In the Spotlight Technical Top Tips

Overheating Engines … the usual culprits

A cooking engine is a problem that no one wants especially in the middle of the day on an off road trail or overland holiday. We have put together a list of the most common factors that are causes of engine overheats, worth a read and you can use it as a check list for your inspections before heading off to the great outdoors.


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 higher temperatures, as your coolant/ water increase in temperature approaching the boiling point, the state of the liquid begins toengine-coolant-flow change to a gas, being steam.  A gas is much less efficient in cooling your engine than a liquid, hence, increasing the pressure increases the boiling point, allowing the coolant and water to do a better job at removing heat.  Remember that coolant running throughout the channels within the engine will come into contact with metals that are well above 210° F so preventing the coolant from boiling is essential. 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 (expansion tank) allowing fluid to be reclaimed upon cooling of the system.
A, 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, checking the radiator cap gasket. The gasket is located on the underside of the radiator cap. The gasket seals the cooling system keeping pressurised coolant in the closed loop system. If the gasket is hard, cracked, split, or missing then it should be replaced.  A bad cap can be the reason for loss of fluids.  A bad cap that is not tightly sealing in cooling fluid can allow the fluid to boil off, especially when the engine is switched 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 off, allowing 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 altogether.
If either the gasket or spring are 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. 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, for the proper coolant & water ratio as well as in your vehicle’s maintenance manual. Coolant should always be mixed with water, preferable distilled water, because water displaces heat better than coolant/antifreeze, however coolant/antifreeze does boil at a higher temperature than water alone and obviously freezes at a lower temperature than water.  Most manufactures recommend a 50/50 mix of water and coolant.  This ratio can vary dependent on your region and expected annual temperature fluctuations.  Coolant also lubricates and prevents corrosion and rust in your system, water alone will corrode 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 eventually clog the channels.  Rust is also damaging to the critical water jacket in the engine block. Most freeze plugs in the engine block are not rust proof and 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 or coolant manufacturers details 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.
Another cause of overheating can be a weak, collapsing hose on the suction side of the radiator, usually the lower radiator hose. With a simple squeeze you can test if a hose rubber is pliable, if it feels very firm then it is time to change due to perishing, replace it before it blows.
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.  Aside 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 deg. above the 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 do it your self, it’s important to carry a spare belt and learn the proper way to install it, to the correct tension.
The water pump, pumps the coolant through your system.  A typical sign of a failed or failing water pump is typically a leak.  A leak will typically occur through the seal and bearing housing.  Leaks will commonly present themselves on the underside of the pump.  Most pumps have a small hole below the pulley, where the coolant will seep out if the seal or bearing has failed. Commonly it is the bearing that fails 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 after market suppliers will move more Gallons Per Minute of water than a standard stock pump. Keep in mind that this may not solve the original problem or solve the overheating problem, but it will assist cooling.
The fan clutch is a coupling device that is located between the water pump shaft and the fan also called the viscos coupling.  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 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 engine speed and 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.
H, 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 even 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 fins can restrict airflow allowing hot coolant to pass through without dissipating enough heat. To test your radiator is functioning well, feel the surface of the radiator when hot, if there are cool spots then areas are blocked within the radiator so cooling is reduced.
Though it is a lesser 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 an off roader, you will occasionally get mud over 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 full of 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. Automatic transmissions will also generate more heat internally than standard transmissions. 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 system. 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 the gasket siting between the engine block and cylinder head. 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 split 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.
Head gasket conditions can be inspected by checking the compression pressure with a pressure gauge.  Also note any indication of combustion gasses in the cooling system, eg bubbling in header tanks etc. Oil mixed with coolant, 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 failure.  Driving with a blown head gasket can cause additional extensive damage due to overheating or loss of engine lubrication through burn off within the cylinder. If you have a diesel engine and it is burning engine oil through the blown gasket, you could experience the frightening phenomenon of a “diesel runaway”.
Too much friction.  Change your engine oil on a regular basis to reduce internal engine friction, synthetic oils and some additives will reduce engine 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.
Engine Compartment Overcrowding
Over crowding under the bonnet with lots of accessories in the engine bay will retain more heat.   A large engine crammed into a small engine bay will not allow a good radiant heat release nor airflow.
Engine Tuning
Proper Engine Tuning. The fuel and 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 and more fuel burn. 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.
Engine Colour
Colour ? OK, you’re laughing, but the colour you paint your engine does matter. We’re informed that black is actually the best colour for dissipating heat from the engine. Don’t ask how this works, I’ve no idea …


keep cool

                                                               Keep your cool, and enjoy your trip …..


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