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Engine Overheating Basics

Top Twelve Things to Check When
Diagnosing Overheating Problems
... plus 4 more Bonus Tips!


Engine Overheating Basics

Overheating Engine Temperature


The cooling system of an engine is designed as a sealed system with the engine coolant held under pressure.  When under pressure, water and engine coolant boil at a higher temperature.  As your coolant and water increase in temperature and approach the boiling point, the state of the liquid begins to change to a gas, seen as steam.  A gas is much less efficient in cooling your engine than a liquid.  So increasing the pressure increases the boiling point, allowing the coolant and water to do a much better job at removing heat.  Keep in mind that internally coolant running through the channels within the engine will come on contact with metal that is well above 210° F so preventing the coolant from boiling is essential.  By increasing the pressure, you are increasing the boiling point and the coolant remains in a state of liquid rather than converting to a gas.  Heated coolant also expands so a closed loop system will require coolant to be held in rather than boiled out.  A closed loop system will also need an overflow tank which is reclaimed upon cooling of the system.

Role of the Radiator Cap

A functioning radiator cap will maintain its rated pressure.  Different vehicles require different pressure ratings so check your owners manual for the correct cap for your vehicle.  A periodic check of your radiator cap is important especially when troubleshooting a cooling problem.  Allow the engine to cool to the touch and remove the radiator cap.   Check the radiator cap gasket. The gasket is located on the underside of the radiator cap. The gasket seals the cooling system keeping pressurized coolant in the closed loop system. Inspect this gasket. If it is hard, cracked or split, or if the rubber is brittle or missing then it should be replaced.  A bad cap can be the reason for lost fluid.  A bad cap that is not tightly sealing in cooling fluid can allow the fluid to boil off, especially when the engine is shut off.  The reason for this is un-circulating fluid will not be cooled in a hot engine and can increase in temperature as it sits, high enough to boil.  The effect of this can cause fluid to be pushed out of the radiator via the radiator cap.  If the cap is properly sealed it will be pushed into the overflow tank.  If the seal is damaged or missing it can spill out of the system.  If the gasket is weak or damaged, it will not hold the pressure necessary to maintain a higher boiling temperature.  Always make sure you have a good cap, an overflow tank and never fill your overflow tank beyond the 'cool' high mark.   Also keep in mind that your cooling system may be leaking pressure somewhere else other than the radiator cap.


It should go without saying but check your fluid level.  Is it full when the engine is cool?  Check the coolant manufacture's information (usually on the container) for the proper Coolant / Water ratio as well as your vehicle's maintenance book.  Coolant should always be mixed with water, preferable distilled water, because water displaces heat better than coolant / antifreeze (coolant and antifreeze are one in the same) however the coolant / antifreeze boils at a higher temperature than water alone and freezes at a lower temperature than water.  Most manufactures recommend a 50/50 mix of water and coolant.  This ratio can vary dependant on your region and expected temperatures.  Coolant also lubricates and prevents corrosion and rust in your system.  Straight water will rust the inside of the engine block's coolant channels and will not provide lubrication to the water pump. Rust is death to a radiator as it will clog the channels.  Rust is also damaging to critical water jackets in the engine block. Most freeze plugs in the engine block are not rust proof will also rust causing leaks or a quick ejection of your coolant at some point.

Engine coolant does degrade over time so it is recommended that coolant be changed every few years.  Check with your vehicle manufacturer for recommended intervals.


The radiator hoses should be in good shape, not cracked, brittle or leaking.  Hose clamps should not be over tightened so that they cut into the hose.  Other than obvious damage, hoses can restrict coolant flow.  Check the routing of your hoses. Make sure they are not routed in a way that causes kinks, pinches or sharp bends causing a partial or complete blockage.  Flexible accordion type hoses will restrict the flow and should be avoided. 

Another cause of overheating can be a weak, collapsing hose on the suction side of the radiator, usually the lower radiator hose.  Located inside of the hose is a spring.  The spring is usually located inside of the hose.  It's job is to prevent the hose from collapsing.  Check this spring and verify that it is in place and that it has not rusted away or simply broke allowing the hose to collapse.  In many cases the hose will not collapse until the engine is at higher RPMs or the coolant is hat high temperatures.  In some cases a stuck thermostat can cause a hose to collapse so check that as well.


The thermostat is a valve located in the cooling system of an engine, which is closed when the engine coolant is cold and opens gradually in response to the engine heating and thereby controlling the temperature of the coolant and rate of coolant flow through the engine block.  When you look at the flow of coolant through an engine, the radiator is the critical component in cooling the fluid in the cooling system.  Coolant must flow through the radiator to be cooled and then back through the block to collect more heat from the engine, then flow back to the radiator to be cooled again.  The thermostat is the device that is governing this flow.  Asside from a small amount of water that may flow through the bypass hose, the water must flow through the opened thermostat to get to the radiator. Thermostats are rated by temperature. Typically a thermostat begins to open at the rated temperature of the thermostat but it is not fully opened until the temperature is 10 to 15 degrees above the rated temperature. So in a sense a thermostat is not an open or closed valve but rather a device that allows a certain amount of coolant to flow depending on the temperature of the fluid. A thermostat that is not opening soon enough or not opening up at all will cause an engine overheat or run higher than normal.  Removing your thermostat all together is not a good solution to a faulty thermostat since the engine must reach a certain operating temperature to run efficiently.  Removing your thermostat can be a temporary fix however in an emergency.  You can test a thermostat by hanging it in a pan of water on the stove (not on the bottom of the pan) and monitor the water temperature and observe the thermostat.  It should open fully at 10 to 15 degrees above it's rated temperature.


The coolant must move to be cooled.  This is the job of the water pump.  The water pump is driven by the fan belts or serpentine belt.  If the coolant doesn't move, coolant within the engine block gets very hot and is not replaced by cooler fluid from the radiator resulting in a quickly overheating engine.

Most cars on the road use a serpentine belt. A serpentine belt is a single belt that drives all the engine accessories. If a serpentine belt fails, all engine accessories will stop turning. That includes power steering, air conditioning compressor, alternator, and of course the water pump. Unless you have an electric fan, the fan belt that drives the water pump also drives the fan.  If you are the do it yourselfer, it's important to carry a spare belt and learn the proper way to install it.  Don’t over tighten v-belts.


The water pump, pumps the coolant through your system.  A typical sign of a failed or failing water pump is bad is typically a leak.  A leak is typically occurring through the seal and bearing.  Leaks will commonly present themselves on the underside of the pump.  Most pumps have a small hole below where the coolant will seep out if the seal or bearing have failed. Commonly it is the bearing that has gone bad and the seal quickly follows. If your pump is leaking in this location, replace the pump as soon as possible.  Pump life depends heavily on maintenance of the cooling system.  Scheduled flushing and changing of the coolant at regular intervals should help the water pump last for the life of the engine.  In some cases a high flow water pump may be the solution to an overheating problem, more particularly when performance modification have altered stock configurations of the vehicle.  High efficiency water pumps from Flow Cooler, Edelbrock, Weiand, and Summit will move more GPM (Gallons Per Minute) of water than a stock pump. Keep in mind that this may not solve the original problem or solve the overheating problem.


The fan clutch is a coupling device that is located between the water pump shaft and the fan.  The fan clutch is designed to improve the vehicle’s cooling system efficiency while reducing the load on the engine and loss of energy caused by the fan itself.  The Fan Clutch allows the fan to operate at lower speeds and effectively detach at higher speeds when the vehicle is moving and air movement due to velocity aids to cool the engine. 

There are two types of Fan Clutches, thermal and non thermal fan clutches, also called centrifugal clutches.  Both types operate on the fluid-drive principle.

Symptoms of a Worn / Defective Fan Clutch that should be Replaced

How do you know when your Fan Clutch is wearing out or has failed to do it's job?  There are a few key symptoms. 

  • Excessive Free-Wheeling when spun manually (when engine is stopped) - With the engine stopped, manually spin the fan.  If the Fan spins excessively, over 3 revolutions, as though there is no resistance it should be replaced.
  • If your air conditioner does not perform well at idle or low vehicle speeds then the clutch may have failed and air is not passing over the A/C condenser efficiently enough to cool the refrigerant.
  • If the fan speed does not increase when engine is running hot or if the fan speed does not increase until engine is excessively hot.
  • Looseness of the Fan - Excessive lateral movement of the fan blades.  If the fan blade moves more than 1/4" front to back measured at the end of the blade.  Some lateral movement is a normal condition due to the type of bearing used in fan clutches. Approximately 1/4" (6.5 mm) maximum lateral movement measured at the fan tip is allowable.  
  • Vibration - Sometimes vibration can be detected due to a failed clutch.  The vibration can increase with with engine speed.   Many times this can lead to water pump failure.
  • When the engine is stopped, turning the fan blade manually turns rough, grinding or does not turn at all.
  • Leaking silicone Fluid - Excessive fluid leakage will cause the clutch to fail to engage.
  • Noise - If you hear excessive fan noise or a roar at all engine speeds.  Noise can be detected when the clutch should be engaged, during initial cold startup or when the engine is hot.  Under high speeds or higher RPMs over 2500, a locked up fan can create a roaring noise.

    More about the Fan Clutch Here

8. Fan Shroud

The fan shroud directs the air from the fan, directly into the radiator making the fan efficient.  A missing or damaged fan shroud will direct air from the fan directly through the radiator. Since it is this moving air that absorbs and removes heat from the radiator and the fluid inside the radiator, a properly fitted, intact fan shroud is important .  If there is not enough air is moving through the radiator to sufficiently cool the fluid, the result is an overheated engine.  The opening in the fan shroud should be just slightly larger than the fan's diameter.  The shroud should also cover about half the fan blade width.  The fan shroud should also encompass a full 360 degrees around the radiator and fan.  A partial fan shroud is allowing air to escape and thus not directed into the radiator.


Obviously the radiator is the key component of any cooling system.  Without it, the heat would not efficiently escape the system.  Several factors of a failing radiator can affect cooling.  Leaks in a radiator can cause fluid to escape and reduce the amount of cooling potential. Look for leaks below, on the sides and in the front and rear in the fins.  Even a small leak can have negative effects in the long term.  As a rule of thumb it is better to fix a leak or replace the radiator rather than use "Fix-a-Leak" concoctions that can plug other things such as cooling ports in the block, channels in the radiator or the thermostat.  Corrosion is another key factor in a failing radiator.  Corrosion in the radiator can block ports or channels in the radiator that flow from one side to the other.  Even if you can look down into the radiator, the corrosion may be at the lower end and out of sight.  Running straight water in the radiator will allow scale and minerals to build up, blocking ports.  Dirt and mud over the fins of a radiator will have a big impact on overheating.  If your radiator is clogged, so get the hose out and clean your radiator from the backside if it is clogged with debris.  Be careful not to damage the fins or channels of the radiator while cleaning it.  Damage is also factor.  Damaged or bent over fins can restrict air flow allowing hot coolant to pass through without dissipating enough heat.


Though it's a less cause, sometimes every little bit helps.  Dirt on an engine can act as an insulator, much like a blanket.  So can mud and if you are a 4 wheeler like we are, you occasionally get mud on the engine.  A dirty or muddy engine will run hotter.  Not by much but a little.  Incidentally, chrome accessories are like heat shields keeping the heat inside your engine.  On the flip side, aluminum is a good conductor of heat and it will actually take heat out of the engine.  Dirt and mud will have more of an impact on overheating if your radiator is clogged, so get the hose out and clean your radiator from the backside if it is clogged with debris.


An automatic transmission will cause an engine to run hotter then a standard transmission. An engine turning an automatic transmission is always turning the torque converter whether or not the vehicle is moving. A torque converter is actually never in neutral and creates a constant load on the engine causing it to run hotter.  An automatic transmission will also generate more heat internally than a standard transmission. Some of the heat will be dissipated through the engine block causing additional heat in the engine.  Installing a good transmission cooler will remove some of the heat from the transmission.  The location of the transmission cooler is also a factor.  If it is located in front of the radiator, heat removed from the transmission via the cooler will pass over the radiator so factor that in when installing a transmission cooler.


A head gasket is a gasket that sits between the engine block and cylinder head in an internal combustion engine. Its purpose is to seal the cylinders to ensure maximum compression and avoid leakage of coolant or engine oil into the cylinders.  Overheating problems due to the head gasket may be due to the Head gasket having a break in it.  Depending on where the break is, you may not see coolant in the oil but the hot coolant may be mixing with cooler coolant and raising the temp. Or possibly the coolant may be entering the combustion chamber and it is burned off resulting in the need for more coolant. If coolant is entering the combustion chamber you may not see it if there isn't much passing through the head gasket however it can still affect temperature.  In mild cases you can buy some time with some sort of leak stopping additive.

Overheating problems due to the head gasket may also be due to a break in the head gasket that is allowing combustion gasses to be pushed into the coolant.  Depending on you engine configuration and the location of the water pump, the result could be trapped air in the water pump. Air can get trapped in a water pump stalls the pump and because automotive water pumps are not "air tight", they cannot always self prime.  This could also explain a loss of coolant. As the engine cools and when the engine goes through the exhaust stroke, coolant is pushed into the combustion chamber.  Some guys I know ran without the cap to hopefully allow the water to not build up pressure and push back into the cylinder but usually the result of that is a lack of "back pressure" in the coolant and the combustion gasses leak into the coolant at a faster rate. Apparently there are better water pumps to prevent air from stalling them. A guy I know would stop his car and wait, sometimes he tried rocking the car to get the air bubble out of the water pump, then start it up again and it would cool quickly.  Ultimately a head gasket replacement is necessary.  Again some sort of stop leak may help and buy some time.

Head gasket condition can be inspected by checking the compression pressure with a pressure gauge, or better yet, a leak-down test.  Also note any indication of combustion gasses in the cooling system. Oil mixed with coolant and excessive coolant loss with no apparent cause, or presence of carbon monoxide or hydrocarbon gases in the expansion tank of the cooling system can also be signs of head gasket problems.  Driving with a blown head gasket can cause additional extensive damage due to overheating or loss of lubrication.



Too much friction.  Change your  engine oil  on a regular basis to reduce internal engine friction.   Synthetic oil and some additives aid to reduce friction.  An engine that had been severely overheated at some point can warp parts, creating severe friction to the point where it will barely run.  Thinking of an old Ford Comet I once owned.

14. Engine Compartment Overcrowding

Over crowding under the hood with lots of accessories in the engine compartment will retain more heat.   A large engine crammed into a small engine bay will not allow good radiant release of heat. 

15. Engine Tuning and Boring

Proper Engine Tuning.  The fuel / air mixture, timing, and spark all affect engine temperature.  An improperly tuned engine can produce more heat.

An engine that has been bored over the stock bore can run hotter because of the reduced wall thickness of the metal in the block.  Less metal means less metal to dissipate the heat.  Typically this is not a problem unless the bore has gone overboard in removing metal or the block has been bored too many times.

16. Engine Color!

Color - OK, now you’re laughing, but the color you paint your engine matters. I’m told that black is actually the best color for dissipating heat from the engine. Don’t ask me how this works, I don’t know.   Here's Why black paint works best at dissipating engine heat


Fan Clutch Diagnosis
How to tell if the Fan Clutch in your Cooling System is failing or has failed.

Why black paint works best at
dissipating engine heat

Engine Overheating Basics




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