How to Determine If a Car's Engine Thermostat Is Stuck

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.

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.

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.

Sounds like the heater core is not working properly. Check to see if the radiator is low on fluid, if not then I would check your heater core.

Check the heater core. It may be blocked or leaking.

I wouldn't suspect the thermostat. I'd look into a blockage in your cooling lines. Coolant isn't reaching one side of the engine. Check to see that the hoses are in place, too.

One simple way to check the thermostat is to remove the housing, which is usually located on the top, front and near the center of the engine. Then, remove the thermostat for inspection. Is it rusty? There's a small stamp on the thermostat reflecting the temperature it should open at. Next, put it in a pot of water and bring the water to a boil. Does the thermostat open when the water reaches that temp? Effects: slow warm-up and overheating.

You may want to check the heater core. It is a radiator inside the car that produces the heat when you turn the heater on. Have you noticed a smell of radiator fluid in your car? Is the windshield getting foggy for no apparent reason? If the heater core checks out then check the water pump. With the radiator cap off, turn the car on. Watch for the water flow inside the radiator. Is it swirling around vigorously as the car warms up? Or is it not moving much at all? As your vehicle warms up the thermostat opens up and the water should start churning around as the water flows through the water pump cooling the engine.

No. The halfway mark on the temperature gauge is not the "go to" position for your car. Some vehicles temperature is regularly at a quarter, some at just below the half mark, or somewhere in between. As long as the gauge does not go above the half mark your engine is fine.

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.

That wouldn't be my first choice. Check the wires first. Start at the gauge and make sure the wires haven't been pinched, detached, loose or broken.

If you do not turn on the a/c will the engine overheat where you have to pull over? Sometimes the engine will get very warm but it's okay. When the a/c is turned on it will push the engine too hot, thus engaging the fans. A/C is notorious for pushing engines too hot. Where yours is overheating enough to kick in the fans only when the a/c is started. Also, check that your thermostat is in working order.

It sounds like your thermostat was adversely affected when the hose fell out. It probably ran the fluid out, causing the thermostat to run dry which in turn froze it in one position.

The temperature sensor detects the temperature of the engine. There is no "stuck" about it. It sounds like you are having a problem with the thermostat. Is that what you were referring to when you said temperature sensor?

The thermostat may be working inappropriately. Have you pulled the thermostat and boiled it in a pot to check when and if it is opening? A vehicle can run just fine if the thermostat doesn't open. It will run on the warm side, not past halfway, and heat the interior just fine. But, when the weather gets warmer outside that thermostat stuck closed will not be able to allow the reservoir water flow and the vehicle will begin to have overheating issues. My first step would be to pull the thermostat, boil it and make sure it is opening at the proper temperature (usually marked on the lip of the thermostat.

The wind created when driving on the highway is what is keeping your engine from overheating. When you slow down the heat becomes cold most likely from low radiator fluid.

Yes, it definitely would be a stuck thermostat.

If I understand your question, the radiator cap is stuck and your gauge reflects 220 degrees? The radiator cap is "stuck" because there is a huge amount of pressure behind it. 220 degrees is overheating. Overheating causes pressure. The pressure presses on the radiator cap, thus making it very difficult, if impossible to remove because you shouldn't remove it. Allow the temperature to reduce and the cap should come off by pressing down and twisting clockwise. Do not attempt to remove the cap when the temperature is high!

It sounds more like a defective gauge to me. When the car starts to move, it sounds like the signal is lost to the temp gauge. I am also thinking maybe when you start to drive the thermostat opens and dumps all the cool radiator fluid into the engine causing it to cool down, but I don't believe it could cool it that much that fast. I'd check your connections first.

Absolutely.

Yes, there should be coolant unless its running low, leaking out, or simply not enough to see it in the bowl. If the thermostat is atop the engine when you open it up the fluid may be just below it.

It may be because you are low on radiator fluid.

There may be something stuck in the coolant lines or your thermostat is bad and when you hit the gas it's forcing it to open which allows the coolant to flow. When you release the pedal it stops flowing and heats up again.

My question for you is, "How far towards hot?" Did you have to stop for fear of over heating? If it just went a little towards the hot side then it was merely due to continued work of the engine causing it to become warmer than mere local driving. If it went a lot towards hot, but not over heating, check your antifreeze fluid level, it may be low.

Not necessarily. I am assuming your vehicle uses an auxiliary fan and not a clutch fan. Before blaming the thermostat check and make sure the fan itself doesn't have any problems.

If your thermostat is not moving, even when the ignition key is turned to the ON position, you have a wiring problem. Somewhere the wire sending the signal to the dashboard is broken, or the fuse is blown.

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.

If I understand correctly the heater only works on one side of the vehicle and not the other. That sounds very odd. Check that the coolant level is full and there are no air bubbles in the cooling system. If the coolant isn't full it can cause poor heating inside the vehicle.

Absolutely. It sounds like the thermostat is stuck in the closed or semi-closed position.

I have an old truck I pulled the thermostat from and still use it without it. It will take your engine longer to heat up without it. Try a new thermostat with fresh radiator fluid, or leave it out.

Check the fluid level in the radiator. Add radiator fluid if necessary. If there is not enough fluid running through the system when you are stopped the temperature of the fluid goes up. When you drive the wind blows across the radiator cooling the fluid down. When there is not enough fluid in the system it cannot keep the temperature down without assistance from the wind passing over the radiator.

Yep. You may want to check the rating on your thermostat as well. There are various temperature settings better for Summer and others are better for Winter.

It is unlikely a problem with the thermostat if it has proven to open and close fine in boiling water. What was the temperature of the water when it did open would be my question? Have you checked for foreign debris? If there is an obstruction blocking the thermostat from moving such as rust buildup at the site of installation. Also, there may be foreign material in the radiator fluid which becomes caught in the thermostat when the engine is running.

Yes, it sounds like the thermostat is not closing properly. An engine warms up fairly quickly because the thermostat restricts the flow of antifreeze to only the engine. Once the engine is warm the thermostat opens allowing all the antifreeze to flow. It sounds like your thermostat is staying open, thus allowing all the antifreeze to flow, all the time, making the warm up process slow.

Check the hose clamps. Something is loose. If you are positive all the hoses are secure, check the hoses themselves. There may be a crack in a hose line.

Make sure the radiator fluid is full with no bubbles. Squeeze the upper radiator hose to press out any air bubbles that may be trapped in the upper hose. The check engine light comes on when the smog system has a problem. Check the EGR valve.

This would be a heater core issue. The heater in your car uses a device called a heater core. It is very similar to a radiator, but it is located under your dashboard. Heated radiator fluid flows from the engine into the heater core and back out to the engine. When the heater is activated in your car a fan blows across the heater core causing the heat from the radiator fluid to flow into the cab. Either the heater core is bad or the fan that blows across the heater core is not working.

This article reflects on the 2007 Mustang, but the process is similar across all vehicles. It will walk you through the thermostat housing change. https://axleaddict.com/auto-repair/2007-Ford-Musta...

When you turn the heater on high can you feel the air blowing into the cab of the vehicle? If yes, you can feel air blowing, but it is cold air, then look to your heater core. The heater core is where the heat comes from. If you do not feel air blowing then your heater fan, or control panel, needs to be replaced. Check the heater core first.

That depends on several variables IE: What is the weather? Cold, Warm, Hot? Is the car being driven right away or parked while it warms up? Is the radiator full of fluid? If everything is on a medium basis, warm weather, parked while warming, full of fluid, I would say within 3 - 5 minutes it will up to warm.