4X4s and Grass Fire Risks. What You Need To Know

Ford has just issued a recall of their hugely popular Ranger because of the risks of fire from grass collecting under the vehicle. Mazda has also acknowledged the problem on their BT-50, but this is a problem that goes well beyond just these two 4WDs.

WHO IS AFFECTED?

Just about everyone, except if you drive a Unimog maybe. Exhaust is very hot by nature, and it runs underneath the body of your vehicle. Grass can catch fire, and there’s a chance it can come into contact with hot exhaust parts.

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This is a Nissan GU Patrol underbody after driving some very remote and overgrown Eagle and Gary highways. You can see how things can potentially get toasty.

If the grass is tall enough, it might start breaking off and collecting in some of the many nooks and crannies of the underbody. Crossmembers and brackets might be designed and fitted in a way that they inadvertently catch lots of grass, and have hot things in close proximity.

DESERT TRAVEL

If you’re travelling out into the far reaches of Australia, then you need to be really switched on when it comes to catching grass underneath your vehicle. It’s probably not as much a problem through more popular areas like the QAA and French lines, but other less-frequented deserts can really up the risks of fire.

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This is the kind of track you need to really worry about. hardly driven, and with plenty of dry grass around.

Spinifex is one of your main culprits. This tough desert grass is full of an oily resin, which burns very, very hot. And once it starts, good luck trying to extinguish it. You’ll see it start to grow tall when it’s going to seed, and can continue to grow in between wheel tracks of lightly-formed (ungraded) tracks.

It’s not just the grasses that collect around exhausts and cross members that you need to worry about. tall grasses and seeds can also get into your radiator, air conditioning and intercooler fins and seriously reduce their efficiency, which all result in a harder-working and hotter engine.

THE DPF PROBLEM

The risk of fire from grass is a potential problem on just about any vehicle out there, but some are more predisposed than others. How crossmembers and the underbody are designed is a big factor in how grass can be caught, but no doubt a major addition to risk is a diesel particulate filter (DPF).

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Let’s play a little game called ‘spot the DPF’ on the current model Ford Ranger.

A DPF is a kind of catalyst that filters the soot out of your exhaust and catches it in the honeycomb structure of the filter. As the engine is running, the DPF will slowly block up with more and more soot.

Once the engine ECU notices the exhaust blocks up to a point, it goes into a regeneration phase. Exhaust gas temperatures are increased by altering the fuel injection timing, and even injecting some extra fuel into the exhaust gasses to really increase the temperature throughout the entire exhaust system.

These very high temperatures (over 600 degrees) are needed to burn off the collected soot in the filter, making room for more. Most (almost all) 4WDs have automatic regeneration cycles as the vehicle computer determines, so it might think it’s a great time to do a regen while your cruising over some tall grass.

How to fix the problem?

In serious conditions, you’d be wanting to do hourly checks under your vehicle to see how much grass is building up in problem areas. You can keep an eye on it, and get a good idea of where and how often you should be checking things out. Keep a long tent peg and some sturdy gloves handy, and use these to fish out the collected debris.

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This is the Ford Ranger once again. You can see how easily and quickly this design could catch combustible stuff, very close to the exhaust.

One great tip here is cutting up a piece of shade cloth or something similar and fixing it to your grille or bullbar, which will let air still flow through to the radiator, but stop the seeds from getting into the fins. It’s also worth checking the gaps between your different cores, because these can often fill with crud as well. I’ve found a long zip tie can help dislodge some of this stuff (between a radiator and front-mounted intercooler) on my own 4X4.

Beyond that, a true ‘solution’ to the problem might be impossible, beyond buying a Unimog. You could go out and buy an older 4WD without a DPF, or one that has exhaust routed in a way that helps keep it clear of collected grass. Your other option is to fashion some kind of plates or skids under your vehicle, which will push the grass away rather than collect it up. But undoubtedly, the best thing you can do is be aware of the problem, and keep an eye on it, no matter what vehicle.

And of course, when you’re travelling remotely, you need to subscribe fully to the idea of self-sufficiency. That means carrying a fire extinguisher, and having it somewhere easy and fast to access if the faeces hit the fan. And, know how to use it.

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It’s happened before, and will probably happen again. It’s all about being tooled up with the right knowledge and skills before going off-road.

What are your tips for reducing the risk of grass fires on your 4X4? Let us know in the comments below.

Article from Pat Callinan’s 4X4 Adventures

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Comments 3

  1. Usually petrol vehicles are much more susceptible to grass fires due to hot exhaust temperature. That is one of the reasons many farmers buy diesel vehicles.

    In SA recently one of the government departments decided to replace the diesel twin cabs utes their staff had been using with petrol SUVs.

    The result of this where grass fires on grain farms the departments staff where visiting to assist the farmer with weed control strategies.

    Because of these fires caused by petrol powered vehicles the owner of the farms banned petrol vehicles from the properties .

    Remember that people working on rural properties often do not have a choice about driving through grass when it is required to access areas of the farms as part of their jobs .

  2. This is great advice, and we have first hand experience of how easily things can go pear-shaped.
    Last year on a Canning trip our travelling companions lost a brand new Prado, which within five minutes ended up looking like the one in your picture.
    We were in spinifex country and had been checking underneath all vehicles regularly, but obviously some had caught fire in an inaccessible place and then set fire to the grass on the track.
    Two fire extinguishers and much shovel work made virtually no impression, and not only did it destroy the vehicle and all contents, but set off a major grass fire.
    Fortunately the wind direction was in our favour and we managed to get out our travelling companions and the other two vehicles. But it was a very unpleasant and dangerous experience (not to mention expensive).
    So, if you’re on remote desert tracks, take the precautions outlined in the article.

  3. A few mates carry a 5 litre pressure garden sprayer, with the long wand can get in to the nooks and crannies and stop smoldering grass with a good soak. The say its the way to make water run up hill under the vehicle.

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