Muddled and Misfuelled?

Putting in the wrong fuel is more common than you think…

KT Insurance
Feb 17 2022

By Allan Whiting of

Especially for those new to RVs with diesel power, it’s not uncommon for people to put petrol into their vehicle’s diesel fuel tank. The results can vary from mild inconvenience to total engine failure, so you need to take extreme care. Interestingly, it’s not so easy to do the reverse – put diesel into a petrol tank – because the diesel nozzle is 25 mm in diameter (high-flow nozzles are even larger), so it won’t fit into a petrol filler neck that’s 23.6 mm across.

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 – say a couple of litres in a 90-litre tank – 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 – might not cause fuel system damage, but any more than that is risky. Should you make a mistake at the bowser, it is best to err on the side of caution and call for roadside assistance rather than run the engine. A tilt-tray job, tank-drain and refill with clean diesel might cost a few hundred bucks, but it’s better than up to ten grand for a new fuel system or twenty-five grand if an engine rebuild is necessary as well!

If misfuelling 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 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 (which might 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 Misfuelling

If you’re worried about potentially misfuelling, there are several simple products on the market to help avoid the trap. 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 this cap but a petrol nozzle won’t. Also, if a high-flow nozzle is the only one available, a plastic key allows the cap to be removed, exposing the standard full-sized filler neck.

Any of these devices is going to greatly reduce the chance of you (or someone else) accidentally filling your diesel tank with petrol. For a modest outlay you might just save a fortune, not to mention an awful lot of mucking around…

KT Insurance