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Tech Briefing: Fire risk


The many burnt-out vehicles abandoned in remote parts all over Africa are grim testimony to the fact that fire is a major risk when overlanding. The culprits are a combination of high ambient temperatures and tall grass, plus heat from the engine and (even more often) from the exhaust. How does one deal with this?

The fact is that even green, wet grass dries out very quickly when exposed to the high heat and ventilation of the exhaust areas under a vehicle. This happens within minutes, not hours. In certain circumstances, you may have to stop every 10 minutes to clean around the exhaust. The newer vehicles which are fitted with catalytic convertors are a lot worse, because of the extremely high temperatures associated with these units. Their owners should think twice about parking in a grassy area when it is hot, as the temperatures are high enough to start a fire remotely.

What to watch out for

There are two aspects to this problem. The first is to identify those places on the vehicle where grass will catch, be torn off, and will then build up close to the exhaust. Exhausts were never designed to prevent grass build-up at all. One can experiment with little guides, and also try smoothing over mounting points, as these are the real culprits. However, this is a rather hit-and-miss affair, as testing circumstances are rather limited.

Guards and bash plates are the second problem. All sorts of hiding-places are created in which grass and soil can build up in layers. This can potentially limit air flow around the engine, the gearbox, and the hottest areas of the exhaust. When this happens, heat build-up quickly becomes an issue. The guards, although they are (without doubt) useful in preventing big-hit damage to components, are also excellent at creating these problem areas. To add to the problem, most are rather inaccessible, and cleaning behind them is difficult. In this scenario, ‘full body armour’ is not a positive addition.

A basic fire risk kit: Seed net, small fire extinguisher, braai tongs and a grass extractor we made up using a metre of 6mm stainless steel rod (cost R30). It doubles as a braai tool, amongst other potential uses. Alternatively, I could have used a length of bloudraad at almost no cost. Keep this kit on hand when driving – it could save your vehicle.

What to do: Make a regular detailed inspection when you are driving through grass – any grass, no matter whether it is tall, or just tall enough to snag the underside of your vehicle. Slide under the vehicle, and inspect areas around the exhaust in detail to determine where your problems are.

Immediately clean out any build-up where a fire is likely to start – this is more likely to be closer to the engine where the temperatures are higher. If you have a catalytic convertor, pay special attention to this.

Determine a safe travel-distance interval at which you must stop and clean out these areas.

Before you leave home, make a hook of 6mm rod, about a metre long. It must have a bend at the front which is 30 millimetres long and an angle of 90 degrees (or more). Add a handle for a good grip at the other end.

This rod should always be next to your seat, as you will need it fast and regularly. Failing this, or in addition to this, make sure you have a pair of braai tongs ready to hand – these will also be useful in grabbing out the dangerous build-ups. You will quickly learn that when you’ve stopped in time, you can actually smell the smouldering grass as you walk around the vehicle. You will also learn where the most critical areas are. Dive under the vehicle, and get that grass out first. Clean out all the areas before you continue driving.

Limited airflow

A blocked radiator that limits airflow is a typical problem arising from a build-up of grass seed. In the rainy season, tall grass grows on the middelmannetjie. This creates multiple problems. The most common occurence is that the seeds and bits of stalk that are broken off by the front of the vehicle, find their way into the front of the radiator. This can build up in a very short period, limiting (or even blocking) the airflow through the radiator. This can destroy your engine.

A second effect is that a tall ‘middelmannetjie’, combined with a shield of tall grass, forms a barrier to airflow − and leaves very little room for the massive amount of air that needs to escape. This is also not helped by the new design of tightly-packaged engine bays, in which there is little place for the hot air to escape.

In addition, manufacturers tend to use sensor readouts that filter engine-temperature indicators, and this has the nasty side effect of substantially masking any variation. They hide pre-warnings, and show that the temperature is high only when it is too late.

To overcome this, make a habit of feeling the bonnet temperature when you stop. (Running a temperature in excess of 80°C is common.) Do this often enough, and you will know how to feel the difference between city driving, and heavy low-range 4×4 work.

Also remember that aircon radiators add to the airflow issues, as do intercoolers on the new diesels. Those top-mounted intercoolers are, in effect, an extra and difficult radiator to protect.

What to do: Prepare for it. The common ‘grass net’ sold is pretty ineffective. There are huge gaps around it, and air will follow the route of least resistance. And when these nets are not wide enough, a significant portion of the grass material will still be carried around the net to block the radiator.

There is some debate doing the rounds that, as they are made of shade cloth for the most part – which blocks wind when used as a wind shelter – these nets will also block airflow. Different results have been reported by the owners of different vehicles, with some reporting a distinct temperature-rise, and others very little effect.

A grass net should cover all openings to the radiator, and seal perfectly all around. It should also be fine netting which does not limit the airflow too much, which means that it is better to use mosquito gauze rather than shade cloth.

The bigger the surface, the longer it will take to block up. It also helps if the net is well-designed and does not form a build-up area at the bottom, and can be removed easily for cleaning.

At regular intervals, take it off, clean it with a stiff brush and/or hose it down from the opposite side. This is a good method of removing the debris sticking to the gauze, but those spiral self-drilling seeds will have to be removed by hand, one by one.

Wet pollen

A fairly unknown phenomenon is wet pollen. This is actually a real nightmare to encounter. If you have tall grass in bloom (and therefore lots of pollen), combined with early morning dew or rain, you have a nightmare scenario.

Avoid driving through this, or try to re-route onto ‘clean’ tracks. The pollen forms a wet putty on the front of the radiator that is really difficult to remove. The most effective way of clearing the inevitable blockage is to use a high-pressure water gun from the rear of the radiator. This is not something available in the bush, and may well entail a lot of stripping of the vehicle.

There are other plans, such as drying out the radiator and using a toothpick to work the gunk off from the front. The pollen is so fine that it goes straight through the finest gauze, and irrespective of your removal technique, this will be really difficult, and time-consuming, to do in the bush.

Driving through pollen-heavy grass when it is bone dry is much less of a problem. It may, however, become a hay-fever issue. The pollen comes over the bonnet like a curtain, and anyone with the slightest allergic reaction will have a very hard time.

What to do: This should not become an issue unless you drive seldom-used tracks, and in the wet season. You can mitigate it to some degree by being the third, or later, vehicle in the convoy.


Pieter runs a 4×4 modification business, focusing on Land Cruisers. Contact him using the form below.

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