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Putting petrol into a diesel tank can cost you thousands.

It’s not uncommon for people to put petrol into their diesel fuel tanks and the results vary, from inconvenience to total engine failure. Here’s what to do.


Accidentally putting petrol into a diesel tank is an all too frequent occurrence, particularly in the case of families and fleets that have mixed-fuel vehicles. It’s not so easy to do the reverse – put diesel into a petrol tank – because the diesel nozzle is 25mm in diameter (high-flow nozzles are even larger), so it won’t fit into a petrol filler neck that’s 23.6mm diameter.

Diesel engine fuel pumps rely on the lubricating qualities of diesel fuel and a petrol-diesel mixture has much less lubricity, potentially causing massive injection system damage.

Before the arrival of high-tech, common-rail-injection diesels a small fraction of petrol in a diesel tank wasn’t as engine-life-threatening as it is today. An older, mechanical-injection diesel could tolerate a small fraction of petrol – a couple of litres in a 90-litre tank, for example – without major drama.

Back then, we had high-sulphur diesel as well, with higher lubricity than today’s very low sulphur diesel, so the diluting effects of petrol were not so detrimental to the lubricating quality of diesel. Old-style injection pumps had wider tolerances and were more tolerant of lower-quality fuel.

A tiny amount of petrol in a diesel tank – say around one litre in a 90-litre tank – may not cause fuel system damage, but any more than that is risky. It’s best to err on the side of caution and call for roadside assistance, rather than run the engine. A tilt-tray job and tank drain and refill with clean diesel may cost a few hundred bucks, but it’s better than up to 10 grand for a new fuel system, or 25 grand if an engine rebuild is necessary as well.

If mis-fuelling happens at a bush service station the proprietor should have a recycling drum into which the tank contents can be drained.

Siphoning out fuel isn’t appropriate for the draining job, because there will always be some fuel left in the tank. Also, since petrol will float on diesel, the residual fuel in the tank is likely to be petrol-rich.

The only draining method is to open the tank drain and catch the fuel mixture in a container that may need to be drained and refilled a few times.

It’s important that you check your tank drain before you go bush and ensure that it can be undone if required.


Preventing mis-fuelling

A brightly coloured fuel cap is a handy aid, in addition to the usual warning stickers.

There are also filler-neck devices in the market that can prevent mis-mis-fuelling petrol into a diesel tank.

Four of them – Diesel Fill, SoloDiesel, Diesel Key and Fuel Angel – are neck fittings that will not allow narrow petrol nozzles to open their neck restrictors.


The only downside with these designs is that they won’t accept a high-flow truck bowser nozzle either and need to be removed if that’s the only available nozzle.

The South African designed Diesel Smart Cap is different in that is a cap only, not a neck fitting. A diesel nozzle will open the cap, but a petrol nozzle won’t. If a high-flow nozzle is the only one available, a plastic key allows the cap to be removed entirely, exposing the standard full-sized filler neck.




























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