Radiator – Common Engine Overheating Causes And Cures

Radiators - It's Primary Task Is To Keep The Engine Cool
Radiators - It's Primary Task Is To Keep The Engine Cool

Your radiator allows your engine to operate within a “normal” temperature range of about 195 to 220 degrees F.

As efficient as today’s engines are, they still waste a lot of the heat energy they produce.

The most common problems a radiator falls prey to are clogging (both internal and external) and leaks.

The average gasoline engine is only about 22 to 28 percent efficient. 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 soften and melt.

Melted Piston
Melted Piston

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.

So, How Hot Is Too Hot

Temperature Gauge Reading Too Hot
Temperature Gauge Reading 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.

A 50/50 mixture of water and ethylene glycol antifreeze in the cooling system will boil at 225 degrees. A radiator cap rated at 15 psi will increase the boiling temperature of a 50/50 mixture up to 265 degrees.

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. As a result, Anytime temperatures climb beyond the normal range, the engine is running in the danger zone.

Consequences Of Your Radiator 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. Consequently, 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.

Overheating can also cause pre-ignition. 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.

A blown head gasket can also be a result of overheating. Heat makes aluminum swell almost three times faster than cast iron.

Blown Head Gasket
Blown Head Gasket

The resulting stress can distort the head and make it swell in areas that are hottest. Like those between exhaust valves in adjoining cylinders. Also 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. This will cause a loss of torque in the gasket allowing coolant and combustion leaks to occur.

Overheating Is Also A Common Cause Of OHC Seizure And Breakage

If the coolant gets hot enough to boil, it may cause old hoses or an age-weakened radiator to burst. 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 valvetrain components. And if coolant gets into the crankcase, you can kiss the bearings and bottom end of the engine goodbye.

Cylinder Head With Broken Camshaft
Cylinder Head With Broken Camshaft

A HOT warning lamp should never be ignored. 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.

Common Causes Of Auto Radiator Overheating

Overheating can be caused by anything that decreases the cooling system’s ability to absorb, transport and dissipate heat:

  • 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
  • Even a defective radiator cap

Heat always flows from an area of higher temperature to an area of lesser temperature. The only way to cool hot metal, therefore, is to keep it in constant contact with a cooler liquid.

So, The only way to do that is to keep the coolant in constant circulation.

As soon as the circulation stops, 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. 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 When It Comes To Maintaining Your Radiator:


Always Replace The Thermostat If The Radiator Overheats
Always Replace The Thermostat If The Radiator Overheats

Severe overheating can often damage a good thermostat. If the engine has overheated replace the thermostat. 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.

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.

Cooling System Leaks

Loss of coolant because of a leak is a common cause of overheating. Possible leak points include:

  • Hoses,
  • 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.


With mechanical fans A faulty fan clutch causes most overheating problems. With electric fans check to see that the fan cycles on when the engine gets hot. 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.

Water Pumps

Any wobble in the pump shaft or seepage would call for replacement. 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.

Belts And Hoses

Check belt tension and condition. A loose belt that slips may prevent the water pump from circulating coolant or the fan from turning fast enough. Recommend new hoses if the old ones are over 5 years old. Sometimes a lower radiator hose will collapse under vacuum at high speed. This can happen if the reinforcing spring inside the hose is missing or damaged.


Car Radiator Fins Image
Car Radiator Fins Image

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.

If clogged, the radiator should be removed for cleaning or be replaced.

Backflushing the cooling system and/or using chemical cleaners can remove rust and hard water scale. 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. You may have to temporarily loosen a heater hose to get all the air out of the system.

Excessive Exhaust Back Pressure

A clogged catalytic converter is usually the culprit here, but don’t overlook the possibility of a crushed pipe. Check intake vacuum at idle. If it reads low and continues to drop, inspect the exhaust system.

Retarded Or Over Advanced Ignition Timing

May also contribute to detonation and preignition.

Dragging brakes

A caliper that’s sticking or a parking brake that isn’t releasing may be making the engine work too hard.

Keeping Tabs on the Heat

One tip is to look for heat tabs on the engine. While OE engines do not have them, remanufactured engines usually have the tabs installed by the rebuilder. These heat tabs are designed to melt if the engine has overheated beyond a certain temperature. Overheating indicates lack of customer care, and often voids a manufacturer’s warranty.

OE Problem Or User Neglect

Overheating problems which generally fall into the categories of either user or technician responsibility include the following:

  1. Radiators – Older copper and aluminum radiators tend to get clogged, restricting the flow of coolant.
  2. Thermostats – Either installed improperly or sticking closed.
  3. Water pumps – Failure to replace older or failing pumps or install a new pump properly.
  4. Heater core leaking – A common problem.
  5. Coolant leaks from either hose connections or from the cylinder head due to overheating.
  6. Bad radiator hoses or belts.
Adding New Engine Coolant
Adding New Engine Coolant

Car owners frequently don’t maintain their cooling systems as well as they should. Coolant should be changed at least every two years or 30,000 miles or it will lose its effectiveness. Also, The mixture of antifreeze and water should always be 50/50. A mixture with more than 50 percent antifreeze will cause the coolant to not perform properly. And, With less than 50 percent, the engine can freeze and crack.


A radiator is important because it is the only way your engine vents heat during operation. Finally, The most common cause of radiator malfunction is physical damage.

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