All cylinder head gaskets and gasket sets come with
detailed installation instructions or at least installation tips from
the manufacturer. Follow these carefully; not matter how many head
gaskets you have replaced. Many modern gaskets come precoated with
either a sealer or a hi-tack coating to assist with correct gasket
placement and sealing. (See Fig 3).
If you were to apply an additional gasket sealer or
gasket maker, you could actually inhibit the ability of the gasket to
seal correctly. In addition, many gaskets have different torquing or
pre-torquing specifications that must be followed to ensure a good seal.
Failure to follow these instructions will most likely result in a
A frequently overlooked component when replacing
cylinder head gaskets are the cylinder head bolts. The best rule to
follow is if in doubt, replace them. (See fig 4). Cylinder head bolts
can stretch during their working life.
constant temperature cycling causes them to expand and contract,
changing their dimensions. All it takes is an extra few thousands of an
inch to provide an incorrect torque reading. Unless you happen to be
working on a brand new or near new engine (less than 10,000 miles)
replace the head bolts. Most head bolt sets can be purchased for under
$25.00. A small price to pay to insure a quality repair.
WHAT CAUSES OVERHEATING?
Overheating can be caused by anything that decreases the cooling
system’s ability to absorb, transport and dissipate heat. A low
coolant level, loss of coolant (through internal or external leaks),
poor heat conductivity inside the engine because of accumulated
deposits in the water jackets, a defective thermostat that doesn’t
open, poor airflow through the radiator, a slipping fan clutch, an
inoperative electric cooling fan, a collapsed lower radiator hose, an
eroded or loose water pump impeller, or even a defective radiator cap.
One of nature’s basic laws says that heat always flows
from an area of higher temperature to an area of lesser temperature,
never the other way around. The only way to cool hot metal, therefore,
is to keep it in constant contact with a cooler liquid. And the only way
to do that is to keep the coolant in constant circulation. As soon as
the circulation stops, either because of a problem with the water pump,
thermostat or loss of coolant, temperatures begin to rise and the engine
starts to overheat.
The coolant also has to get rid of the heat it soaks
up while passing through the block and head(s). So the radiator must be
capable of doing its job, which requires the help of an efficient
cooling fan at slow speeds. Finally, the thermostat must be doing its
job to keep the engine’s average temperature within the normal range. If
the thermostat fails to open, it will effectively block the flow of
coolant and the engine will overheat.
WHAT TO CHECK
If your engine overheated and the cause hasn’t been determined, all of
the following should be checked to make sure the engine doesn’t
Severe overheating can often damage a good thermostat. Therefore, if
the engine has overheated because of another problem, the thermostat
should be tested or replaced before the engine is returned to service.
One way to check the thermostat is to start the engine
and feel the upper radiator hose. The hose should not feel uncomfortably
hot until the engine has warmed-up and the thermostat opens. If the hose
does not get hot, it means the thermostat is not opening.
Another way to test the thermostat is to remove it and
dip it into a pan of boiling water (it should open). The exact opening
temperature can be checked by using a thermometer.
If the thermostat needs to be replaced, install one
with the same temperature rating as the original. Most cars and light
trucks since 1971 require thermostats with 192 or 195-degree ratings.
Using a cooler thermostat (160 or 180 degree) can increase fuel and oil
consumption, ring wear and emissions. On newer vehicles with
computerized engine controls, the wrong thermostat can cause major
performance and emission problems if the engine fails to reach the
proper operating temperature. Cooling system leaks Loss of coolant
because of a leak is probably the most common cause of overheating.
Possible leak points include hoses, the radiator, heater core, water
pump, thermostat housing, head gasket, freeze plugs, automatic
transmission oil cooler, cylinder head(s) and block.
Make a careful visual inspection of the entire cooling system, and
then pressure test the cooling system and radiator cap. A pressure
test will reveal internal leaks such as seepage past the head gasket
(usually due to warpage in the head or block, too rough a surface
finish on the head or block, or improperly torqued head bolts) as well
as cracks in the head(s) or engine block. If there are no leaks, the
system should hold pressure for at least a minute or more.
It’s important to pressure test the radiator cap too, because a weak
cap (or one with a pressure rating too low for the application) can
allow coolant to escape from the radiator. Fan With mechanical fans,
most overheating problems are caused by a faulty fan clutch — though a
missing fan shroud can reduce the fan’s cooling effectiveness by as
much as 50% (depending on the fan’s distance from the radiator) which
may be enough to cause the engine to overheat in hot weather or when
The fan clutch disengages the fan when less cooling is needed to
reduce the parasitic horsepower drain on the engine as well as fan
noise. Inside the clutch is a special silicone fluid that acts like a
fluid coupling to turn the fan. Above a certain r.p.m., the resistance
created by the fan exceeds the shear characteristics of the fluid and
the fan ceases to spin any faster. “Thermal” fan clutches also have a
bimetal thermostat spring on the front that increases or decreases the
amount of slippage depending on the temperature of the air flowing
through the radiator. This allows more or less cooling as needed.
Defective fan clutches are a common and often
overlooked cause of overheating. The shear characteristics of the clutch
fluid gradually deteriorate over time, with an average loss in drive
efficiency of about 200 r.p.m. per year. Eventually, slippage reaches
the point where effective cooling is no longer possible and overheating
results. (On average, the life of a fan clutch is about the same as a
water pump. If one needs to be replaced, the other usually does too.)
If the fan clutch shows signs of fluid leakage (oily
streaks radiating outward from the hub of the clutch), spins freely with
little or no resistance when the engine is off, or wobbles when the fan
is pushed in or out, it needs to be replaced.
With an electric cooling fan, check to see that the
fan cycles on when the engine gets hot and when the air conditioner is
on. If the fan fails to come on, check the fan motor wiring connections,
relay and temperature sensor.
Try jumping the fan directly to the battery. It runs,
the problem is in the wiring, relay or sensor. If it fails to run, the
fan motor is bad and needs to be replaced.
Any wobble in the pump shaft or seepage would call for replacement. In
some instances, a pump can cause an engine to overheat if the impeller
vanes are badly eroded due to corrosion or if the impeller has come
loose from the shaft. The wrong pump may also cause an engine to
overheat. Some engines with serpentine drive belts require a special
water pump that turns in the opposite direction of those used on the
same engine with ordinary V-belts.
Check belt tension and condition. A loose belt that slips may prevent
the water pump from circulating coolant fast enough and/or the fan
from turning fast for proper cooling.
The condition of the hoses should also be checked.
Though not leaking now, internal corrosion or old age may make them
vulnerable to sudden failure. Radiator and heater hoses should be
replaced if leaking, cracked, brittle, mushy feeling or otherwise
damaged. Make sure the clamps are tight, too.
Sometimes a lower radiator hose will collapse under
vacuum at high speed and restrict the flow of coolant from the radiator
into the engine. This can happen if the reinforcing spring inside the
hose is missing or damaged.
The most common problems radiators fall prey to are clogging (both
internal and external) and leaks. Dirt, bugs and debris can block
airflow through the core and reduce the radiator’s ability to
Internal corrosion and an accumulation of deposits can
likewise inhibit coolant circulation and reduce cooling. “Backflushing”
the radiator and cooling system when changing coolant is highly
recommended to dislodge accumulated deposits and to flush the remaining
coolant from the engine block. Backflushing is running water back
through the radiator and engine in the opposite direction to which it
normally flows. After the coolant has been drained from the radiator, a
T-fitting is installed in the heater inlet hose. The fitting is then
connected to a pressurized water hose or power flusher. The water is
turned on and the system is reverse flushed. The flushing should be
continued until only clean water emerges from the radiator. Cleaning
chemicals may also be used to remove accumulated deposits from the
When the cooling system is refilled, use a 50/50
mixture of ethylene glycol antifreeze and water. This will give freezing
protection down to -34 degrees Fahrenheit, and boiling protection to 265
degrees F. in a pressurized system with a 14-psi radiator cap. A 70/30
mixture will protect against freezing down to -84 degrees F. and
boilover up to 276 degrees F. Do not use more than 70% antifreeze
because antifreeze carries heat less efficiently than water. Straight
water should never be used in the cooling system because it offers no
boilover or freezing protection and no corrosion protection (which is
extremely important in today’s bimetal and aluminum engines).
When refilling the cooling system, be sure you get it
completely full. Air pockets in the head(s), heater core and below the
thermostat can interfere with proper coolant circulation and cooling.
Some cars (mostly front-wheel drive) may have one or more “bleeder
valves” for venting trapped air from the cooling system. On some
vehicles, it may be necessary to temporarily loosen a heater hose to get
all the air out of the system.
Other factors that can contribute to overheating
include retarded ignition timing, detonation/pre-ignition, a lean
air/fuel mixture, exhaust restrictions (partially plugged converter or
muffler), a radiator that’s too small for the engine, and overworking
the engine (towing, mountain driving, etc. in unusually hot weather).
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Produced by the National Automotive Radiator Association (NARSA)