Engine Cooling System

Engine cooling system on a hard working 4×4

The engine cooling system of an off-road vehicle are just like the cooling systems found on other types of vehicles, except that they generally have bigger fans, larger radiator cores, contain more coolant and work much harder at their core function; regulating the engine temperature rather than merely cooling it down.

Correct engine temperature is crucial for complete combustion on the one hand, and to minimise mechanical wear of internal components on the other. Just like any other engine, running a 4×4 engine too cold can be just as bad as running it too hot, so how does the engine cooling system go about regulating engine temperature? While it is easy to say they do it by utilising the excellent heat absorption properties of water, there is a little more to it than that, so, let us break the engine cooling system down into its component parts:


Aengine cooling system - radiatorlso called a heat exchanger, since it dissipates the heat of the coolant passing through it into the atmosphere, it consists of a core made up of small diameter oval or rectangular tubes connected to each other with copper or aluminium vanes to further increase its surface area.

Since a small volume of water cools down quicker than a large one, and copper and aluminium being good conductors of heat, atmospheric air passing through the matrix of small diameter tubes and vanes absorbs the heat transferred from the coolant to the metal of the radiator, carries the heat away in to the atmosphere.

The cooled water is collected in the bottom tank of the radiator and returned to the engine, where it collects heat from the block and cylinder head(s) before again being passed through the radiator to cool down.

Expansion tank

Since water expands as it heats up, some provision must be made for space, or room for the water to expand into. This is normally accomplished by the inclusion into the engine cooling system of a container that is linked to the radiator. This container, being only about half full when the engine is cold, accommodates the added volume of the expanded water.

This container also contains the relatively high pressures (around 15 lbs/sq. in.) by means of a cap containing a pressure relief valve, which has the function of releasing excess pressure and coolant, should the expansion tank inadvertently have been overfilled.Engine cooling system - Radiator cap

The pressure mentioned earlier is important, since it prevents the coolant from boiling. Even with the addition of additives such as antifreeze/anti-boil, which contain ethylene glycol to raise the boiling point while at the same time lowering the freezing point of water, the coolant can still boil if there is insufficient pressure inside the engine cooling system to prevent it, which from a 4×4 perspective, is a very bad thing, especially in Africa or the Australian Outback, where the next water could be hundreds of km’s away.

A cap that fails to maintain the correct pressure will also allow coolant to escape past it, compounding an already serious problem, and while coolant can be replaced, the demands of serious off-road driving are such that it may not be possible to stop to do so. You may be in the middle of a long muddy stretch where stopping could mean sinking into the ooze up to your floor pan in a matter of seconds.

When the engine is at operating temperature, the tank appears to be full, however, as the engine cools down, the coolant contracts and the level returns to normal. However, even if all is well with the engine cooling system of your 4×4, never open this cap while there is still some residual heat in the engine; the residual pressure may just be enough to keep the coolant from suddenly boiling when the pressure is removed, very possibly causing severe scalds and burns. The effect is exactly like the pressure in a shaken up beer can, which keeps the liquid from fizzing until it is opened.

Water pump

engine cooling system - electric water pumpThe water pump on a 4×4 vehicle is not just a pump; it is a pump designed to displace coolant at a very specific rate. While this rate differs from engine to engine, the pump has to be able to force the coolant through the engine cooling system at a rate that allows the coolant sufficient residual time in the radiator to cool down.

Apart from the fact that coolant that passes through the radiator too quickly will not cool down sufficiently, a too high displacement rate also contributes the already significant parasitic power losses, which result from friction, coolant and oil pumping, electricity generation and air conditioning, among many others.

A pumping rate that is too low, due for instance, to a drive belt is slipping, has the converse effect; coolant cools down too much can paradoxically, can lead to overheating, because while the coolant in the radiator is cool, the rate of circulation may be too low to permit effective temperature control.

Many new developments and research programs on electric water pumps and thermostats have been initiated in recent years. However, while many electric systems are currently available, some research is required to get the most effective and reliable engine cooling system for any particular 4×4 vehicle.


If the heart of the engine cooling system is the water pump, the thermostat is the central nervous system. Usually located as close to the engine as is practical, the thermostat is a valve that is controlled by the expansion rates of two different metals.

Engine cooling system - thermostatIts primary function is to regulate the flow and circulation of the coolant and it does this by opening and closing a shutter in direct response to the coolant temperature. However, this method of operation is as old as the motor car itself and while a thermostat can operate flawlessly for years, it can also fail suddenly and without advance warning. Hardly a 4×4 driver has not had this happen to himself, or not heard of it happening to someone else.

Should a thermostat fail and cause overheating, removing it may not be the answer since this will allow the coolant to circulate at too fast a rate which can in many cases cause overheating in itself. On diesels, it could also cause the engine to run far too cool, (in some cases under 40 deg C coolant temperature), which could potentially cause the glow plugs to operate, since on most older engines the glow plugs are solely controlled by the coolant temperature.

If you are in the hinterland of Africa for instance, and you have overheating issues, a quick test is to check the temperatures of the large diameter radiator hoses. If the thermostat is stuck closed or partially closed, the hose returning cool water to the engine will be significantly cooler to the touch than the hose leading from the engine to the top radiator tank.

It is then that you will be glad you brought along a replacement thermostat; they are compact, weigh next to nothing and are in most cases easily replaced. As far as emergency replacement parts go, a thermostat, (or two) should be at the top of the list, along with spare drive and V- belts.

Radiator fan

The radiator fan is the single biggest cause of parasitic power loss and in efforts to minimise this, manufacturers have come up with the idea of viscous couplings, which is a way of transferring power from the driving hub to the fan itself via oil, much like the way torque converters work.

Some fans are further equipped with a sort of clutch, Engine cooling system - flex mechanical fanwhich engages only when the engine has reached a pre-defined temperature. While both methods have the effect of reducing parasitic power losses, and allow the fan to operate mostly only when needed, their design make them prone to failure or inefficient operation. In the case of the purely oil driven fan, the oil can escape past worn seals, causing the fan not to rotate fast enough, which is a leading cause of overheating and engine failure on off-road road vehicles. The same thing can happen to the oil/clutch operated fans, with the same disastrous effect.

To test the correct operation of a viscous fan, try to manually rotate it in the usual direction of rotation with the engine switched off, of course. It must rotate but with a significant resistance; if there is no or very little resistance, or if it spins freely, the fan is worn and must be replaced, since there are no reliable repair methods, especially in the off-road environment. Another method of checking a viscous fan is to have someone switch off the idling engine while you watch the fan; a properly working fan will stop rotating when the engine stops. If it does not, and continues to spin after the engine has stopped, replace the fan.

Electric fan(s)

While it may be possible to install electric fan(s) to some 4×4 vehicles, the advantages should be weighed against the several disadvantages.

Advantages:Engine cooling system - Electric fan and radiator

  • Parasitic power loss is greatly reduced, if not totally eliminated.
  • Reduced (slightly) fuel consumption.


  • Electric fans must be shrouded and since there may not be a suitable shroud available for your proposed modification, you may have to make one. This is a very tricky thing to do if you want the most out of electric fan, so some experimentation may be required.
  • Fan size is critical; two small fans are never as efficient as one large fan, so if want to do the modification, learn everything you can about the airflow through the grille and radiator of your particular 4×4. The installation in your mates’ off-road-road vehicle (that is different from yours), may not be a suitable guide.
  • Fool proof wiring is required; electric fans usually require about 40 amps to start and 20 amps or so for continuous running. The installation should be wired by a qualified auto electrician to make it reliable, which could make the exercise more expensive than it is worth.
  • If you are doing the conversion on a new 4×4, bear in mind that modifications to the electrical system could lead to the dealer repudiating your guarantee should something go wrong sometime in the future.
  • In the off-road context, insulation against water is essential if you are planning on doing deep water crossings. Your newly installed electric fans may not survive immersion, which is when you will be sorry you did not stick with the original belt driven version.


By far, the most commonly used ratio between crankshaft- and water pump speed is 1:1. This ratio strikes the best balance between parasitic power loss and coolant flow rate, however, an overdrive system, meaning a smaller diameter pulley on the water pump to increase its rotational speed relative to the crankshaft, is possible provided the overdrive is kept below 30-35%. This modification will increase coolant flow but it will also increase fuel consumption and cause a significantly increased parasitic power loss.

The future of the 4×4 cooling system

While the traditional water engine cooling system has served the off-road fraternity well over the last 100+ years, the imperatives of increasing fuel economy coupled with the relative scarcity of oil, has forced engine manufacturers to look at more efficient ways to keep their engines cool. Many innovations are being investigated and developed, anEngine cooling system - Hotd while variable speed electric water pumps and electronically controlled thermostats are already available, there are other, more exotic systems on the way.

From the 4×4 perspective, any method that keeps engines cool while at the same time reducing fuel consumption and causes less power loss is a GOOD THING. However, while the engine cooling system using computer controlled actuators to control flow rates, internal heating control and all other aspects of engine cooling are being tested, the overall design of engine blocks and cylinder heads needs to be improved to make full use of the advantages of these systems; something that is sadly, still some way off into the future.