How To Tell If Your Thermostat Or Water Pump Is Bad

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Sometimes, it is not the water pump that went bad, it could be the thermostat or the pulley bearings. So, you need to be sure of what component went wrong, then you can know how best to go about the fixing. This article explains how to tell if your thermostat or water pump is bad.

A bad thermostat exhibits similar symptoms as a bad water pump would do; this includes overheating, loud whining noise, and a significant drop in engine performance. When you experience any of these occurrences, just after checking the water pump, check the thermostat too.

How To Tell If Your Thermostat Or Water Pump Is Bad

The thermostat is connected to the water pump system. Both of them have to be in good condition for your car’s engine to work as designed – and even more efficiently. But when things go wrong, what should you do?

In all vehicles, the thermostat is designed to open up and close back at intervals. The process occurs several times – at intervals – for as long as you keep driving.

When the thermostat opens, it oozes out hot steams and allows the coolant liquid to get into the engine to cool it down and maintain operating temperature.

So, if the thermostat is stuck and won’t open, the engine is most definitely going to overheat excessively, which may cause a fray of most of the engine’s components, resulting in more expensive repairs.

It’s a bit tricky to spot if your car engine’s thermostat is stuck or not; however, mechanics know how to get around this.

On the contrary, the thermostat may be stuck open, which would cause the coolant liquid in the reservoir to drain quickly; but the good news is, your car would never overheat. However, having an uncontrolled flow of coolant into the engine is not a good scenario.

You’d know you have a stuck open thermostat when the temperature gauge doesn’t reach the “hotness” region, and also, the coolant liquid drains faster, even though the water pump is not leaking. The location of the thermostat differs from vehicle to vehicle, to find the one in your car, check the vehicle’s manual.

When you know where it is, check to see if it is stuck open or closed. Either way, you have a thermostat to replace, but it’d be more comfortable if it’s stuck open, than if it’s stuck closed. Replacing a bad thermostat costs around $400 on average.

Troubleshooting a Faulty Thermostat

Before proceeding to check or touch the thermostat, ensure that your car is completely cooled down. Touching the thermostat when the car is still hot would result in serious burns.

  • Remove the thermostat from its housing
  • Heat water to the temperature stamped on the thermostat (usually around 180-212oF); use a meat thermometer to measure the heated water temperature.
  • Put the thermostat in the water – use needlenose pliers so you hands won’t get into the hot water too.
  • The thermostat should open when the water heats up to high temperatures; if it doesn’t open up, that means the thermostat is bad, you need to replace it with a new one.

What About a Bad Water Pump?

A bad water pump would leak out coolant, and you’d see the leak when you look under your car. If the water pump is cracked, coolant liquid would drain quickly because it’d be leaking out.

Other bad water pump symptoms include overheating and whining noise coming from the engine bay. After inspecting the water pump to ensure it doesn’t have a physical crack, then you can check the pulley, thermostat, and bearings to know which is causing the issue you’re facing.


In conclusion, this article explains how to tell if your thermostat or water pump is bad. Changing both the thermostat and water pump would set you back $600 on average depending on various factors, including the mechanic workshop you went to for the repair.

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