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PREVENTING CYLINDER HEAD GASKET AND COOLING SYSTEM FAILURES
One of the most important parts of the
cooling system is also the most invisible
A cylinder head gasket is required to effect a seal
between the cylinder head and block of a gasoline or diesel engine. It is an
integral component of the engine and is requires to perform many functions
at the same time during engine operation.
The head gasket must maintain the seal around the
combustion chamber at peak operating temperature and pressure. The gasket
must seal against air, coolants, combustion and engine oil at their
respective peak operating temperature and pressure. The materials used and
design employed must be thermally and chemically resistant to the products
of combustion and the various chemicals, coolants and oils used in the
When assembled, the head gasket becomes an important part
of the total structure of the engine.
supports the cylinder head along with its operating components. It must be
able to withstand the dynamic and thermal forces that are transmitted from
the head and block. The type of engine application will be the determining
factor in cylinder head gasket design. With engines ranging in size from one
cylinder gasoline fired engines up to twelve cylinder, turbocharged or
supercharged high-compression diesels, the material and design of the gasket
is paramount to it’s functional life span.
Every application requires a unique cylinder head gasket
design to meet the specific performance needs of the engine. The materials
and designs used are a result of testing and engineering various metals,
composites and chemicals into a gasket that is intended to maintain the
necessary sealing capabilities for the life of the engine.(See Fig. 1).
The most widely used materials are as follows:
|Steel and stainless steel of various grades and forms. |
|Fiber based composite materials. |
|Graphite in various densities. |
|Chemical formulations containing polytetrafluorothylene, silicone,
nitriles, neoprene, polymeric resins and others. |
HOW TO PREVENT HEAD GASKET FAILURES CAUSED BY ENGINE OVERHEATING
Engines are designed to operate within a “normal” temperature range of
about 190 to 220 degrees F. A relatively consistent operating temperature
is absolutely essential for proper emissions control, good fuel economy
If the engine overheats and exceeds its normal operating
range, the elevated temperatures can cause extreme stress in the cylinder
head, which may result in a head gasket failure. This is especially true
with aluminum cylinder heads because aluminum expands about two to three
times as much as cast iron when it gets hot. The difference in thermal
expansion rates between an aluminum head and cast iron block combined with
the added stress caused by overheating can cause the head to warp. This, in
turn, may lead to a loss of clamping force in critical areas and allow the
head gasket to leak.
What else can happen when an engine overheats? Coolant can
boil out of the radiator and be lost. Pistons swell inside their cylinders
and can scuff or seize. Valve stems can swell in their guides and also scuff
or seize. This, in turn, may damage valve train components (broken rocker
arms, bent pushrods, etc.) or possibly result in damaging contact between
the valve head and piston if the valve sticks open. Valve lifters can also
stick, possibly causing a valve to remain open a little too long. Bearings
can seize. Cylinder heads can crack (especially if someone dumps cold water
into the radiator in an attempt to “cool off” the engine). Combustion
chambers can become so hot that a spark is no longer needed to ignite the
fuel, leading to a condition known as “preignition” where the engine
misfires and runs erratically. Air/fuel mixtures are upset, and gasoline
becomes less able to resist detonation. Oil thins out and is less able to
protect the engine’s internal components against friction and wear.
When a localized hot spot forms, it causes the surrounding metal to swell
excessively. This, in turn, can crush the head gasket causing the gasket
to leak, erode and/or eventually burn through. Hot spots also create added
stress in the head itself, which may cause the head to warp (go
out-of-flat) and/or crack.
Aluminum cylinder heads with Siamese exhaust valves (such
as the Chrysler 2.2L and Honda 1.3L and 1.5L) seem to be especially
vulnerable to localized overheating in the area between adjacent exhaust
valves. This is typical of head designs that restrict or limit coolant flow
and circulation in critical areas. Some engine blocks with siamesed
cylinders also provide minimal cooling between the cylinder bores. Even
engines like smallblock Chevy V8s that have adjacent exhaust valves in the
two center cylinders can experience hot spots if other factors are present,
such as overheating, detonation and/or pre-ignition.
As long as the coolant level is okay and the cooling
system is functioning normally, there should be no problems. But if there’s
a loss of coolant due to a leak, an air pocket in the cooling system, a
cooling problem that causes the engine to overheat or some other type of
engine problem that causes normal combustion temperatures to soar (such as
loss of EGR, incorrect ignition timing, vacuum leak, lean air/fuel mixture,
exhaust restriction, etc.), the result can be the formation of localized hot
spots and head gasket failure.
PINPOINTING HOT SPOTS
A head gasket that has failed because of excessive crush created by a
localized hot spot will be measurably thinner in the damaged area when
checked with a micrometer. By comparison, a gasket that has failed due to
detonation or pre-ignition will usually have cracked armor around the
combustion chamber, which leads to burnthrough. (See Fig. 2).
The corresponding surface areas on both the head and
engine deck where the gasket failed should be inspected for damage (erosion,
pitting or cracks) as well as flatness. If either surface is damaged or is
not flat, the head and/or engine block must be resurfaced otherwise the new
head gasket may not seal properly. What’s more, the same conditions that
caused the original hot spot to develop may still be present, which will
only make matters worse.
AIR POCKET DAMAGE
One of the most common causes of localized hot spots is air in the cooling
system. Air pockets can form when the cooling system is being refilled
after a coolant change or when other engine repairs are being made (valve
job, replacing a water pump, thermostat, etc.). As coolant is being poured
into the radiator, the thermostat often blocks the venting of air from the
engine leaving air trapped in the upper portion of the block and/or heads.
Some thermostats have a small bleed hole or jiggle pin to prevent this
from happening, but many do not. Some engines also have special bleeder
valves on the thermostat housing or elsewhere to help vent trapped air
from the system.
If the trapped air isn’t removed, it may cause localized
hot spots to form when the engine is started. The trapped air may also
prevent the thermostat from opening and cause the engine to overheat. That,
in turn, may lead to additional damage such as head cracking or warping.
Another symptom of air trapped in the cooling system would be little or no
heat output from the heater when the engine is warm.
IF AN ENGINE HAS OVERHEATED...