Engines often tend to overheat in off-road driving conditions. The speed is usually low and the engine has to work hard in a lower gear. The lack of airflow means that the cooling system struggles to cope. Jake Venter discusses the things that can lead to overheating
There are several things that can lead to an engine overheating on an overland trip, such as seeds and debris clogging up the spaces between the radiator fins.
A common cause of overheating is that the radiator hoses may be leaking. Hoses should be checked from time to time for signs of old age, such as excessive softness, or fraying and cracking at the edges. The engine’s water level may be low because you haven’t noticed that there is a leak somewhere.
A mysterious cause of water loss is a leaking cylinder-head gasket. This tends to leak in such a way that it is not easily noticed.
One should always inspect the water level before going off-road, because you may not be in a position to replenish the water later.
A third type of leak that is difficult to trace occurs when the radiator cap seal is too old to seal properly. The water level will drop in the radiator but you will not find any leak.
Another reason for overheating is a drive belt that has started to slip. This can usually be detected by feeling the pulleys after the engine has done some work. Any slip will heat up the pulley to such an extent that it will become untouchable.
During servicing, the belts should be examined carefully, not just by feeling the play and looking at the outside, but by getting a mirror and looking at the underside of the belts because this is where the cracking starts.
The water pump may be on its way out. Water pumps tend to leak when they get old and should be examined, fixed or replaced. Sometimes the impeller inside the pump gets corroded and worn to such an extent that it will no longer circulate enough water to keep the engine cool.
The radiator fan may be malfunctioning. The fan is needed only at speeds below 80km/h. At higher speeds there’s enough airflow to keep the radiator functioning correctly. Off-road motoring often takes place at low speeds, with the result that the cooling fan becomes vitally important.
These days, this fan is driven electrically and a thermostat is wired into the circuit to switch the fan on and off. When these units fail, the fan’s operation will be erratic, or it will no longer work at all.
When a vehicle tends to overheat easily, which is sometimes the case with an older vehicle, the presence or absence of an antifreeze in the cooling water may make the difference between overheating or not. Antifreeze not only lowers the freezing point but also raises the boiling point of water. It also contains substances to reduce rust in the cooling system.
Incorrect ignition timing may cause overheating. If it’s too far advanced, detonation may set in during the combustion process. This will raise the temperature of the engine internals, causing the water to get hotter. It may even cause engine damage.
Using fuel with a low octane value can also cause detonation and overheating. If, however, the timing is too far retarded, the combustion process will occur too late in the cycle, with the result that a lot of heat will be discarded via the exhaust system. Once again the water temperature will rise.
An incorrect fuel mixture can also cause overheating. In a petrol engine, a lean mixture will cause the temperature of internals to rise, but a rich mixture will have the opposite effect.
If a diesel engine doesn’t get enough fuel it simply develops less power without any side effects. Diesel engines always run with at least 20% excess air, and a rich mixture is referred to as over-fuelling. This occurs when the engine doesn’t get enough air and overheats very rapidly, especially if the engine is turbocharged.
The above explanation implies that, for example, if you cross a stream and the air intake filter gets wet so that it swells up, the result will be that your engine will get less air.
In a petrol engine, this means that the mixture will be rich, resulting in a slight loss of power and an increase in fuel consumption but no other harmful effects. In a diesel engine, combustion temperatures will rise, and engine damage as well as turbo damage may occur.
Bad driving habits, or the terrain, may lead to overheating. If you climb in first or second gear the vehicle may eventually overheat, especially if it is a part-time 4WD soft-roader that obviously hasn’t been designed for that kind of work.
On the other hand, using large throttle openings in a high gear at low vehicle speeds may also cause overheating. Neither engines nor gearboxes like this kind of driving because it causes the crankshaft to vibrate torsionally and will in the long run result in engine and transmission damage.
Finally, some imported vehicles, especially those that were bought overseas and then brought here, have been known to overheat because they’ve been built with radiators too small for our conditions.