5 Symptoms of a Bad Thermostat

(Updated on December 14, 2020)

When you get too cold or hot in your house, you probably adjust a thermostat to change the temperature in the room. Engine blocks can also be too hot or too cold; they too need a thermostat to manage their temperature.

How does a Thermostat Work?

Unlike the thermostat for your house, most thermostats used in cars do not require electricity. Inside your thermostat, there is a wax element.

As this wax heats up, it melts and expands. This expansion pushes a valve open in the thermostat, which allows coolant from the radiator to enter the engine block. Coolant in the radiator is roughly the same temperature as the ambient air.

This cooler coolant allows the wax to cool back down, eventually closing the thermostat again. This cycle repeats to keep the engine within a specific temperature range.

Thermostats come in different temperature ranges depending on the engine and application. A 180° thermostat will begin to open when coolant in the engine block is within 3 degrees of 180° Fahrenheit. The thermostat will be fully open when engine coolant is 15-20 degrees above 180° F.

If you’re looking to buy a new thermostat, be sure to choose it according to the OEM’s specifications for your vehicle. Different engines run best at different operating temperatures.

Bad Thermostat Symptoms

1) Overheating

engine overheating symptoms

The most obvious symptom that your thermostat has gone bad is your vehicle overheating. If a thermostat is stuck closed, you will almost certainly run into overheating issues.

Overheating from a stuck thermostat is caused by hot coolant that gets trapped in the block, and ambient coolant from the radiator will not be able to make it into the block to replace it.

2) Poor Engine Performance

car hesitates when accelerating - gas pedal

When your engine is cold, it doesn’t run as efficiently as it does at operating temperature. Your engine is built with specific tolerances that are designed for running within a specific temperature range.

As metal in the engine heats, it expands. This fills up some of the empty space between the piston and the cylinder wall, creating a better seal inside the combustion chamber. A better seal means the engine will make more compression, and therefore be more efficient.

If your exhaust has a strong fuel smell, your car may be running rich because the engine is cold. A car that runs rich will yield poor fuel economy and excessive carbon build up over time. Spark plugs may become fouled as well.

Poor fuel economy also increases your vehicle’s emissions, which is more harmful to the environment. 

3) Temperature Gauge Stays Cold

cold engine temp gauge

If your thermostat is stuck in the open position, your car may never reach operating temperature. This is especially true on a cold winter day, where engine heat dissipates very quickly.

If your temperature gauge is stuck on the cold side or barely moves off of “C”, you may want to check your instrument cluster and coolant temperature sensor as well. 

4) Heater Doesn’t Work

hvac system

Heat inside your vehicle’s cabin is provided by a heater core. The heater core is very similar to the radiator at the front of the car and works on the same principle.

Inside a heater core, coolant flows from one side to the other, heating up the core. As air passes over this warm core, the air is warmed up. This warm air then makes its way into the vehicle’s HVAC system and blows out of the vents.

Depending on how the thermostat fails and where your heater core sits with respect to your thermostat, your heater core may go cold when your thermostat fails. This is because the heater core only has access to ambient temperature coolant and has been cut off from the hot coolant in the block.

If the thermostat fails while open, it may take an unusually long time for the cabin to heat up. You may also notice the heat from the vents fluctuate, going from hot to cold to hot again without touching the HVAC temperature settings.

These symptoms may also be experienced if there is an air pocket in the heater core. 

5) Blown Radiator Hose

bad radiator hose
radiator hose ready to burst

As temperature builds in the engine block, pressure builds as well. The cooling system is a pressurized system, but it is only designed to hold so much pressure. If your engine gets too hot, you may start to notice some of the rubber parts start to leak or even burst.

If you experience this symptom, check your radiator cap as well to make sure it is the correct pressure for your vehicle. A radiator cap that holds too much pressure will also exhibit these symptoms.

Can You Drive Without a Thermostat?

A short term solution to a thermostat that is stuck closed is to remove the thermostat entirely.

While it is possible to drive without a thermostat, it is not wise to do so long term. Engines run very poorly before they reach operating temperature.

If you are missing your thermostat, your engine will probably not be able to reach operating temperature (unless you are running your car hard on a race track, and it’s blisteringly hot outside).

Other Causes With These Symptoms

head gasket leak
The carbon deposit on the wall of cylinder #4 (right) is a textbook sign of a head gasket leak.

Cooling system problems can be tricky to diagnose. This is because many parts will exhibit similar symptoms when they fail. If your vehicle is overheating, there are several parts of the cooling system that may be the cause.

You may also want to consider checking or replacing any of the following items, depending on your symptom above:

Check your coolant level. If it’s low, also check for coolant leaks.

Burp your radiator. Properly bleeding the radiator of air ensures there are no air pockets that can create hot spots in the engine.

Check or replace your radiator cap.

Replace the radiator.

Replace your heater core.

Replace your head gasket(s). If you have a Subaru, you can learn more about Subaru head gasket failures here.

Replace your intake manifold gasket(s).

Since there are many components that make up the cooling system, it’s best not to throw parts at the car (unless they’re cheap parts you haven’t replaced in a long time, such as the radiator cap).

If you’re unsure how to test your cooling system, consult a professional to help you narrow down the problem. A good mechanic with a mature troubleshooting process will be able to help you diagnose and pinpoint the root cause without spending money on unnecessary parts.

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