Petrol in Diesel

Petrol in a Diesel Engine

What happens and what to do if you put the wrong type of fuel in your car

Putting the wrong fuel in our cars has become an ever more increasing occurrence. With so many more diesel powered cars on the road and with technology advancing so rapidly it is sometimes it is difficult to know if you are driving a diesel car or not. It is estimated that over 150,000 of us put petrol in a diesel powered car or vice versa, but what actually happens to the engine?

The most basic piece of advice to give or remember is, do not switch on your ignition. The further the fuel is pumped into the engine the more expensive the repair will be.

How does Petrol Damage a Diesel Engine?

The first engine component to be damaged will be the fuel pump. Diesel fuel pumps require more lubrication than a petrol pump. The diesel pumps are lubricated by the diesel fuel itself, when petrol is mixed with diesel it acts like a solvent and this reduces lubrication. The overall effect is that the metal components of the pump will rub together causing the parts to wear and become damaged very quickly. Other parts of the pump such as seals may also be damaged due to the incompatibility with petrol.

If you are unfortunate enough not to realise your mistake whilst filing up your car and have switched on the ignition the damage will be more costly. Fuel will be contaminated by the pump wear caused by lack of lubrication and if this reaches the injectors, low and high pressure pumps, filters and the fuel tank it can mean that all of these parts will need to be replaced.

If you have managed not to turn on the vehicles ignition you may be able to remedy the situation quite simply and at a comparatively lower cost.

If I have put Petrol in a diesel engine

Do not turn on the ignition or start the vehicle

Realistically a small amount of petrol in a diesel engine should not cause to much damage as long as you have not started the ignition and can top up with diesel.

The AA advice is as follows:

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.

If your car is still within its warranty period you should first seek advice from the manufacturer as their guidelines may be different.

If I have put diesel in a petrol engine

Do not turn on the ignition or start the vehicle

This is a much less common occurrence for the simple reason that diesel filling pumps have a bigger nozzle than the petrol filler neck and so it is extremely difficult to push it in. If you have managed to do it then the advice from the AA is as follows:

If the car's still under warranty you should check with the dealer regarding their advice and correct action.
Realistically a small amount of diesel in a petrol engine should not cause too much damage as long as you have not started the ignition and can top up 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.

There are also many companies who will carry out the work of pumping out the fuel tank, flushing out the system and refilling the car. Many of the well known roadside assistance companies offer a specialised service to do this whether you are a member or not and as they will come to you there is no excuse for turning on that ignition! The cost of such services is likely to be high but compared to replacing an engine in some instances it will be a much cheaper repair.