Misfuelling What to do if you've put the wrong fuel in your car You're not alone. It's estimated that at least 150,000 drivers put the wrong fuel in their car each year – one every three and a half minutes! And the scale of the problem has been getting bigger too, especially over the last five years with growth in diesel car sales – modern diesels are so quiet it's easy to forget you're driving one, particularly if it's a second family car or hire car. If you've put the wrong fuel in your car, don't turn on the ignition or start the engine as this will circulate the contaminated fuel and increase the risk of damage. Potential damage Diesel fuel pumps operate on very fine tolerances and are lubricated by the diesel fuel itself. Petrol in diesel acts as a solvent – it reduces lubrication and can cause damage to the pump through metal to metal contact. Some fuel system seals can be affected by the compounds in petrol too. The further the contaminated petrol goes in the system the more expensive the repair – it can be cheaper in more extreme cases to fit a new engine. The newer common rail (or HDi) diesel engines are particularly vulnerable – if fuel contaminated by pump wear debris gets as far as the common rail system you may have to replace the low and high-pressure fuel pumps, injectors, fuel rail, line filters and the fuel tank too. Many systems use a low-pressure electric pump in the tank or sender unit and this begins to work as soon as the ignition is switched on, circulating the contaminated fuel through the pump and rail – so it's important not to turn the ignition on. Petrol in Diesel Do not turn on the ignition or start the car If the car's still under warranty you should check with the franchised dealer regarding their advice and correct remedial action – running with even a small amount of petrol in the tank might invalidate the warranty while some manufacturers advise that seals and filters should be renewed even if the engine has not been run. Generally a small amount of incorrect fuel should not damage the engine as long as you have not started the car and top up fully with diesel fuel. If you've added more than 10% (5 litres in a 50 litre tank) petrol – Drain the tank and refill with diesel If you've added less than 10% petrol (5 litres in a 50 litre tank) – Top up with diesel and run normally unless the manufacturer has advised otherwise Diesel in Petrol Do not turn on the ignition or start the car This is much less common because the standard diesel nozzle at fuel stations is larger than the fuel filler neck on modern petrol cars – you have to be pretty determined and patient to misfuel with diesel as a result. If the car's still under warranty you should check with the franchised dealer regarding their advice and correct remedial action. Generally a small amount of incorrect fuel should not damage the engine as long as you have not started the car top up fully with petrol. If you've added more than 10% (5 litres in a 50 litre tank) diesel – the car shouldn't be run and the tank should be drained If you've added less than 10% (5 litres in a 50 litre tank) diesel – the tank can be topped up with petrol and the car run normally unless the manufacturer has advised otherwise