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Once winter sets in the weather turns cold, and diesel owners start to worry: in freezing conditions diesel can wax, and once it does the car may not start at all. Why does diesel wax in winter?
At its cloud point, diesel develops a haze. The high-melting-point paraffin waxes in the fuel crystallise out as plates or needles; the crystals grow and interlock at their edges into a three-dimensional network that traps the lower-melting-point fuel, so it stops flowing. When only 0.5 to 2.0 per cent of the wax has separated out, the fuel has set.
Some owners resort to dangerous methods to deal with it. Two in particular.
Wrong method one: heating the tank with an open flame
Some owners try to raise the temperature by playing a flame under the tank. This is extremely dangerous: it can ignite the fuel and set the vehicle on fire. It is slow, hard work and highly risky.
Wrong method two: mixing petrol into diesel
Some say adding petrol prevents waxing, but that is very risky too. Diesel ignites at about 220°C and petrol at about 427°C, and petrol burns far faster. Mixed together, the car becomes hard to start, the engine knocks and its life is shortened. Petrol is also far more flammable than diesel, so mixing them is a genuine safety hazard.
The right answer: use a proper diesel antigel, added at the ratio given on the pack before the fuel waxes. Because it lowers the pour point and cold filter plugging point, it keeps the fuel flowing and the engine starting.