Can a Bad Thermostart Cause a Blown Head Gasket

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There are some makes or models of cars that have a reputation for blowing head gaskets.  One such vehicle has such a bad reputation for head gasket problems there is a group of owners trying to get the manufacturer to have a recall on them.  If you happen to own one of these vehicles, don’t worry it does not mean that you have a bad car or that you are going to spend lots of money.

When you think head gasket, I want you to think of it a lock that forms a seal between the engine block and the cylinder. 

Meaning that it’s the duty of head gasket to seal both high-pressure and extremely hot combustion gases, as well as engine coolant.

And also, this can be anywhere, whether cold ambient temperatures or the normal operating temperature of an engine.  

So because of this, it is not unusual for a head gaskets to develop leaks as time goes by since it has wide range of temperatures and relatively large surface space.

Can A Bad Thermostat Cause A Blown Head Gasket?

Any thermostat that doesn’t sends coolant to the engine radiator has the ability to cause overheating, and this can result in complete head gasket failure.

This means that if the thermostat stays in the closed position and causes the engine to overheat, then yes, it can cause a blown head gasket.

What is a Head Gasket?

Let’s start by talking about what a head gasket is.  The head gasket lives between your engine block and cylinder head.  Every modern car has this gasket but they vary in their thickness and construction based on the manufacturer’s design of the engine.  

The head gasket is so important because it seals the combustion chamber allowing your car to build the proper compression and contain exhaust gases both of which aid in maintaining the efficiency of your engine. 

The head gasket also keeps coolant and oil out of the combustion chamber which is important for the same reasons you wouldn’t want coolant or oil leaking anywhere else.

Now that we understand what a head gasket it, it’s important to learn what head gasket failure symptoms look like to avoid higher expenses due to additional engine damage caused by driving with a blown head gasket.  To understand the symptoms it can be helpful to understand why a head gasket might fail.

Why Do Head Gaskets Blow?

The head gasket forms a seal between the engine block and the cylinder head.  This means your head gasket has to seal both extremely hot, high-pressure combustion gases as well as engine coolant which can be anywhere from cold ambient temperatures to the normal operating temperature of your engine.  Due to the wide range of temperatures and relatively large surface area, it is not unusual for head gaskets to develop leaks over time.  This can happen regardless of the make or model of your car or the type of head gasket used.  To better understand why a head gasket might blow, check out this article on why head gaskets blow.

Since the head gasket seals the coolant passage both from the atmosphere and the combustion chamber you can’t see much of the head gasket on a vehicle with the engine installed.  Because much of the gasket can’t be seen without disassembling the engine, blown head gaskets symptoms can be very difficult to diagnose.  Since a visual inspection usually will not prove a head gasket leak, it is important to know the other symptoms so you can accurately diagnose a head gasket problem.

How To Tell if a Head Gasket Is Blown:

  • Coolant leaking externally from below the exhaust manifold
  • White smoke from the exhaust pipe
  • Bubbles in the radiator or coolant overflow tank
  • Overheating engine
  • White milky oil
  • Fouled spark plugs
  • Low cooling system integrity

You can watch this video for more symptoms of blown head gasket.

Effect of an External Head Gasket Leak

A head gasket leaking external would cause coolant to come from below the intake or exhaust manifold and often only happens when the engine is completely warmed up.

 If there are no other cooling passages or hoses near the head gasket you may be able to positively identify the leak as a head gasket leak, but if there are other cooling passages nearby, you may need to add a UV dye to the coolant  then watch the head gaskets with a UV light to positively identify the leak.

White Smoke From Tailpipe

Most head gasket leaks are internal to the engine allowing coolant to flow into the combustion chamber on every intake stroke.  

When this happens to coolant burns/evaporates with the combustion process and appears as white smoke coming from the tailpipe.  

This smoke can be differentiated from moisture during a cold start by a sweet smell and will continue even when the engine is warm.  If the leak in the head gasket is large this white smoke can often be excessive and billow from the tailpipe.

Bubbles in the Radiator

Besides allowing coolant into the combustion chamber, an internal head gasket leak allows exhaust gases into the coolant. 

This can cause bubbles to be in the radiator or coolant reservoir making the coolant look like it’s boiling even when it’s cold.  

The bubbles are exhaust gases that force their way into the cooling system during the combustion process. 

An easy do-it-yourself test for a blown head gasket is to perform a chemical test using this type of tester on your coolant to check for the presence of exhaust gases to see if this is happening in your car.  This is the most effective blown head gasket test and can give you a positive sign of a blown head gasket.

Overheating Engine

If you’ve got a blown head gasket, your engine usually will overheat after longer drives.  This happens both due to the lack of coolant as your engine consumes it, but also the efficient combustion process, the excess heat from the exhaust in the coolant and the inability of your vehicle’s radiator to cool the dirty coolant.  If your engine overheats it can cause lots of problems. 

The biggest concern is the expansion of metal components past what they were designed for which can cause cracks and warping. 

Also, it can permanently damage seals and gaskets cause other leaks in your engine.  Both of these problems often require a full engine rebuild to remedy.

White or Milky Oil

As coolant leaks into your combustion chamber, it will seep past your piston rings into your oil.  Over time oil and water will mix and cause the oil to turn a milky white.  You can look for this on your dipstick and around your engine oil cap.  Having water in your oil will make your oil ineffective in properly lubricating your motor which will quickly allow wear on your cylinder walls and on the crank and camshaft bearings. 

Even if you don’t drive the vehicle, the presence of water in the oil can cause rust on machined surfaces which can lead to pitting in the metal and necessitate and engine rebuild.

Fouled Spark Plug

As coolant burns in your combustion chamber, it will leave tiny white deposits on your spark plug usually around the ground strap and electrode.  

Other problems can cause these white deposits so this isn’t a conclusive blown head gasket symptom but if others are present it could give you more proof. 

Low Cooling System Integrity

If there is a leak from your head gasket, pressurizing your cooling system and watching for pressure loss can help prove you have a blown head gasket as well.  

Since there could be other leaks you don’t know about, this also isn’t a conclusive test, but again just more indications that you may have a blown head gasket. 

Similarly, you can perform a leak down test which pressurizes the combustion chamber with compressed air and measured the amount of air that leaks out through the head gasket or any other opening the combustion chamber.

Related Article:

Can I Drive With a Blown Head Gasket?

If you have multiple blown head gasket symptoms, it is important to drive your vehicle as little as possible.  The hot gases and cold coolant moving through the hole in the gasket can quickly erode or warp the metal head or engine block leaving you with costly machining bills or even having to purchase new heads or a new engine and having water in your engine oil can destroy bearings quickly.

How to Prevent a Head Gasket Failure

There are some ways you can reduce your chances of blowing a head gasket.  The first is to keep combustion chamber pressures as low as possible. 

If your vehicle is turbocharged or supercharged, make sure your boost level is set to the factory setting to keep combustion pressures in check. 

Also, make sure your engine doesn’t have any pre-ignition or knocking from overly advanced timing or carbon build up.  Similarly, maintaining low engine RPMs will reduce stress and heat on your head gasket so avoid running your engine at high RPM. 

Lastly, if your car has a manual transmission avoid downshifting to slow the car and rev-match anytime you can to reduce pressure on your head gasket.

If you are rebuilding an engine, you can reduce the risk of a blown head gasket in the future by using properly lubricated head studs torqued in the proper sequence to the correct torque setting.  You can also make sure your block deck and cylinder head mating surface are properly prepared for the new gasket.  Similarly, using a multi-layer steel or other metal head gasket can improve the reliability of your engine.  For more information, check our complete article on how to prevent a blown head gasket. 

These are just a few different steps you can take to prevent a head gasket failure, but sometimes even following these steps you may still end up with a blown head gasket.

If not taken care of right away, blown head gaskets can turn into bigger engine problems that will end up being even more costly to fix down the road.

A thermostat opens and closes to regulate the engine coolant temperature. Let’s say the thermostat is a 195* F thermostat, if the engine coolant temperature is below that the thermostat will remain closed, which keeps the coolant from flowing thru the radiator, coolant will still flow thru the vehicles heater core allowing the cabin to warm with the engine. Coolant will also circulate around the engine block to help it warm up as well.

Now when the thermostat opens coolant will flow thru the radiator and back into the engine this allows the outside air to cool the engine, and when the engine cools down below the 195* F, the thermostat closes, stoping the flow they the radiator again.

This process repeats over and over, on a hot summer day, most likely your thermostat will remain open as the engine is constantly trying to remain cool.

With that being said, if your thermostat sticks open if will take an increased amount of time for your engine to warm up and in winter it may never reach operating temperature, and the cars heater will not blow hot air, warm air at best. If your thermostat remains closed and stuck closed the coolant will not flow thru your radiator, resulting in increased engine temperature which results in overheating the engine.

Blown head gaskets most of the time is caused by the cylinder head warping, not always mind you, sometimes the gaskets do give out.

The cylinder head and block should have a precision machined mating surface, to make them as flat as possible and within manufacturers tolerance.

When an engine is overheated the heads may warp, causing them to twist, or just become uneven at the mating surface.

Generally, the head is taken off and machined and installed with new gaskets and new head bolts.

Now the engine block surface area that the head is mounted to can also become uneven from the extreme temperature changes. Both the block and he’s head should be checked.

Can a Bad Thermostart Cause a Blown Head Gasket

Now back to the question can a stuck thermostat cause a blown head gasket, my answer is this: the stuck thermostat caused the increased coolant temperature however it does not directly blow the car head gasket if the driver were observant to stop driving when this coolant temp is high. On the other hand, the driver who was not watching the gauges or warning lights that continued to drive, while the coolant temperature was too high, is the reason the head gasket is blown.

Yes, that happens all the time. If you drive even for a few minutes with the engine overheating, it is quite possible to blow a head gasket.

The older the car, the more likely that can happen. When the engine gets past 100,000 miles (260,000 kilometers), they are not quite as tolerant of you running out of oil, running out of coolant, placing excessive loads on them, etc.

As a home mechanic, you don’t generally “test” for a blown head gasket.

A leaking, or “blown” head gasket will come with some pretty obvious symptoms. That is because what is happening in your engine is that two parts of the engine containing two different fluids – that are normally kept separated by the gasket – now have a path between them for fluids from one part to spill over into the other.

The head gasket sits between the block, where the cylinders and pistons are; and the head, where the intake and exhaust manifolds, spark plugs, and valves are.

Oil circulates around the head, lubricating everything. Coolant circulates in a space around the cylinders called the “coolant jacket”, which helps the engine maintain a steady operating temperature.

When the gasket develops a leak, oil vapor can be forced into the coolant stream (along with combustion and exhaust gasses from the cylinders), and coolant will find it’s way into the oil sump and valve cover atop the head. Coolant may also find it’s way into the exhaust, and you may notice a rapid loss of coolant when this happens.

The injection of hot oil vapor and really hot combustion gasses into the coolant will lead to an overheating problem. You may notice a fair and steady loss of coolant as well.

Since coolant is water-based, oil traces will float on top of the coolant in the radiator, and in the coolant tank next to the radiator.

Coolant in the oil will deposit in the oil sump and inside the valve cover atop the head as a greasy brown “chocolate milkshake gunk”.

Oil in the coolant can (sometimes) leave a “mayonaise” residue on the radiator cap. Same idea, only whitish instead of brown.

That said, there are actual tests you can perform to confirm a leaking head gasket. Most of these tests require specialty tools not generally available to a DIY mechanic

In my experience, the first thing you notice is the vehicle just isn’t running right. Maybe a little rough at idle, feels better as you come up to speed. But the power is down a little too, it feels like it’s working a little harder going up hills.

That’s from the loss of power. At least one cylinder is not running at full power, due to having lower compression, due to a leaking gasket.

That, and potentially coolant in the cylinder, damping combustion.

That can go on for a long time.

The next step is when you start to notice low coolant. You don’t see any leaks, but either you get a “temp” light or you serendipitously discover that your coolant is low when you’re under the hood for something else.

That’s never a good thing, so you top up, and check your oil. Whew. No “mayonnaise” in the crankcase from mixing oil and water, keep driving.

But it keeps happening. Low power, getting worse. Low coolant, frequently.

And now, when you take off the cap to the coolant reservoir, there’s this “whoosh” sound as air escapes. That’s not supposed to happen.

You might get a situation where air is rushing INTO the reservoir (cooling liquid makes a vacuum as it contracts) if there’s a blocked line somewhere, but never OUT of the reservoir. That means something is pressurizing it.

What To Read Next: Blown Head Gasket Diagnosis, Symptoms and Preventive Measures

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