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How Do I Know If My Thermostat or Water Pump Is Bad?

How Do I Know If My Thermostat or Water Pump Is Bad?

If you're wondering, how do I know if my thermostat or water pump is bad? You need to inspect the two units physically. Typically, the thermostat might be stuck open or stuck closed. To test a bad thermostat, monitor the reading on the temperature gauge. A Stuck opened thermostat prevents the engine from reaching the minimum operating temperature, while a set closed thermostat leads to a significant overheating in a couple of minutes. A bad water pump, on the other hand, can be measured by testing the coolant pressure.

Auto Repairs Are EXPENSIVE


The water pump and thermostat are critical components in any vehicle cooling system. They're both need to be in good condition for your engine to operate within the optimum temperature range. If any problem happens to the thermostat or the water pump, you'll end up with an overheated engine that could get completely damaged.

Sometimes it can be a little tricky to determine whether it's the thermostat or the water pump going bad. Therefore, this article clarifies the main differences between a bad water pump and a bad thermostat. Next, it provides clear symptoms indicating when either the water pump or the thermostat goes bad. Finally, it highlights rough estimates about how much it will cost you to replace the water pump or the thermostat.

How does the cooling system work?

Before we get into the details about how to differentiate between a bad water pump or the thermostat, you must understand how the cooling system works in the first place. Once you know the main rule of each component within the cooling system, you can easily answer the question, “how do I know if my thermostat or water pump is bad?”

The cooling system starts when the engine temperature exceeds a maximum threshold. Then, your vehicle's computer sends a signal to the thermostat to open and allow the coolant to run around the engine. Then, with the full capacity of the water pump, the water pump pumps coolant without proper speed and pressure to get around the engine on time and drop its temperature down.

As you might notice, both the thermostat and the water pump work together to get the coolant to the engine at the right time with the right speed. If the water pump doesn't work properly, the coolant won't have the right amount of pressure, and therefore, it won't reach the engine with the right capacity, and your engine won't cool down as efficiently as before.

Similarly, when the thermostat is not working properly, the coolant will not run around the engine because the thermostat is responsible for blocking the coolant and not allowing it to pass through until the thermostat receives the signal from the vehicle’s computer. Finally, of course, the thermostat might get stuck open, which means that the coolant will run around the engine continuously, but the worst scenario happens when the thermostat gets stuck closed.

How do I know if my thermostat or water pump is bad?

Differentiating between a bad thermostat or a bad water pump can be a little tricky and requires a physical inspection to confirm the problem. Let's take a closer look at what would you see when each component is bad :

1.    A bad thermostat

When the thermostat is not working properly, then you're dealing with two common situations:

A stuck open thermostat

If the problem has to do with the stock open thermostat, you are lucky. This is because while a stuck open thermostat is not good, it's not as bad as a stuck closed thermostat.

The thermostat needs to open and close whenever the internal computer sends a signal. If the thermostat is continuously opened, the coolant will be running around the engine all the time, which means that the engine cannot reach the minimum operating temperature. Therefore, you'll notice that the vehicle is not providing the right amount of power, and it might use fewer than usual.

To confirm its stuck open thermostat, you can look at the temperature gauge. If you notice that the temperatures gauge is not exceeding 50 to 60 Celsius, then there is an issue with the thermostat stuck open.

If you have the right mechanical skill sets, you might even take the thermostat out and perform a simple thermostat test. There are plenty of available tutorials on how to destroy vehicles' thermostats to help yourself answer the question, “how do I know if my thermostat or water pump is bad?”

You can put the thermostat inside a pot filled with water. Then, start heating the water gradually and monitoring the behavior of the thermostat. If the thermostat closes as the temperature increases, it's working properly, and there is no problem. However, if it does not respond and is kept open, you will need to install a new thermostat.

A stuck closed thermostat

When the thermostat gets stuck close, then this is a very critical situation. One quick way to determine if it's a bad thermostat is to monitor the temperature gauge on the vehicle's dashboard. If you notice a significant increase in the temperature beyond the maximum operating temperature, this might give you some hints about internal problems.

Keep in mind that increased temperature gauge reading does not clearly say that it's a bad thermostat, and therefore, you need to perform a physical inspection to confirm the culprit. Also, sometimes if there is a coolant leak or a coolant drop for any other reason, then you're always going to deal with and overheating.

Thus, the only way to confirm is to take out the thermostat and perform the pot test that we clarified earlier. If you're taking your vehicle to a mechanic, they might have some other tools to help them detect a bad thermostat as it gets stuck closed.

2.    A bad water pump

On the other hand, when the water pump is the culprit, you won't see the same behaviors as when the thermostat gets stuck closed. This is because typically, a sock closed thermostat leads to significant quick engine overheating within minutes.

However, if there is a simple issue with the water pump, there might be some engine overheating, but it's not going to be when the thermostat gets stuck closed. However, there are some instances where a completely failed water pump leads to a significant engine overheating in no time.

Therefore, if you want to be 100% sure about whether it's a bad thermostat or a bad water pump, you need to inspect the water pump physically. Your mechanic has certain tools that help him check the coolant pressure around the engine. This way, he can determine if there is not enough pressure on the coolant, which means there is an issue with the water pump.

Sometimes if the water pump fails, it might lead to seals breakage around the water pump connections allowing coolant to leak to the ground. Therefore, performing a visual inspection of the water pump and looking for coolant leak symptoms helps you confirm whether it's a bad water pump or a bad thermostat.

Thermostat or water pump: Possible repairs

Depending on the culprits, you must take care of the problem immediately. In other words, whether it's the water pump that's bad or the thermostat, you must get the problem resolved as quickly as possible to prevent engine damages.

1.    How much does it cost to fix a bad thermostat?

Although a bad thermostat might lead to significant catastrophic results, replacing the thermostat is not a big deal and is considered relatively cheaper than other Car repairs. Typically, a bad thermostat might cost you between $200 and $300, depending on your vehicle's time and the location where we get the job done.

2.    How much does it cost to fix a bad water pump?

On the other hand, a bad water pump is a decent repair. Automotive experts indicated that replacing the water pump should cost between $461 and 638 dollars. Those repair costs depend heavily on the vehicle type and the independent shop or the dealership where you get the job done.

3.    How much does it cost to fix coolant leaks?

Sometimes a bad water pump might even lead to some coolant leaks, which means that you won't only need to install a new water pump but also take care of all coolant leaks and internal cracks.

Unfortunately, it's hard to tell how much exactly fixing the coolant leak will cost you because a coolant leak can happen anywhere in any pathway around your cooling system. Roughly, a coolant leak might cost you as low as $100 and as high as about $800.

Before you spend money installing a new water pump, you must understand the overall repair costs. First, add up all repairs and maintenance costs and then compared them to your vehicle's value. If you don't start repair costs approaching 75% or more from your vehicle's value, you'd better sell the car and use its money towards a better vehicle that doesn't have major problems.

Should the thermostat be replaced with a water pump?

Automotive experts recommend that whenever you decide to install a new water pump that you install a new thermostat as well. Although here are typical, both components might be impacted by each other, and since the thermostat is not very expensive, it might be worth installing a new one.

You can always consult with your mechanic and confirm whether the thermostat is in good condition and doesn't require installment. Still, it won't hurt to invest an extra couple of hundred dollars in preventing dealing with engine overheating anytime soon.

How often should you replace your water pump?

The water pump needs replacement between 60,000 and 100,000 miles. You also need to monitor any symptoms of a bad water pump if you need to replace it earlier because that could happen.

Thermostat or water pump: Conclusion

It can be a little tricky to differentiate between a bad water pump and a bad thermostat, but our expert team got the right answer for your question “how do I know if my thermostat or water pump is bad?”

In both scenarios, you will still need to perform a physical inspection, but there are some hints that you can collect as you're monitoring the vehicle's behavior. For example, the bad thermostat can be detected by monitoring the temperature gauge, and if you notice that the gauge is reading low all the time, it might indicate a stuck open thermostat. However, if the temperature gauge increases significantly in a very short time, it indicates a stuck closed thermostat, but it could be related to something else.

Therefore, we advise that you take the thermostat out and perform the pot test or reach out to your mechanic. For the bad water pump, unfortunately, the only option would be to have your mechanic inspect the coolant pressure to confirm if it's the water pump or not.

As we indicated in this article, if you notice that the repair costs are piling up, it might not be worth fixing the vehicle, and if you got to that point, you might want to reach out to cash cars fire to sell your car.

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