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Overheating
Causes and Cures, Larry Carley |
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Underhood Service Magazine |
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Internal combustion engines run on heat. Chemical energy
in the fuel is transformed into thermal energy when the fuel
burns, which produces mechanical energy to push the pistons,
spin the crankshaft and drive the vehicle down the road.
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As efficient as today’s engines are, they still waste a
lot of the heat energy they produce. The average gasoline
engine is only about 22 to 28 percent efficient. That means
over two-thirds of the heat produced by each gallon of fuel
either goes out the tailpipe or is soaked up by the engine
itself. Diesels squeeze a little more bang out of each
buck’s worth of fuel with efficiency ratings of 32 to 38
percent, but even that leaves a lot of waste heat that must
be managed and carried away by the cooling system.
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Ironically, the hotter an engine runs the more efficient
it becomes. But there’s a limit because aluminum pistons
and heads can only get so hot before they start to soften
and melt. The same goes for cast iron. Engineers have been
tinkering with exotic ceramic materials and metallic-ceramic
alloys in an attempt to build high-temperature, super
efficient engines. They’ve realized some significant gains
but ceramics are still too expensive for everyday
applications.
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How Hot Is Too Hot?
Most engines today are designed to operate within a "normal"
temperature range of about 195 to 220 degrees F. A relatively constant operating
temperature is absolutely essential for proper emissions control, good fuel
economy and performance.
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A 50/50 mixture of water and ethylene glycol antifreeze in the cooling system
will boil at 225 degrees if the cap is open. But as long as the system is sealed
and holds pressure, a radiator cap rated at 15 psi will increase the boiling
temperature of a 50/50 coolant blend up to 265 degrees. If the concentration of
antifreeze to water is upped to 70/30 (the maximum recommended), the boiling
temperature under 15 psi of pressure goes up to 276 degrees.
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So does this mean a cooling system with a maximum concentration of antifreeze
in the coolant (70 percent) can run as hot as 276 degrees without boiling over?
Theoretically yes - but realistically no. The clearances in most of today’s
engines are much, much closer than those in engines built in the 1970s and early
1980s. Piston-to-cylinder clearances are much tighter to reduce blowby for lower
emissions. Valve stem-to-guide clearances also are closer to reduce oil
consumption and emissions, too. Plus, many engines today have aluminum heads
with overhead cams. Such engines don’t handle higher than normal temperatures
well, and are very vulnerable to heat damage if the engine gets too hot.
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Anytime temperatures climb beyond the normal range, the engine is running in
the danger zone.
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Consequences of Overheating
If the engine overheats, the first thing that will happen is a gasoline
engine will start to detonate. The engine will ping and start to lose power
under load as the combination of heat and pressure exceed the octane rating of
the fuel. If the detonation problem persists, the hammer-like blows may damage
the rings, pistons or rod bearings.
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Overheating can also cause preignition. Hot spots develop inside the
combustion chamber that become a source of ignition for the fuel. The erratic
combustion can cause detonation as well as engine run-on in older vehicles with
carburetors. Hot spots can also be very damaging and burn holes right through
the top of pistons.
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Another consequence of overheating may be a blown head gasket. Heat makes
aluminum swell almost three times faster than cast iron. The resulting stress
can distort the head and make it swell in areas that are hottest, like those
between exhaust valves in adjoining cylinders, and areas that have restricted
coolant flow like the narrow area that separates the cylinders. The typical
aluminum head swells most in the middle, which can crush the head gasket if the
head gets hot enough. This will cause a loss of torque in the gasket allowing
coolant and combustion leaks to occur when the head cools.
Overheating is also a common cause of OHC seizure and breakage.
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Wait, there’s more. If the coolant gets hot enough to boil, it may cause
old hoses or an age-weakened radiator to burst under the increased pressure.
Pistons may swell up and scuff or seize in their bores, causing serious engine
damage. Exhaust valve stems may stick or scuff in their guides. This, in turn,
may cause valves to hang open which can damage pistons, valves and other valve train
components. And if coolant gets into the crankcase, you can kiss the
bearings and bottom end of the engine goodbye.
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A HOT warning lamp should never be ignored. Though a few high tech cars like
Cadilac's
with the Northstar engine can disable cylinders to
"air-cool" the engine and keep it running at reduced power in the
event of coolant loss, most engines will suffer serious damage if they overheat.
So advise your customers to stop driving at the first sign of overheating. Turn
the engine off, let it cool down and try to find and fix the cause before
risking further travel.
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Causes of Overheating
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Overheating can be caused by anything that decreases the cooling system’s
ability to absorb, transport and dissipate heat, such as 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.
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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.
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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.
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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.
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What To Check
Thermostat - Severe overheating can often damage a good thermostat. If
the engine has overheated because of another problem, the thermostat should be
tested or replaced before the engine is returned to service.
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One way to check the thermostat is to start the engine and feel the upper
radiator hose (or use an infrared noncontact thermometer to read its
temperature). 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.
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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.
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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) in an attempt to "cure" a tendency to overheat can increase fuel
and oil consumption, ring wear and emissions. On newer vehicles with
computerized engine controls, the wrong thermostat can prevent the computer
system from going into closed loop resulting in major performance and emission
problems if the engine fails to reach its normal operating temperature.
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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.
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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 as well as cracks in the
head or block. A leak-free system should hold pressure for at least a minute or
more.
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It’s important to pressure test the radiator cap, too, because a weak cap
(or one with too low a pressure rating for the application) will lower the
coolant’s boiling point and can allow coolant to escape from the radiator.
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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 percent (depending on the fan’s distance from
the radiator), which may be enough to cause the engine to overheat in hot
weather or when working hard.
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Defective fan clutches are a common and often overlooked cause of
overheating. The shear characteristics of the clutch fluid gradually
deteriorates over time, with an average loss in drive efficiency of about 200
rpm 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.)
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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.
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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. If 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.
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Water pump - 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.
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Belts & hoses - 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.
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The condition of the hoses should also be checked. Recommend new hoses if the
old ones are over 5 years old.
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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.
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Radiator - The most common problems radiators fall prey to are clogging
(both internal and external) and leaks. Dirt, bugs and debris can block air flow
through the core and reduce the radiator’s ability to dissipate heat. Internal
corrosion and an accumulation of deposits can likewise inhibit coolant
circulation and reduce cooling. A good way to find clogs is to use an infrared
thermometer to "scan" the surface of the radiator for cold spots. If
clogged, the radiator should be removed for cleaning or be replaced.
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Backflushing the cooling system and/or using chemical cleaners can remove
rust and hard water scale, but may do little to open up a clogged radiator
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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. If the cooling system has no bleeder
valves to vent air, you may have to temporarily loosen a heater hose to get all
the air out of the system.
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Excessive exhaust backpressure - A clogged catalytic converter is usually
the culprit here, but don’t overlook the possibility of a crushed pipe or a
collapsed double wall pipe. Check intake vacuum at idle. If it reads low and
continues to drop, inspect the exhaust system.
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Retarded or overadvanced ignition timing (may also contribute to
detonation and preignition).
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Overheated incoming air - On older vehicles with a carburetor or throttle
body injection, check the operation of the heated air intake system on the air
cleaner. If the temperature control valve is stuck so only heated air from
around the exhaust manifold is drawn into the air cleaner, it may contribute to
detonation and/or overheating. Also check the heat riser valve for manifold heat
on older V6 and V8 engines. If stuck shut, it may be overheating the intake
manifold.
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Dragging brakes - A caliper that’s sticking or a parking brake that
isn’t releasing may be making the engine work too hard.
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Overworking the engine. The cooling systems in many passenger cars today
are marginal and have little excess capacity to handle extra heat generated by
towing or high speed mountain driving in hot weather.
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A
special thanks to UnderHood Service Magazine
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