Logo

WHEN AN ENGINE OVERHEATS

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 and performance.

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.

Use a straight edge and feeler gauge to check the flatness of the face of the cylinder head and block deck for warpage before replacing the head gasket.

IF AN ENGINE HAS OVERHEATED...

If a head gasket has failed as a result of severe engine overheating, both the face of the cylinder head and block deck should be checked for warpage -- and resurfaced if needed to restore flatness prior to replacing the head gasket. If the face of the head and/or block deck are not flat and are not resurfaced when the head gasket is replaced, the new head gasket will be unevenly loaded and will likely leak or fail.

Flatness can be checked by placing a straight edge on the face of the cylinder head or block, and then using a feeler gauge to check any gaps between the straight edge and head or block. If the amount of warpage exceeds the following maximum limits, the head or block is not flat enough to hold a good seal against the head gasket and should be resurfaced:

The surface finish on the face of the head and block is also important. The surface finish should be 54 to 113 RA microinches (60 to 125 RMS), with a recommended range of 80 to 100 RA (90 to 110 RMS).

If the surface is too rough (more than 113 RA), it may be too rough to seal properly and the head gasket will leak. If the surface is too smooth (less than 54 RA), it may not provide enough "grip" to prevent the gasket from flowing or scrubbing.

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 overheat again:



Thermostat
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 working hard.

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 deteriorates 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 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.

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.

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.

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.

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. "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 system.

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/preignition, 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).