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Cooling Systems

The water pump circulates water around the block and head(s) of the engine. When the engine has warmed up, and the water temperature is sufficient, the thermostat opens and allows water to pass into the radiator where it cools and is drawn back to cool the engine. - © L. A11ison.


wax filled

A wax-filled thermostat (right) will usually (but not always) fail in the open position which allows you to drive to a convenient place to get it replaced. It should be replaced as soon as is convenient because excessive cooling leads to increased engine wear.

The other kind of thermostat usually (but not always) fails 4wd.sofcom.com --> in the shut position, which generally leads © L. A11ison --> to overheating. If this happens the thermostat must replaced as soon as it is safe to do so.


An older vehicle probably has a brass radiator. The end caps are soldered on and can be removed by a radiator specialist for inspection and cleaning of the inside of the cooling tubes.

If one or two tubes have holes, from corrosion or damage (e.g. from sticks and stones etc.), those tubes can be cut and the ends pinched off or soldered.

A modern vehicle probably has an aluminium radiator core with plastic end tanks. This is perhaps less likely to corrode (with the correct coolant), but it is much harder to repair leaks and damage, particularly in the bush - it could be a tow home.


In all but the oldest cars, cooling systems run at temperatures that are very close to the boiling point of water; this improves engine efficiency. The radiator cap is made to resist a pressure typically in the range of 10 to 20 lbs/in2; this effectively raise the boiling point of the coolant. Water expands when it gets hot. To prevent loss of coolant, an overflow tank collects any driven out of the main system by expansion. Coolant from the overflow tank is returned to the main system when the engine cools down. The overflow tank is only partly filled (to the marks) when the engine is cold.

In some systems the overflow tank is not pressurized. Here the main-radiator cap only allows water out under pressure into the overflow tank, but offers little resistance to its return. The cap must have a good lip-seal to stop air being drawn back in instead. In other systems the overflow tank is pressurized and the radiator cap is fitted to it rather than to the radiator. Here the tank's pressure helps overflow water to main system as the engine cools.


Because the cooling system is pressurized when hot, removal of the radiator cap may allow the coolant to boil and blow scalding water into the air and over bystanders. Therefore the radiator cap should not be removed when the engine is hot, but if this is absolutely necessary precautions must be taken such as allowing the engine to cool for as long as possible, releasing the pressure gradually by turning the cap slowly, covering the cap with a heavy cloth, keeping bystanders away etc..

Loss of Coolant

If a serious leak develops (e.g. split hose) the coolant can be lost in a matter of seconds. The temperature gauge may or may not show a significant rise in temperature because the sensor will now be surrounded by steam which does not transmit heat as well as water. If and when you become aware of the problem, the natural reaction, after turning the motor off, is to pour water back into the engine. Resist this temptation until the engine has cooled down for a long time because the temperature shock may do severe damage, perhaps even cracking the block.

It is quite possible that the head may be warped and other severe damage done to the engine if run for any time without coolant.

It is possible to fit after-market sensors that measure coolant water level, and the temperature of the engine block, some fitted with audible alarms, to guard against engine damage.

Electric Water Pump.

[c1999] Davies Craig (au) developed an electric water pump (EWP) for use in car cooling systems, also making a conversion kit for current vehicles. The thermostat is removed, and the existing engine-driven water pump is either disconnected (with a shorter belt being used to drive the alternator etc.) or its impeller is removed. The new pump is plumbed into the lower radiator hose. It is claimed that a conventional water pump absorbs up to 6kW at high engine revs whereas the new electric one uses a maximum of 6amps, i.e. about 80W. A "smart" control box varies the current drawn between 4v and 12v to control water flow, thus removing the need for a conventional 4wd.sofcom.com --> thermostat. The electric pump can deliver maximum cooling independent of engine speed and can be made to run for a time after the engine is stopped. The pump's body is made of plastic, as now used in header tanks.

As of 2001, the company has also introduced a lighter duty electric booster pump (EBP).

Possible Causes of Overheating

From the cheap to the expensive:

  • Loose fan belt.
  • Radiator air flow obstructed, e.g. by insects, spinifex grass, mud, etc.
  • Low coolant level, but why is it low?
  • Faulty radiator cap unable to retain pressure. (Engine may boil when turned off.)
  • Thermostat failed - replace.
  • Collapsed hose restricting water flow (e.g. interesting [70-series] problem c1998.)
  • Engine incorrectly tuned - timing, mixture too lean, etc..
  • Faulty viscous coupling (if fitted) on radiator fan.
  • Faulty electric radiator fans (if fitted).
  • Corroded impeller in water pump.
  • Blocked tubes in radiator core. Changing coolant, back flushing, and use of correct inhibitor may help.
  • Head gasket failed.
  • Block and/or head cracked.


Daily to weekly depending on terrain and distance travelled:

  • Levels in radiator and overflow tank, when cold - see safety.
  • Hoses for signs of softness, leaks etc.
  • Radiator for damage and possible blockage by insects, vegetation etc..
  • Inspect for drips and leaks.

Spares to Carry

The 4wd.sofcom.com minimum:

  • Top radiator hose and hose clamps (worm drive, strap type).
  • Spare fan belt (and tools to fit).
  • Some water for emergency top-ups.

In remote areas:

  • Bottom radiator hose.
  • Length of heater hose ond hose clamps.
  • Radiator stop leak.

Consider these spares:

  • Radiator cap.
  • Thermostat.


Read the manufacturer's handbook and change the coolant, using the recommended inhibitor and anti-freeze, at the recommended intervals.

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