5 Signs & Symptoms of a Failing Car Thermostat

The thermostat in your car controls the flow of coolant through the engine and is an important aspect of its operation. A malfunctioning thermostat is the most common cause of overheating or no-heat in your car.


Given the low cost of the thermostat, it makes more sense to replace it than to spend hours trying to figure out what's wrong. You've only spent approximately two hours learning how to change a thermostat if that doesn't work.


Signs of a Faulty Thermostat

Fortunately, a thermostat failure is one of many automotive issues that you can diagnose and repair on your own. The following are symptoms that your car's thermostat is failing.


  • The engine overheats

  • Changes in interior temperature

  • High temperature reading

  • Temperature fluctuating

  • Coolant leakage around the thermostat

  • Low coolant level

  • Abnormal sounds


1) Engine Overheating

The following is an explanation of how thermostats function. In a copper cup known as a pellet, the producers inject a combination of ground-up brass and wax. Then they insert a highly polished metal rod into the wax via a rubber "O-ring" gasket. A crimp ring is used to seal the pellet.


The pellet is then encased in a metal "skirt." When the engine is cold, a spring presses the skirt up against a seat, preventing the flow of coolant (much like a closed faucet). The wax melts and expands as the engine warms up.


The wax seeks to "spit out" the metal rod as the expansion pressure develops. The rod, on the other hand, is immobile. It's connected to a "bridge" on the thermostat's opposite side.


The pellet eventually moves and overcomes the spring's pressure. As a result, the thermostat opens and coolant may flow. Until the metal rod corrodes, the whole system functions well. The wax oozes out due to the rusted rod damaging the rubber seal.


The thermostat will cease opening, coolant will stop flowing, and your engine will overheat. The end consequence might be a catastrophic engine failure that costs possibly thousands.


2) Changes in Interior Temperature

When the air temperature inside your car abruptly changes, this is a common sign. It may begin by decreasing to a very low temperature before abruptly rising to a very high temperature.


Any change in air temperature that isn't consistent with your present interior climate settings indicates that the thermostat is malfunctioning.


car air vent

3) Temperature Variations

Keep an eye on the temperature monitor on your dashboard. A thermostat failure might be at fault if the needle spikes and dips during driving.


Note that some slight change in temperature may occur due to normal operation but it should be very slight, it'll be larger dips and spikes if the thermostat is faulty.


car temperature gauge

4) Leaking Coolant

Check for leaks in the thermostat housing. Coolant keeps your engine running at the proper temperature, and a shortage of it might compromise its performance.


thermostat diagram
The typical location and design of the thermostat housing.

5) Abnormal Sounds

If the temperature variations weren't unpleasant enough, odd rumbling sounds may start to appear. Your radiator, engine, or both will be making these sounds. The sounds might alternatively be described as a banging, boiling, or bubbling sound.


Basically, if you're hearing unusual sounds like the ones mentioned above and you're also experiencing one of the other symptoms listed here, you've got a thermostat issue.


How to Check a Thermostat in a Car

Check the coolant flow and temperature before removing the thermostat from your car.


Idle the engine after removing the radiator cap. Your thermostat is jammed open if the coolant flows immediately. Wait till your engine heats up if it isn't flowing. After 10-20 minutes, the coolant should reach the proper temperature and begin to flow.


Your thermostat is stuck closed if it does not start to flow yet the temperature indicator on your dashboard increases. If your coolant is flowing regularly, there might be another reason for your engine overheating.


Start with a cold radiator and engine to check the temperature of your coolant. Idle the engine and use a thermometer to check the temperature of the engine block or cylinder heads. The temperature of the top radiator hose should then be checked.


After five minutes, repeat the test. Complete the exam three times in total. Your thermostat is stuck open if the temperature does not increase much. If the temperature of the radiator hose stays constant yet the dashboard gauge increases considerably, your thermostat is jammed closed.


If the top radiator hose reaches a temperature similar to that of the engine block, your thermostat is permitting appropriate coolant flow. It's possible that a separate issue is causing your cooling problems.


Place the thermostat in a pot of water on your kitchen stove to test it outside of the vehicle. Do not allow the thermostat to come into contact with the pot's bottom. To determine the temperature at which the thermostat opens, use a thermometer.


Take note of the temperature when it first starts to open and when it has fully opened. Take it out of the pot. As it cools, keep an eye on it to observe how it progressively closes. Compare your notes to the instructions in your owner's handbook.


Thermostat Replacement

Don't merely replace your thermostat and fill out the coolant reservoir if it fails. It's alerting you to a more significant issue: your coolant's corrosion inhibitors have also failed. When you replace the thermostat, cleanse the system and add new coolant.


Also, don't attempt to save a few bucks by purchasing an inexpensive thermostat. Corrosion resistance is included into premium thermostats. When compared to the possible engine damage, the price difference is negligible.


To avoid engine overheating, several manufacturers provide "fail-safe" variants that fail in the completely open position.

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