot!) engine test: Open hood and remove radiator cap. Start engine. Observe coolant action. It should be swirling vigorously.
When an engine is warm, the thermostat is in the open position. This allows the coolant to flow through the radiator causing vigorous movement.
Cold engine test: Start the engine. Time how long it takes the engine to warm up enough to produce heat when the heater is turned on.
If it takes more than five minutes for the heater to produce heat, it is a sign that the thermostat is stuck open, allowing all the coolant to flow all the time. This causes it to take a good deal longer for the engine to warm up.
You can find your car's thermostat by starting at the upper radiator hose and following it to where it enters the top of the engine. The thermostat housing is right where the radiator hose meets the engine housing. It looks like half of a metal baseball.
When the thermostat becomes stuck, it is reasonable to simply replace it. Also, when the thermostat housing is removed, the housing gasket should be replaced even if you're just checking it. A housing gasket needs to be able to seal fully, and it won't if it's been used.
The thermostat and housing together cost less than $20. Keep reading, however, to find out how to be sure that your thermostat is the problem.
Removal and Testing
If you are still unsure whether the thermostat is stuck, perform the following procedure to get an accurate diagnosis.
- Disconnect the negative battery cable.
- Drain some of the coolant (a gallon should be enough) so the fluid won't pour out when you perform the next step.
- Remove the upper radiator hose.
- Remove the thermostat housing.
- Remove the thermostat.
- Locate and write down the temperature stamped on the lip of the thermostat.
- Fill a pot with cold water and a thermometer and place on a stove burner.
- Place the thermostat into the cold water.
- Turn the burner on.
- Watch the thermometer. When the temperature rises to the number you recorded in step #6, the thermostat will start to open. If it does not, or if it doesn't open until reaching a different temperature (hotter usually), then the thermostat is bad and should be replaced. If it opens at the designated temperature, then the thermostat is just fine.
Another Way to Tell If the Thermostat Is Faulty
The thermostat is such a vital component to the well-being of your vehicle, yet it is so cheap. Replacing it is far less costly than repairing a warped engine. A warped engine must be removed and either resurfaced, if the damage isn't terribly bad, or completely replaced.
Sometimes car owners remove but do not replace their car thermostat. This is not a good idea, because a cold engine does not function efficiently and a constant, slow warm-up will take years off of the engine's life.
Be safe and replace!
Thanks for stopping in!
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
Questions & Answers
My car is not running hot, but I have no heat going inside my car. Is this my thermostat not working?
It's your heater core or the fan that blows across the heater core. Replace the heater core and/or fan and you will have heat.Helpful 72
I let my car warm up, and when I get going, I'll turn my heater on and then my temperature gauge goes back down to cool. Then, I'll turn the heat off, and the gauge goes back up. What could be happening?
This anomaly is normal. In some vehicles, it is more obvious than in others. Some drivers use this anomaly to aid them if the car starts to overheat. To understand the cause a little cooling system background is in need. The heater in a vehicle uses the radiators cooling system. Yes, really. The radiator fluid is routed through the heater core in your vehicle and back to your engine. Check back for a hub entitled Vehicle Engine Cooling System. It will go into depth about what you are experiencing.Helpful 28
My temperature gauge is between midway and cold and never seems to get hot but I have heat in the cabin. Is my car's thermostat going bad?
Between midway and cold sounds like a great reading to me. Is it normally hotter? If so, yes, check your thermostat. As long as the engine is running cool that's fine. You don't want it running cold, or hot. Anywhere in between, especially towards the cold side is good.Helpful 23
If I have a 98 Dodge Ram 5.9 my gauge stays between cold and halfway after warm up but the heater only blows cold air could this be the thermostat stuck open?
If the thermostat was stuck open the heater would eventually warm up. In between cold and halfway sounds like a fine engine temperature. This is more likely a problem with the heater core.Helpful 2
I have a 2000 Honda Civic. Can a thermostat that is stuck closed cause an old radiator to crack?
Not necessarily. It would crack from overheating or freezing. The thermostat could be the culprit, but low coolant would be more likely. Coolant not only cools the engine it also prevents the fluid from freezing. Do you have coolant in your system or just plain water? Water will freeze, expand, and crack engine parts. That's what freeze plugs are all about. They are supposed to pop out before your engine cracks if it becomes too cold.Helpful 7
© 2012 Joanna