with everything in modern engine mechanics, cylinder head gasket design
has become more sophisticated than in years past. Aluminum generally
expands at a slightly higher rate than cast iron and, because the
cylinder head usually runs hotter than the cylinder block, the head
expands faster than the block and actually slides across the cylinder
head gasket as the engine temperature increases.
This small amount of
slippage eventually wears out the gasket and causes it to fail. In
addition, because aluminum heads expand faster than the steel head
bolts that clamp the gasket to the cylinder block, the head bolts tend
to lose torque when the head gaskets crush or extrude during an
To combat head gasket scuffing and maintain
cylinder head bolt torque on modern engines, graphite-based designs
have been introduced that allow the aluminum cylinder head to slip
smoothly over the gasket surface.
The most recent development in
cylinder head gasket design is the multi-layer steel (MLS) gasket. The
more expensive MLS gaskets use multiple layers of coated spring steel
shim gaskets to accommodate cylinder head slippage while maintaining
cylinder head bolt torque and cylinder sealing. The MLS design is least
susceptible to loss of head bolt torque after an overheat condition.
GASKET FAILURE PATTERNS
head gasket leakage can be difficult because a failure pattern very
much depends upon operating conditions, the amount of cylinder head
warping present and the relative tension of the head bolts. In
addition, a cracked cylinder head often mimics a head gasket failure,
the only difference being that a cracked cylinder head seldom results
in relatively minor, but consistent, cylinder sealing failure.
failure patterns are the most common. As always, the primary symptom of
a leaking head gasket is an increasing consumption of coolant with no
apparently visible external coolant leakage.
If cylinder leakage occurs
during cold-engine operation, combustion gases accumulate under the
engine’s thermostat during cold engine operation, forcing the coolant
back through the radiator and resulting in an overflow condition at
the coolant reservoir. If, on the other hand, leakage occurs only under
full throttle, hot engine operation, the coolant may become aerated
with combustion gases, which reduces cooling system efficiency and
increases operating temperatures.
When the head gasket completely
fails to seal the cylinder, the cylinder loses compression and the
spark plug becomes fouled with coolant, causing an engine misfire. The
engine oil may also become contaminated with coolant. In most cases,
coolant-contaminated oil exhibits a milky, viscous appearance. In
addition, the underside of the oil filler cap and filler tube tends to
accumulate a viscous, milky residue that indicates a high degree of
moisture in the crankcase.
In still another type of failure pattern,
if an intermittent misfire code is recorded on an OBD II engine
management system, always check the freeze-frame data to determine the
conditions under which the misfire occurs. If the misfire occurs during
a specific set of conditions, and the spark plug in question exhibits
an uncharacteristic coloration on the combustion side insulator, the
misfire might be caused by coolant leaking into the combustion chamber.
most common method of detecting head gasket leakage is to remove the
radiator cap, warm up the engine to open the thermostat and look for
gas bubbles circulating with the coolant. A combustion gas detection
tool can also be used to detect traces of combustion gas in the cooling
The tool is filled with a blue-colored fluid to the
specified level and, being careful not to inhale coolant, the rubber
bulb is used to draw air from the radiator head tank or coolant
reservoir through the fluid in the tool.
The presence of combustion gas
in the coolant is confirmed when the fluid turns from blue to yellow.
An exhaust gas analyzer can also be used, but care must similarly be
taken to prevent damaging the equipment by letting it inhale liquid