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Head Gasket


 

The head gasket is located between the cylinder head and the engine block (short block).

Head gaskets are mainly made of a fiberous material, much like paper.  They somtimes include thin layers of metal.  The edge around combustion chambers are usually surrounded by metal.

The most common cause of a head gasket leak (also know as a "blown" head gasket) is overheating.  Overheating may cause the head gasket to deform or break.  Another cause of head gasket failure is deterioration.  Over time, metal in head gaskets may corrode or the fiberous head gasket material may deteriorate.

A failed head gasket will have different effects, depending on where the break/breach occurs.  The following are types of breaches and effect:

  • Combustion Chamber > Coolant Passage:  This type of failure causes combustion gases to enter a coolant passage.  The gases will displace (push) coolant out of the cooling system.  In extreme cases, you will be able to smell exhaust fumes in the radiator or coolant reservoir.  In less obvious cases a "block test" is required to detect combustion gases in the cooling system.  A block test uses a dye that changes colors when it detects combusiton gases.

More on this subject: It is also possible for coolant to enter the combustion chamber (see below, "Coolant Passage > Combustion Chamber") but this isn't always the case when the defect in the head gasket is very small.  Pressure inside a typical cooling system is around 15 psi.  Pressure in the combustion chamber can 150 psi to 1,000 psi.  A very small defect may be able to withstand 15 psi of coolant pressure from forcing coolant into the combustion chamber, but can't resist the extreme pressures created by combustion.  So, it is common to find combustion gases in the cooling system without finding coolant in the combustion chamber.

  • Coolant Passage > Combustion Chamber:  This type fo failure causes coolant to enter the combustion chamber.  When combustion occurs, the coolant is turned into steam and exits the exhaust pipe.  In extreme cases, the steam will be visible from the tailpipe.  In less major cases, such small amounts are burned that the steam may be invisible.  You will experience a loss of coolant.  Removing the spark plugs will reveal one or more spark plugs that are wet with moisture.

  • Coolant Passage > Oil Passage:  This type of failure causes coolant to enter an oil passage.  Oil contaminated with coolant will appear milky or cloudy.  Checking for residue under the oil filler cap or dipstick will reveal this condition.

  • Combusiton Chamber > Combustion chamber:  This type of failure causes combustion pressure to leak into another combustion chamber.  A compression check will reveal loss of pressure in one or more cylinders.

  • Oil Passage > Coolant Passage:  This type of failure causes oil to enter a coolant passage.  Checking the coolant reservoir for oil residue or checking coolant in the radiator for floating oil (with engine off) will reveal this condition.

  • Combustion Chamber > Oil Passage:  This type of failure causes combustion gases to enter an oil passage.  A compression check will reveal loss of pressure in the affected cylinder.

  • Coolant passage > Outside: Coolant leaks out of the engine between the cylinder head and engine block.

  • Oil passage > Outside: Oil leaks out of the engine between the cylinder head and engine block.  Note: This is rare because oil passages are usually "return" passages which are under very little pressure.  Return passages are the passages that allow gravity to pull oil from the top of the engine, back to the bottom (oil pan).