Automotive Engine Overheating – Common Causes And Consequences

Automotive Engine Overheating - Common Causes And Consequences
Automotive Engine Overheating - Common Causes And Consequences

Not operating within a “normal” temperature range of about 195 to 220 degrees F. will cause automotive engine overheating.

So, 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 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.

A 50/50 mixture of water and ethylene glycol antifreeze
A 50/50 mixture of water and ethylene glycol antifreeze

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? This is amazing. Theoretically yes – but realistically no.

There is way more to take into consideration when trying to solve automotive engine overheating:

  • The clearances in most of today’s engines are much tighter. 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. These engines don’t handle higher than normal temperatures well.

Consequences Of Automotive Engine Overheating

If the engine overheats, the first thing that will happen is a gasoline engine will start to detonate. As a result, 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.

Automotive Engine Overheating - Common Causes And Consequences
Hammer-like blows may damage the rings, pistons.

Automotive engine 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 the result of overheating.

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.

Automotive Engine Overheating - Common Causes And Consequences
Cylinder Head Cracking – Overheating Is The Most Common Cause

Causes Of Automotive Engine Overheating

Automotive engine 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 or 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, 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 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. So, 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 Cooling System:


Severe automotive engine overheating can often damage a good thermostat. Replace the thermostat if the engine has overheated for any reason. 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. Most cars and light trucks since 1971 require thermostats with 192- or 195-degree ratings.

Overheating can often damage a good thermostat.
Overheating can often damage a good thermostat.

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.

Cooling System Leaks

Loss of coolant because of a leak is probably the most common cause of automotive engine overheating.

Possible leak points include:

  • Hoses
  • Radiator
  • Heater core
  • Water pump
  • Thermostat housing
  • Head gaskets
  • 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.

Leaking coolant at hoses.
Leaking coolant at hoses.

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. It’s important to pressure test the radiator cap; because a weak cap (or one with too low a pressure rating for the application) will; lower the coolant boiling point and can allow coolant to escape from the radiator.

Cooling Fan

With mechanical fans, most overheating problems are caused by a faulty fan clutch. Even 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.

Engine Cooling Fan
Engine Cooling Fan

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 automotive engine overheating results.

On average, the life of a fan clutch is about the same as a water pump. 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.

Water pump

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 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 ;or the fan from turning fast for proper cooling. Check the hoses. Recommend new hoses if the old ones are over 5 years old.

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 air flow through the core and reduce the radiator’s ability to dissipate heat.

Automotive Radiator
Automotive Radiator

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.

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.

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.

Excessive Exhaust Back Pressure

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.


Retarded or over advanced ignition timing (may also contribute to detonation and preignition).

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.

Dragging brakes

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

Overworking The Engine

The cooling systems in many passenger cars today are marginal and have little excess capacity to handle extra heat. This is a huge factor when it comes to automotive engine overheating.


Car owners frequently don’t maintain their cooling systems as well as they should. Consequently, If coolant isn’t changed, the coolant passages can experience buildup of scale and sludge.

Radiator And Cap With Rust Contamination
Radiator And Cap With Rust Contamination

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. Also, the system needs to be flushed when the coolant is changed.

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