What’s a common cause of head gasket failure?
A. There can be numerous causes, but one of the most common is engine
overheating. If the engine gets too hot for any reason (loss of
coolant, failure of the water pump, thermostat or cooling fan, breaking
the serpentine belt), the cylinder head can swell so much it will
literally crush the head gasket (usually between the cylinders since
this is the thinnest point on the head gasket). The extruded material
and/or cracked combustion armor then provides a leak path for coolant
and/or combustion gases.
Replacing the head gasket will fix the
leak, provided the head hasn’t warped and isn’t out-of-flat. However,
the underlying cause that made the engine overheat in the first place
also needs to be diagnosed and repaired so the newly installed gasket
will seal properly and won’t leak.
The cooling system on many
late-model vehicles is just barely adequate (to reduce weight and
cost), so only a little loss of coolant or cooling efficiency can make
an engine run hot and overheat. A low coolant level, a dirty cooling
system, a cooling fan that is slow to kick in, a sticky thermostat,
even a plugged converter can all make an engine run hot and overheat.
Q. Can detonation damage a head gasket?
A.
Yes. Detonation is another common cause of head gasket failure.
Detonation causes a sharp spike in combustion chamber pressure, which
over time can overload and crack the gasket armor that surrounds the
cylinder. This leads to burn through and loss of compression.
Detonation
can be caused by a variety of problems. One is an accumulation of
carbon in the combustion chamber that increases compression. Many
late-model engines run fairly high-compression ratios, and some require
premium octane fuel. If compression reaches a point where the fuel
ignites spontaneously before the spark can set it off, the engine will
knock and ping under load.
Mislabeled fuel that does not have the
octane rating claimed on the pump can also lead to trouble in high
compression or turbocharged engines, especially when the engine is
working hard under load or high boost pressure. If there isn’t enough
octane-boosting additives in the fuel, the engine may experience mild
to severe detonation.
Other factors that may increase the risk of
detonation include a EGR system that isn’t working (by diluting the
air/fuel mixture slightly with exhaust, EGR actually helps cool
combustion temperatures under load). Over-advanced ignition timing can
also cause detonation, as can an overly lean air/fuel mixture or a
defective knock sensor. Any problems in the cooling system that make
the engine run hotter than normal will also increase the chance that
detonation may occur.
One way to tell if detonation or overheating
caused a heat gasket failure is to carefully measure the head gasket
with a micrometer in the area where it failed. If overheating or a hot
spot was the cause, the gasket will be crushed and measurably thinner
in the damaged area.
If detonation caused the failure, the armor
around the combustion chamber will usually be cracked. When a customer
is replacing a head gasket, its important to make sure the mounting
surfaces on both the cylinder head and engine deck are clean, smooth
and flat — otherwise the gasket may not seal properly, leak or fail. If
flatness is not within specifications, the head or block will have to
be resurfaced.
For engines with aluminum heads, flatness should be
0.002 in. (0.05 mm) in all directions. If an engine has torque-to-yield
(TTY) head bolts, the old head bolts should be discarded and replaced
with new.