Symptoms of a Bad or Failing Thermostat : Causes and Fixes!

A thermostat is an essential component in the car’s cooling system that monitors and regulates the flow of coolant in the entire engine block. You may hear a phrase like ‘the thermostat was stuck open or closed. When the car engine is running and gets to a specific temperature, the thermostat will be forced to open, enabling the coolant to travel to and from the radiator and keeping the engine at a standard temperature. In contrast, the coolant keeps recirculating back and forth the engine. In addition to other cooling components, this recirculating process helps the engine work at a standard temperature.

The opening and closing of the thermostat enable your car engine to run at optimum temperature. In case the thermostat becomes stuck open, there will be a steady flow of coolant to the engine, which will not allow the engine to reach a certain level of heat, resulting in performance issues. If the reverse is the case, there will be no coolant passage to the radiator for cooling purposes, resulting in engine Overheating. Knowing the symptoms of a bad thermostat will help safeguard the life of the thermostat and engine.

How does the Thermostat Work?

Like every other water-cooled engine, a car’s engine works efficiently within a designated operating temperature range.

what causes a thermostat to fail

While all car manufacturing companies have their manufacturer’s specifications, all car engines should operate between 195f and 220f temperature. If the engine temperature gets below 195f, the engine oil will be thick and will not give the right lubrication required by the engine to perform optimum, which could rust and cause engine failure. On the other hand, if the temperature goes above 220f, the engine will overheat.

As mentioned earlier, the thermostat enables the car engine to maintain an operating temperature. The thermostat works as a valve that allows the engine coolant to travel at a given temperature.

For instance, when you turn on your engine and rev it, the thermostat will prevent the coolant from passing through the radiator because the radiator cools the coolant. But when the engine gets heated up, the thermostat will gradually open up, allowing the coolant to pass to the radiator for cooling purposes.

Once the coolant and engine temperature stick below 195f, the thermostat will gradually start closing to provide the normal engine temperature. The thermostat functions in this open-close cycle to ensure the engine runs smoothly.

Symptoms of a Bad Thermostat

Just like other mechanical components, the thermostat is bound to fail at one point or the other. Let’s look at the signs you’ll get when the thermostat stop functioning properly.

Engine Overheating: the first and potentially symptom that you will notice when there are issues in your cooling system is the temperature gauge reading very high and engine Overheating. Like every other cooling component, a bad thermostat or water pump will result in engine blow-by if not given proper attention.

Temperature gauge fluctuations: temperature changing erratically indicates that you have a bad thermostat which could result in poor engine performance. You may see the temperature gauge abnormally high in this state and shortly comes down to an unusually low level.

Coolant leaks around the thermostat housing or around the thermostat housing or under the vehicle: coolant around the thermostat housing is a significant sign of a bad thermostat in the home. When you see a coolant leak around the thermostat housing, it is likely to be caused by a stuck-close thermostat not allowing coolant to pass through. The leak can occur in many locations that the coolant passes through, but it is commonly noticeable around the thermostat housing. This can cause other coolant hoses to leak as well.

Other significant symptoms that will pop when your thermostat goes bad might be as follows:

  • The cylinder head might not work appropriately.
  • A temperature warning light popped up on the dash.
  • Your automatic transmission might not be able to shift freely to hear 4 and 5 due to a lack of power.
  • It might result in a head gasket blown
  • It can finally cause catastrophic engine damage.

How to Fix Bad Thermostat Issues

Fixing or replacing bad thermostat issues might be a pretty easy job for most DIY lovers, while others don’t know where to start.

I will show you how to replace a bad thermostat in a simple step-by-step process. Before you proceed, you will need some common mechanic tools to carry out this job easily.

Tools Needed

  • A wrench
  • A socket set
  • Screwdriver
  • Plastic gasket scrapper
  • Pliers

Other things Needed

  • New thermostat gasket
  • Drain pan

You have to make sure that you’re using the right coolant. Check your vehicle repair manual and see the specified coolant for your car.

Step 1: Park your car. Park your car in a safe place and engage the hand brake.

Step 2: Allow your car to cool. Like I always recommend, when working under your car hood, ensure you allow your car to cool down.

Step 3: Remove battery terminal cables. Ensure you remove your battery terminal cables when working under your hood. However, if your car computer requires programming after removing the battery terminal cables, you may consider leaving the terminal cables in place.

Step 4: Place a Drain pan under your car. You want to place a drain pan under your vehicle and catch the coolant that will escape when you pop off the thermostat.

Step 5: Locate the thermostat housing. The thermostat housing can be easily traced by following the upper radiator hose. However, the housing may be in a different spot in your car.

Step 6: Unbolt the thermostat housing bolts. Usually, the thermostat housing is fastened with two or three 10mm or 12mm bolts. Grab the right socket and break the bolts free, then unfasten the bolts with your hand.

Step 7: Pull off the faulty thermostat. Once you take off the thermostat housing, you easily pull off the faulty thermostat by sliding it back and forth. Some might easily pull off, while some might prove stubborn. If the thermostat proves stubborn in your case, grab your pliers and tap them back and forth so they’ll easily come off.

Step 8: Clean the mounting surface. Take your plastic gasket scraper and scrape off debris or rust on the surface so the new thermostat will not leak after replacement.

Step 9: Install the new thermostat. Ensure you reinstall the thermostat the same way you pull it off. You can check your owner’s booklet to see how to install the thermostat if you lose track of how you pull it off.

 Step 10: Tighten the Bolts. Tighten the bolts the same way you take them off. Run the bolts with your hand and torque them down with your wrench.

Step 11: Cross-check your work. Fix other things like the battery terminal cables. Cross-check your work, and add coolant to your reservoir. You can start your engine and examine the thermostat again.

Symptoms of a bad thermostat

Frequently Asked Questions (FAQs)

Q: How do you know if a car thermostat is working?

The easiest way to know if your car’s thermostat is working is by turning on your engine and allowing it to idle. Look into the radiator filler and observe if the coolant is flowing. Since you’re just starting up the engine, the coolant should not be flowing now because it has not reached its operating temperature to open up. If the engine runs for a while, the coolant is not flowing; it means the thermostat is stuck close.  When you hear that the thermostat is stuck open or close, it means the thermostat is bad.

Q: What Happens When a thermostat fails?

A bad thermostat will not allow adequate coolant flow when required, resulting in engine overheating or lowering your engine temperature if it stays at a stuck open position, affecting your engine lubricating force. When the engine is not functioning correctly due to low engine oil temperature caused by a stuck-open thermostat, the engine will experience rough idle; hence, the phrase, bad thermostat rough idle.

Q: How do I test my thermostat?

See how to tell if the thermostat is bad in a car. There are several ways to know if a car thermostat is working or not. You can tell if a thermostat is working by checking the coolant flow.

  • Open the radiator back.
  • Turn on the engine and allow it to idle.
  • When you start the engine, the thermostat will not allow coolant passage to the radiator because the engine has not heated up. If the thermostat is passing through, you have a stuck open thermostat.
  • If there is no coolant passage at first, and even when the car heats up, you have a stuck close thermostat.

Q: Can a bad thermostat cause engine to Overheat?

A stuck-close thermostat will not allow coolant passage to the radiator for the cooling process, thereby causing the engine to heat up swiftly and eventually cause an engine to Overheat.

Q: Why would the thermostat stop work?

What causes a  water pump to fail? Several factors could cause the thermostat to stop functioning correctly, such as a blown a fuse in your fuse box, or tripped breaker, or a dead battery. If these components are working in order, it could be a faulty inner component.

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

A bad water pump and thermostat will give similar symptoms when any of them go bad since both components help recirculate and allow coolant passage in the engine, respectively. When you see any of the following signs, it is an indication that you have a water pump or thermostat.

  • Engine Overheating
  • Temperature gauge fluctuations
  • Coolant leaks from the front of your car
  • Steam coming from the front of your car
  • Nosey or loosened water pump pulley
  • Coolant leaks around the thermostat housing

Final Thought

At this point, you must have known the symptoms of a stuck open thermostat, symptoms of a bad thermostat, and how to fix bad thermostat problems.

You might notice just one or two of the outlined symptoms when your thermostat goes bad. So, once you notice any of these symptoms, you want to diagnose and replace a bad thermostat, or better still, contact a professional mechanic for proper inspection and repair.

Osuagwu Solomon

Osuagwu Solomon is a certified mechanic with over a decade of experience in the mechanic garage, and he has over five years of experience in the writing industry. He started writing automotive articles to share his garage experience with car enthusiasts and armature mechanics. If he is not in the garage fixing challenging mechanical problems, he is writing automotive repair guides, buyer’s guides, and car and tools comparisons.

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