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 comeback.

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. The 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.

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.

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.

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.

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.

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/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)