Bad Thermostat Or Water Pump? What’s The Cause of Your Car Overheating

An overheating car is never good news, especially when you are in the middle of a long trip.

A bad thermostat or water pump is one of the most common reasons behind an overheating engine and it’s hard to pinpoint the issue.

Generally, a bad thermostat either overheats or underheats your engine while a water pump issue does the same while affecting your coolant pressure.

Understanding the Operations of the Thermostat and the Water Pump

Before we start the discussion on how to tell apart a bad thermostat or a water pump, it’s better to understand how they both work.

Both the water pump and the thermostat are essential components for your vehicle’s cooling system.

Your car can’t operate if it’s below the optimum temperature. Similarly, it won’t operate properly if it overheats.

Moreover, you’ll also be opening yourself to more significant damage in the engine, which will need time and more money to repair.

So, let’s take a look at the operations of both the thermostat and the water pump.  

The Main Operations of a Thermostat

Take any internal combustion engine, it’ll operate properly between the range of 195F to 220F. Below that, your oil won’t lubricate the engine properly.

Similarly, anything over 220F would start deteriorating the gasket, seals, and permanently affect your engine.

The thermostat stops both of these things from happening. It helps your vehicle’s cooling system maintain the temperature range by restricting the flow of your coolant.

The thermostat opens and closes at regular intervals which keeps the entire system operational.

For example, consider a situation where you just started a cold engine.

If the coolant starts to flow at the same time, your engine would never reach the operating temperature of 195F and above.

The thermostat restricts the flow, which is primarily responsible for keeping the temperature in check.

However, if the coolant flow remains restricted the entire time, the temperature of the engine would go well above 220F.

Which would damage your engine and may even cause it to become unusable.

To prevent that, the thermostat opens up and allows the coolant to flow properly and maintain the temperature of the engine.

How Does a Water Pump Work?

The water pump has a simple, but essential function. That is to circulate the entire coolant throughout the system.

The cooling system of any vehicle consists of different components out of which the radiator, lower hose, and upper hose are essential.

Consider the lower hose as the first step of the cycle.

The cold coolant enters the engine through the bottom hose and takes away the heat generated by fuel consumption.

That hot coolant then exits the engine through the top hose and enters the radiator where it cools down and is ready for the lower hose.

In this cycle, the water pump is responsible for circulating the fluid throughout the system. It’s a simple centrifugal pump that operates with minimal power input but plays an essential role.  

Symptoms of a Bad Thermostat or Water Pump

One of the main reasons why it’s so hard to tell a bad thermostat or water pump apart is the overlap in their symptoms.

Most symptoms of a bad thermostat and water pump are similar.

They’ll both cause the car to overheat, the coolant to leak, or visible temperature gauge fluctuations frequently.

Here are the 4 main symptoms that show that your thermostat or water pump is not operational.

1. An Overheating Engine

An overheating engine is a giveaway sign that your vehicle’s cooling system is not working. There are several reasons why that happens, but the most common ones are a bad thermostat or water pump.

This means that they are the first thing you should check if you see your engine overheating.

Other than the issues we are discussing, your engine may overheat due to a plethora of other reasons as well.

Some of them are a blown gasket, a clogged radiator, a bad radiator fan, or insufficient coolant levels, that might have happened because of leaks, which is a symptom as well.

2. Coolant Leaks

The next sign that may indicate a stuck thermostat or a bad water pump is a coolant leak. In both cases, the position of the leak might give you a slight idea of the problem, but it’s not a guarantee.

In most cases, both the thermostat and water pump are equally likely to be a culprit in case of a coolant leak.

So, how does a stuck thermostat or a non-operational water pump cause the coolant to leak?

In the case of a bad thermostat that’s stuck closed. The outgoing coolant from the engine will be hot enough and the closed thermostat will pressurize it.

Your hoses, seals, and other connection sustain that pressure and might fail midway, which causes a leak.

For a bad water pump, the failure to get enough pressure from the coolant can compromise the overall cooling system.

The heat generated can again affect the seals and hoses but cause rust in the radiator. The combined effect of these issues can many problems in your vehicle’s cooling system which includes leakage.   

3. Little or No Heat from the Cabin Heater

Your cabin heater uses the engine heat to provide you with the heat required to maintain the temperature inside.

If the cooling system isn’t working properly, the heat exchange process won’t be effective enough to increase the cabin temperature.

Little to no heat from the cabin heat can happen due to both a bad thermostat and water pump.

You’ll see your temperature gauge rising, but won’t feel that heat in the cabin heater. In the case of a bad thermostat, the reason behind it would be a restricted flow.

If the coolant is unable to flow properly, the cabin heater would obviously not work.

The water pump, on the other hand, is directly responsible for circulating the coolant throughout the system.

Without that circulation, the hot coolant won’t reach the heat exchanger of your cabin heater and you won’t feel the heat despite the temperature gauge showing normal readings.

4. Temperature Gauge Fluctuations

Another sign that indicates a bad thermostat or a problem in the water pump is a fluctuating heat gauge.

Erratic temperature changes indicate that the cooling system doesn’t have the flow buildup necessary to keep the temperature of your vehicle in check.

The Tests for Water Pump and Thermostat

If you are experiencing the aforementioned symptoms in your vehicle, then it’s highly likely that the problem is in the thermostat or the water pump. So, the next question is how you can check them.

Fortunately, checking if your thermostat works is extremely easy. All you need to do is to inspect it visually by removing it.

Generally, the thermostat is located somewhere near the engine, but the exact location depends on the make and model of your car.

Therefore, the best course for you is to consult the owner’s manual of your vehicle.

You can also find the thermostat manually. Simply follow the hose until you see the housing unit for your thermostat. It’s extremely simple.

If your thermostat is stuck, then you have probably found the source of the issue. A stuck thermostat prevents the coolant to flow properly and causes your engine to overheat.

The thermostat is a small component and doesn’t cost much.

So, it’s viable to replace it even at the remotest chance of an issue. Doing so would save you from any major problems that stem from an overheating engine.

Checking the water pump is not that simple. Most people can’t tell a bad water pump apart from a good one unless there is major damage. So, visual inspection is not viable.

There are two methods to know if your water pump is operating.

Both come with a major prerequisite that you must be 100% sure that your thermostat is working perfectly.

If that’s not the case, you’ll know that there is something wrong but won’t be able to pinpoint it. In other words, you’ll be still at square 1 with little to no progress.

The two ways that help you know for sure if your water pump is operational or not are:

1. Checking the Flow in the Radiator

This is a very simple process. Once you know for sure that your thermostat is working, follow these simple steps:

  • Start your car and keep it on idle
  • Open the hood and remove the radiator cap
  • Keep on revving the car until it reaches the operational temperature range for the engine
  • Check if the coolant is flowing in the radiator

The process may sound simple. However, there is always a catch. This method will not tell you exactly if your water pump needs replacement or not as a clogged radiator will also give the same results.

2. Checking the Pressure in the Hose

Another excellent method is to check the pressure inside the cooling system while it’s still in operation. Follow these steps:

  • Idle the car until it reaches the operational temperatures
  • Open the radiator cap carefully or you’ll burn your hand.
  • Press the incoming hose firmly
  • The water pump will be fully operational if you feel a rush of coolant in the pressed hose

Bad Thermostat Or Water Pump: Possible Repairs

As stated before, it is more viable to replace the thermostat rather than repair it.

While it is a simple component, repairing it would consume a lot of time and may even cause some complications. It’s better to replace the part instead and get on with it.

Doing so wouldn’t take up a lot of your time or money as thermostats are not that expensive.

The water pump, on the other hand, is a little more expensive than the thermostat.

However, it should also be replaced because that’s a better alternative.

Water pumps usually work for a very long time, around 1000 miles, so this one-time investment would save you from a lot of trouble in the long run.

Bad Thermostat Or Water Pump: Final Thoughts

To conclude, both the thermostat and the water pump are essential components for a vehicle cooling system as they maintain the coolant flow responsible for keeping your car’s temperature in check.

They can go bad with time and are very easy to replace.

However, you need to recognize the signs of a bad thermostat or water pump.

Generally. an overheating car, coolant leakage, or deteriorated performance of your engine or heat-based systems like the cabin heater indicates some sort of problem.

In such cases, you can check your coolant flow and pressure to ensure that the thermostat and water pump are operating properly.


Dean Alvarez, TireForge Head Author

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