Car Thermostat Problems? 6 Most-Common Issues & Fixes

Your car’s thermostat regulates the coolant flow in and out of the engine to keep a consistent temperature. If the thermostat starts to have issues, it can cause multiple issues with your car – including overheating.

Your car overheating is a common issue faced by drivers, and it can result in potential damage to the engine and other components. 

This article covers the most common issues associated with the thermostat and we’ll explain how to fix each as you diagnose them.

#1 – Your Engine is Overheating

The primary function of the thermostat is to regulate temperature. The thermostat does that by opening and closing to allow coolant to flow into and out of the engine.

Each thermostat is calibrated to open at a certain temperature specific to the car. If the coolant temperature is below the opening temperature, the thermostat stays closed.

Coolant bypasses the thermostat until the temperature reaches the calibrated opening temperature. When that occurs, the spring in the thermostat changes length and the coolant is allowed to flow through the thermostat and circulate throughout the complete coolant system.

How to Fix It

Your radiator uses air to transfer heat away from the cooling system. The thermostat allows coolant to circulate through the complete coolant system.

Before you assume the thermostat must be the reason your engine is overheating, double-check the coolant level in your cooling system. With the engine cold, you can open the radiator cap or the overflow bottle to verify the coolant level.

If the level is below the full mark, add coolant until the level reaches the full “cold” mark. A low coolant condition can trap an air bubble in the cooling system and prevent the coolant from circulating when the thermostat opens.

If the coolant is full, you should be able to see it circulating inside the radiator cap opening when the engine is warm. If the system is warm, and the coolant is stagnant, the thermostat has stopped opening.

You can remove the thermostat from the engine and verify that it does or doesn’t open by placing it in boiling water. The thermostat should open within 15 seconds of being placed in the boiling water. If it doesn’t, then the thermostat should be replaced.

#2 – Your Engine Doesn’t Reach the Normal Operating Temperature

Your thermostat regulates the temperature of your engine by allowing coolant to enter and then circulate to the radiator to dissipate extra heat.

When the thermostat doesn’t open at the normal operating temperature, it can cause the engine to overheat due to a lack of coolant circulating. It can also keep the engine from reaching the normal operating temperature if it stays open all the time.

If the thermostat stays open due to contamination or because the spring has stopped working, the coolant will circulate through the system without regulation.

The coolant should circulate slowly through the engine to absorb excess heat and then transport it to the radiator for dissipation through the radiator fins. Without the thermostat acting as a flow regulator, the coolant won’t stay in the engine long enough to collect heat as designed.

How to Fix It

Thermostats aren’t meant to be rebuilt or repaired.

If you remove the thermostat and it is full of contamination and corrosion such as rust or dirt, you may be able to soak it in cleaner overnight to dissolve the contamination or corrosion.

Once it is clean, verify the thermostat still works by placing it in boiling water. You can also verify the temperature it opens by placing a thermometer in the water.

Thermostats aren’t expensive, so it may be a better option to buy a new one if they are available.

If your Check Engine Light (CEL) is illuminated in your gauges, it may be for a trouble code P0128. That code is associated with the engine not achieving proper operating temperature.

#3 – The Engine Temperature Fluctuates Up and Down

Your thermostat regulates the flow of coolant through the cooling system and engine by opening and closing at a determined temperature.

If the thermostat is bad, it may open and close continuously and cause the coolant temperature to fluctuate up and down. You may see this on your temperature gauge as the needle moves up and down too.

How to Fix It

Fluctuating temperature is caused by low coolant levels or a bad thermostat. You can check the coolant level when the engine is cold by opening the radiator cap or inside the overflow bottle.

If the coolant is at the proper level, then the thermostat needs to be checked.

You can verify if the thermostat works by placing it in boiling water. It should open within 15 seconds in boiling water. If it doesn’t, it should be replaced with a new thermostat.

#4 – You Have a Coolant Leak

A bad thermostat can prevent coolant from circulating through the engine and cooling system. When this happens, the engine can reach temperatures that will cause damage due to warping metal or gaskets that fail.

You may see external leaks at cooling system connections where the hoses connect to the engine. You may also start to mysteriously lose coolant inside the engine if the head gaskets are damaged.

The coolant seeps into the combustion chamber, which will cause white smoke to come out of the exhaust pipe with a sweet smell.

How to Fix It

If your car is reaching extreme temperatures, you should turn the engine off immediately. Allow it to cool down to room temperature before you start diagnosing the issue.

If you have a leak, you will most likely be low on coolant. When the engine is cool, you can refill the coolant. You can then start the engine and the diagnostic process to find the cause of the low coolant.

An external leak should be easy to spot and trace back to the source. An internal leak won’t be visible outside of the engine, but you can check the exhaust for white smoke and a hint of a sweet smell.

If you detect both of those conditions, the head gasket needs to be replaced. That is a large job that may require a certified mechanic to complete.

While the repairs are being completed, the thermostat can be checked for proper opening and closing and also replaced if it’s bad.

#5 – You Hear Rumbling or Knocking Sounds from the Engine

Your engine shouldn’t make any strange sounds like rumbling or knocking. If it is making weird sounds, the thermostat may be stuck closed.

The thermostat allows coolant to circulate through the engine, and without the circulation, it can reach extreme temperatures. Those temperatures can reach boiling levels and steam can develop inside the engine and cooling system.

Steam can cause damage to the engine and other components while it travels through the engine causing rumbling and knocking sounds.

How to Fix It

If your car is reaching high temperatures, such as the temperature needle reaching the red part of the gauge, you should turn the car off.

The engine needs to cool down to prevent damage due to the extreme heat. Once it is cool, the coolant level can be checked and refilled as needed.

You may have burned some coolant if it reached the boiling point. If the thermostat is bad, it should be replaced. You can check if it opens and closes by placing it in boiling water.

#6 – Your Heater Works Intermittently

Imagine if you jump in your car, turn on the heat, and nothing but cold air comes out of the vents. Your heater gets the warmth from the cooling system, which requires the thermostat to open and divert hot coolant into the heater core.

If the heat works intermittently, the thermostat may be opening and closing causing warm coolant to flow intermittently through the heater core.

How to Fix It

A heater that doesn’t work all the time may be due to a low coolant volume. Check that your cooling system is full of coolant and that it doesn’t have air pockets trapped somewhere in the system.

Next, check that the thermostat opens and closes by starting the engine and watching if the coolant circulates when it reaches operating temperature.

If the coolant doesn’t circulate, the thermostat is most likely stuck closed. It needs to be removed and replaced with a new one. They are inexpensive and a new thermostat may bring back the heat on those cold days.

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