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Blown head gasket. To the uninitiated, it sounds like just another car repair term – an innocent phrase that might be mentioned alongside things like low tire pressure and warped rotors. The reality is that it’s a major car repair, one that can result in catastrophic engine damage if the early warning signs are ignored.

Don’t discover that your car suffered a head gasket failure only after steam starts billowing from the engine bay on your drive to work. By arming yourself with a bit of knowledge about what a head gasket is, how it can fail, and the telltale symptoms that usually accompany it conking out, you’ll be that much less likely to be caught off guard should your car indeed blow one of its most critical gaskets. If you’re a Subaru owner, consider this required reading.


What Is a Head Gasket?

A head gasket isn’t just any old rubber plug – it’s a precision-formed mechanical seal that’s typically made out of thin layers of steel woven together. Your engine needs this gasket in order to provide an airtight seal between the cylinder head and the engine block. Colloquially, these two portions of the engine are known as the top end and bottom end, respectively.

The bottom end of an engine is the engine block itself. This is essentially the foundation of the engine; imagine something roughly the shape of an anvil that’s been plumbed and routed to make room for the crankshaft, pistons, and other major components. 


Think of the top end of an engine as the cylinder head. The cylinder head’s purpose is twofold: provide compression and house the valves. The valves let the air/fuel mixture into the cylinder and release the exhaust gasses out once the pistons have compressed this mixture enough so that a single jolt from the spark plug causes combustion. 

This intake, compression, combustion, and exhaust process typically happens thousands of times a minute and has been the foundation of almost every four-cycle automotive engine ever made. The underlying principle allowing this concept to function so flawlessly is a vacuum—and you won’t have sufficient vacuum without an airtight seal between the top and bottom ends of an engine. The head gasket provides this seal, enabling reliable performance even when you wring out your engine to redline.


What Is a Blown Head Gasket?

Here’s the short and sweet answer: a blown head gasket is when the gasket that secures the cylinder head to the engine block has failed, compromising the airtight seal necessary for adequate performance and reliability. 

In reality, a head gasket failure is a bit more complicated. It’s easy enough to say the seal is blown out, but it takes a bit more finesse to determine where exactly the gasket failed and what the resulting impacts are. This is because the engine block isn’t just bored out for the pistons –  it’s also crisscrossed with coolant and oil passages that snake between the top and bottom ends of the engine to provide critical cooling properties. Like the veins that bulge in your forehead when you realize you’ve lost yet another 10mm socket, these passages are conduits transmitting the lifeblood of your engine – and they have to run right through the head gasket in order to reach the cylinder head. 

When the head gasket fails, the exact point where it becomes compromised depends on whether you’re now dealing with oil leaking externally, coolant seeping into your engine, or a cylinder that isn’t as airtight as it should be. Of these options, the best-case scenario is an external leak. The worst-case scenario? An internal leak that leads to oil and coolant mixing together.


Head gasket failure doesn’t just allow coolant and oil to go where they shouldn’t – it can also compromise what had been a pressurized, airtight combustion system, which can lead to reduced compression in one or more cylinders. If that’s the case, your car will suddenly feel sluggish and weak, as if it’s suffering from a perpetual hangover.

We know what you’re thinking: why can’t we just bolt the top half of the engine right to the lower half and call it a day? Can’t engineers just relax and do it the easy way for once?

If only that were the case. The reality is that the head gasket is a critical element that tempers the effects of something called thermodynamics. If you’ve never heard of this concept before, that’s alright – most of us also played hooky from science class that day. Let’s quickly revisit it here:


Head Gaskets & Thermodynamics

Boiled down to its most basic definition, thermodynamics refers to the relationships between heat, energy, and the environment. In an engine, temperatures can get high enough to put your oven to shame, which can cause internal and external components to expand and contract. Warpage at high temperatures is another concern as well—just put a cheap cookie tray in the oven and see how flat it is after 20 minutes at 450 degrees.

Warpage, contraction, and expansion all become critical to manage when you have an engine that features different materials for its top and bottom ends, like when automakers pair iron blocks with aluminum cylinder heads. Without proper heat management, these materials will expand or contract at different rates, which could ultimately impact that airtight fitment so critical to proper engine operation. 


That’s where the head gasket comes in. Even in the merciless environment of an engine bay, the head gasket makes sure the top and bottom ends of the engine stay firmly cemented together. The airtight unity of these parts ensures that oil and coolant can flow happily between the block and head and that compression will hold steady regardless of the thermal forces the engine is subjected to. Think of the head gasket like super glue, and you’ll have the right idea.


Symptoms of a Blown Head Gasket

When you blow a head gasket, a portion of the gasket itself becomes crushed, blown out, or otherwise damaged. Depending on where exactly the gasket fails, you’re looking at oil, coolant, or compression leaks—maybe even a mix of all three. Here are the classic consequences of a head gasket failure: 

  • External coolant or oil leak
  • Internal coolant or oil leak
  • Loss of compression


What Happens When A Head Gasket Fails?

Let’s discuss compression first. You can think of this failure like a garden hose. A proper hose with a quality nozzle can deliver impressive water pressure. Now imagine the hose suffers a small puncture. What happens? Water being what it is, it’ll escape through the puncture, reducing the available pressure you have to water your gardenias. 

Enlarge the puncture and the pressure from the nozzle will be reduced further. It’s the same story with losing compression from a blown head gasket. It doesn’t matter whether the gasket blows out between two cylinders or between a cylinder wall and an outside edge – the airtight seal necessary for correct operation is gone. The affected cylinders won’t be able to maintain effective compression, leading to lackluster or spongy performance. 

A head gasket failure along a coolant or oil passage comes with its own set of woes. These fluids act like an automatic climate control system, helping regulate temperatures to keep the engine from overheating. When they become compromised, fluid can leak either out of the engine or into the cylinder. 


To determine if a gasket failure has resulted in the latter, check to see if your tailpipe is emitting plumes of white smoke – a classic sign that oil or coolant is getting burned up in combustion. Just keep in mind that there are a few possible causes for oil or coolant getting into the combustion chamber, so consider your head gasket innocent until proven guilty if your car starts spewing white or blue smoke.

Oil and coolant can also intermix thanks to a blown head gasket. You’ll know if this is happening if you find a milkshake-esque under your oil cap or on your dipstick. This is a tell-tale sign of a blown head gasket, not proof that your car is developing a sweet tooth.


Why Does a Head Gasket Fail?

Head gasket failure can stem from a variety of causes. The simplest is just a subpar gasket – pour one out for all the Subaru owners out there. Perhaps the most common cause of a blown head gasket? Overheating.


We already discussed how thermodynamics can affect your engine. Once an engine begins to overheat, things can get precarious. Engine materials like metal and aluminum will begin to react to the excessive temperatures. Symptoms include warpage, which we’ve discussed, and head lift, which is when the cylinder head literally begins to lift off the head gasket due to heat and stress.

When the engine and head begin to succumb to these forces, the head gasket is put under immense, immense pressure that exceeds what it was reasonably designed to accommodate. Eventually, it can no longer uniformly bind the head to the block. The cylinder head then shifts or warps, and where exactly this occurs is where the head gasket will be crushed or blown out.

Keep in mind that because engine tolerances are so precise,  all it takes is a few excessive millimeters of cylinder head movement to cause a blown head gasket. As for the temperature required to trigger this series of events? Every engine will have a different threshold, but generally, anything over 250 or 260 degrees celsius puts you at severe risk of blowing a head gasket. 

To avoid this scenario, watch your temperature gauge. If you ever see it start to climb out of its normal operating range, stop the car as soon as you can and let the engine cool. Whatever you do, don’t let the temp gauge reach its uppermost reading, which is marked in red for a reason. At engine temperatures that high, you’re in serious danger of blowing the head gasket. 


Detonation could be another potential cause of head gasket failure. There’s plenty that could be said about detonation, but we’ll leave it at this: it’s combustion that occurs when it shouldn’t. Unlike when fuel is properly ignited by the spark plug, detonation results in an uncontrolled explosion that can cause cylinder pressure to spike dramatically. If detonation is bad enough, the excessive cylinder pressure can actually cause the gasket to fail from the inside out.


Your best bet to prevent detonation? Use quality fuel that’s the appropriate octane rating for your engine. As for overheating, don’t go too long without giving the coolant system a quick once-over – just a quick check of the overflow tank is enough to ensure there’s adequate fluid in the system. Also, don’t forget to replace the coolant regularly – like all fluids, coolant will break down and lose effectiveness over time, so regular replacement is recommended.


Final Thoughts

The head gasket is a deceptively simple part – a simple-looking bit of woven steel that can have severe consequences if it fails. Anything from underwhelming performance to major overheating issues can be caused by a blown head gasket, and the potential ramifications can be bad enough to do in your engine for good if you don’t catch and rectify the problem right away.

Our advice? Stay vigilant. You already read this article, so you’ll know what to look for should your head gasket decide to retire early. In the meantime, treat your coolant system well. If you can prevent any major overheating incidents, you’re reducing the risk of triggering warpage or excessive expansion or contraction of the cylinder head, which is a leading cause of these failures (if it isn’t shoddy manufacturing). An ounce of prevention is worth a pound of cure, as they say – and when it comes to car repair, that old adage couldn’t be more true.

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Written by :
Anthony Sophinos

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