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Head Gasket
The daddy of poor
running, is the failure of the cylinder
head gasket, this might be noticed via the need of having
to top up the water in the coolant tank on a regular basis, especially if there are no
visible leaks, allowing the water to run low will worsen the
situation over time, in general a failed head gasket might show up
when the car is started from cold..
When cold and started
up does it sound like its mis-firing?
then it clears itself when warm? or after a few seconds?
it sounds like a daunting job but
in 99% of cases it is easily fixable, a gasket kit and a skim of the
head to ensure flatness is usually all that is needed and the time to
do the job, this is where many garages will sting you in
their labour costs, as you then wonder whether the car is worth the
spend...
Why and where does
it fail
The head gasket often fails between the cylinder fire ring [a metal
ring around the large hole [combustion chamber] and the water jacket
[where the water circulates around the block and the head], the
reason it fails can simply be old age and general deterioration of
the gasket material, this allows water to seep through when the
engine is not running and there is no compression in the cylinder to
push the water back, when running and warm the engine might feel
alright, in a bad case the compression from the cylinder will force
its way past the failed gasket, pressurising the cooling system and
this will push water out of the cap
Other failures can
happen if the water is run low, overheating can damage the gasket as
well as the cylinder head itself, another place the gasket can fail
is between the fire rings of no2 and no3 pistons, here the fire
rings can crack leaving a tiny gap where combustion passes through
one way then another, leading to poor running but no water loss.
Some pics, click pics
for bigger version:
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Here the gasket has failed
between the fire ring and the water jacket, this gasket
wasnt very old [about 18months] so that shows that its not
always age related, the engine had never run short of water,
this is cheap gasket and the makers think some sort of
acidic reaction had taken place between the coolant and the
gasket material |
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Here is a new high quality
head gasket as supplied by my local engine machinist, its an
ELRING, a note worthy of a mention here is that this gasket
was more-or-less the same cost as the low quality gasket in
the pic above |
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Before re-fitting, ideally
the cylinder head needs to be "skimmed", this is a process
where the head is put in a huge machine that grinds the head
surface flat, ready to accept a new gasket, the valves can
remain in place but while the head is off the car its an
ideal time to replace the valve stem seals |
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A close up pic of the head
after skimming, if you look closely you can see the circular
marks left by the grinding process, these marks are totally
flat and do not affect fitting of new gaket |
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