What Does It Mean to Blow a Head Gasket?

Source | Tony Harrison/Flickr

“Blown head gasket" is one of those phrases that transcends car culture into everyday terminology. Even non-car people know the term, and know it can be an expensive fix. Let's examine common head gaskets, how to tell if you blew a head gasket, and steps you can take to prevent this serious problem.

A closer look at head gaskets

First, a head gasket is a simple mechanical seal sandwiched between the surfaces of the engine block and the cylinder head(s). It has two primary purposes:

  1. As part of the combustion chamber, the head gasket helps contain the combustion process so you can have a properly running engine.
  2. The head gasket provides a physical extension of the fluid passages from head to block. Fuel, oil, and coolant all serve a different purpose and need to stay separated while operating in very close quarters.

Being this close to the combustion process, the head gasket has to be tough. Most modern gaskets are made of composite materials or steel, with copper used in high-performance applications. Despite the critical importance of this part, new head gasket sets are surprisingly affordable. Most of the cost of a head gasket replacement is due to the extensive labor involved in removing the cylinder head.

Blown head gasket symptoms:

  • Coolant leaking from below the exhaust manifold. This is the most obvious and easy diagnosis. The head gasket has failed along an outer portion, and a water passage is leaking to the outside of the engine.
  • White smoke from the exhaust pipe. This time, the water passage is leaking internally, letting coolant pass into the hot combustion chamber. Most of what you are seeing is steam and burned glycol. Other than the white smoke, the engine may operate normally until the coolant is gone.
  • Loss of coolant with no visible leaks. Sometimes the gasket barely leaks fluid into the combustion chamber, so even a watchful driver doesn't notice burning fluid. In this case, the first symptom would be low coolant, with no sign of a leak. This is a good reminder to routinely check your fluids.
  • Overheating engine. The coolant either leaked or burned away until there isn't enough left to cool the engine, and it overheats. Hopefully you get a check-engine light before things get too hot.
  • Bubbles in the radiator or overflow tank. This can occur when the gasket failed between the combustion chamber and a water passage. Air can get forced into the cooling system, causing bubbles. This is more serious than it sounds, as the bubbles can build into an air pocket and not allow coolant to pass.
  • White, milky oil. This is a head gasket leak between the oil passage and the water passage. The antifreeze/coolant mixture is forced into the oil, and it's not a great mix. The cooling system takes a double hit here, as the coolant runs low while the oil's ability to pull heat is also compromised.
  • Low power or poor running engine. The gasket has failed to the point that the combustion chamber is compromised. The engine can't maintain proper fuel burning cycle due to lack of compression, so the result is a rough idle and dramatically decreased power.

Prevention pays off

The easiest way to prevent head gasket failure is to ensure your engine does not overheat. Check the radiator and coolant overflow tanks anytime you check your oil level. Add coolant as needed, and watch for any unexplained fluid loss. Check the radiator hoses for splits or frays, and replace at the first sign of damage.

Watch for fluid leaks under your vehicle and any unusual smoke. Signs of a blown head gasket will vary by the engine and the particular way the gasket failed, so take any of the above symptoms seriously. Despite your best efforts, the gasket may fail anyway. Due to its construction and the environment it is in, it won't last forever.

While blown head gaskets can turn into serious engine problems, a properly installed replacement gasket should get your engine as good as new and last several years.

Have any head gasket advice? Ever tried head gasket repair in a bottle? Let us know your thoughts in the comments below.

Last updated June 6, 2018