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Are caps to stop petrol being used in diesel vehicles reliable?

Asked by Duncan

I am looking to buy a 'misfuelling cap' for my Kia Sorento so that petrol can't accidently be placed into the diesel Kia. Is the technology reliable? Can you name a product, for example Diesel Key, Diesel Fill, Solo Diesel Cap? Do they work.

Answered by CarsGuide

3 Mar 2021 David Morley

This is a real problem and many road service call-outs are, in fact, caused by this very problem. It’s vastly more common for petrol to be put into a diesel vehicle than the other way around, simply because a petrol bowser nozzle will fit into the diesel car’s filler neck, but not the other way around. But should you mistakenly put petrol into a modern, common-rail diesel engine, the entire fuel system needs to be cleaned as a result. And that’s the best-case scenario, because if you drive any distance with petrol in the system, repairs can top $10,000 in some cases.

The devices you have listed usually work in the same way; they replace the car’s standard filler neck and act as a physical barrier to an unleaded petrol nozzle being inserted into the car. Unless the nozzle being presented is a diesel-sized nozzle, you won’t be able to put anything into the tank. Installed correctly, they should present no problems, but as with any part of a car’s fuel system, the installer needs to know what they’re doing. But they’re popular with fleet vehicles (which are driven by a variety of people who may or may not know the vehicle is diesel-powered) and families with a fleet that uses more than one type of fuel.

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