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Can a Bad Thermostat Cause Rough Idle? (Quick Answers)

A thermostat is an essential automotive part responsible for coolant flowing in and out of the engine. This helps the engine to reach an optimal temperature and maintain it for smooth driving. 

But just like other automotive parts, a thermostat can go bad and cause trouble for your car.

Can a bad thermostat cause rough idle?

A bad thermostat doesn’t usually cause rough idle. It can cause a faster idle but not rough. A thermostat stuck close can cause overheating of the engine. On the contrary, a stuck open thermostat can cause overcooling of the engine. This overcooling can result in fast but smooth idling.

A thermostat can get stuck open and close both ways. When it is stuck open, the engine never gets warm enough or takes a long time to reach the operating temperature. But cold vehicles don’t cause rough idle. This can cause your car to idle faster but it’ll still be smooth.

If the thermostat gets stuck close, the engine will overheat as the coolant won’t be able to reach the radiator. An overheated engine can cause several other problems for your car. 

In some cases, the idle can get a bit rough when there are other problems involved or if you’ve been letting your car run with a bad thermostat for a long time. Whatsoever, a bad or stuck thermostat usually doesn’t cause a rough idle.

How does a bad thermostat cause rough idle?  

A bad thermostat is not responsible for rough idle. If you are facing problems with the idling, it may not be because of a faulty thermostat. A thermostat is responsible for coolant flowing in and out of the engine. 

A stuck open thermostat causes engine overcooling, whereas a stuck close one can cause engine overheating. But, a bad thermostat can’t cause rough idle. It can make your engine idle fast, but it won’t be rough. 

If the engine suddenly starts to idle roughly, there may be other issues with your car. The two most common reasons for rough idle are vacuum leak and a faulty fuel system.

Will a bad thermostat cause car to run rough?

A bad thermostat doesn’t make your car run rough. It can interfere with the idling but only when the thermostat is stuck open. When the valve stays open, the coolant keeps flowing to the radiator causing overcooling of the engine. 

As your engine can’t reach its ideal temperature, it can idle fast but not rough. After being idle for a long time, the temperature may slowly build up under a steady load.

What are the symptoms of a faulty car thermostat?

A few symptoms of a faulty car thermostat are given below –

Overheating engine:

The job of a thermostat is to supply coolant to the radiator to maintain an optimal temperature level of the engine. A failing thermostat won’t be able to do the job properly hence causing the engine to overheat. 

If you see a coolant temperature warning in your car’s dashboard, there might be something wrong with the thermostat.

Abnormal temperature gauge reading:

When a thermostat fails to maintain proper timing of the coolant flow entering and exiting the engine, the engine temperature will be unstable. This can display abnormal readings in the temperature gauge.

The fluctuating temperature inside the car:

The coolant is responsible to warm up the air inside your car. When a thermostat fails, it can’t keep the engine’s temperature steady causing the temperature inside the car to fluctuate. When both the temperature inside your car and the temperature gauge fluctuate, there’s a high chance that the thermostat has gone bad.

Leakage and steam from the engine:

When the thermostat is stuck closed, the coolant inside tries to find a way to escape. As the temperature of the engine begins to rise, the coolant starts heating up. At one point, the pressure increases so much that it strikes the weak points to breakthrough. 

You may often see white steamy smoke coming out.

Full radiator tank:

Another sign of a bad thermostat can be a full radiation tank. When the thermostat doesn’t work properly and gets stuck closed, the coolant can’t get out. The trapped coolant begins to heat up and eventually starts steaming. 

You will find the radiator tank filling up with water.

What causes a car thermostat to go bad?

Some reasons that can cause a car thermostat to go bad has been discussed below –


A thermostat is designed to open up when the engine reaches the optimum temperature. But sometimes, due to several reasons, the engine can overheat causing the thermostat to fail.

Sludgy coolant:

Since coolant is an automotive fluid, it can also go bad over time due to contaminations. Oftentimes, the coolant becomes sludgy and restricts the flow. The thermostat fails to take the precise readings. 

This can slow down the closing and opening of the valve leading to overheating or overcooling of the engine.

Wearing out of components:

With time, the internal components can wear out leading to the thermostat going bad. Like most other automotive parts, the thermostat loses its power over time. The temperature at which the valve opens starts getting higher with time and reaches a position of overheating.

Defect and incorrect installation:

Since the thermostat is a mass-produced item and needs a human inspection before installation, it can sometimes come with defects. These defects can be very small but cause problems in the future. Also, incorrect installation of the thermostat can cause it to go bad.

How long can you drive with a bad thermostat?

You can drive with a bad thermostat but not for very long. Also, it can be hazardous for your car. Especially if the thermostat is stuck close, it will completely restrict the coolant flow causing overheating of the engine. 

An engine overheating can be very dangerous and cause other parts of your car to be damaged.

If the thermostat gets stuck open, it won’t make the engine overheat. Instead, it will try to keep it cool all the time with a continuous circulation of coolant. 

You may not find any trouble running your car like this, but your car’s heater will find it very hard to balance the temperature inside the car. If you don’t take action soon and keep running your car with a bad thermostat, the engine may fail.

So, you should not drive your car with a bad thermostat.

Final thoughts

A bad thermostat doesn’t cause rough idle. It can cause your car’s engine to idle fast but smooth. This happens due to the overcooling of the engine. When the thermostat gets stuck open, it can cause the engine to over cool, whereas, a stuck close thermostat can cause the engine to overheat.