If you have put petrol in a diesel car, do not start the engine and do not turn on the ignition. Leave the car where it is if you can, tell the forecourt staff, and call a mobile fuel drain service. As long as the engine has not been run, a straightforward drain and flush will almost always sort it out with no lasting damage.
Petrol in a diesel is the most common, and the most damaging, misfuelling mistake in the UK, with around 150,000 incidents every year. The good news is that catching it early makes all the difference. Here is exactly what to do, why it matters so much, and what happens next.
What to do right now, step by step
The actions you take in the first few minutes decide whether this is a minor inconvenience or an expensive repair:
- 1. Stop fuelling the moment you realise. Do not add any more fuel of either type.
- 2. Do not put the key in the ignition or press start. On most modern cars, switching the ignition on runs the fuel pump and begins circulating the contaminated fuel through the system, which is exactly what you want to avoid.
- 3. If it is safe, put the car in neutral and push it to a parking bay clear of the pumps, with help from forecourt staff. Do not start it to move it.
- 4. Tell the petrol station staff, they deal with this regularly and will expect a recovery or drain vehicle to attend.
- 5. Call a mobile wrong fuel drain service. They can attend the forecourt, drain the system, and get you on your way, usually within the hour.
Why petrol damages a diesel engine
A diesel fuel system relies on the diesel itself to lubricate its precision components, in particular the high-pressure fuel pump and the injectors, which operate at extremely high pressures and very fine tolerances. Diesel is naturally oily and provides that lubrication as it flows.
Petrol does the opposite. It acts as a solvent and strips that protective film away. Run an engine on petrol-contaminated diesel and you get metal-to-metal contact inside the high-pressure pump, which wears rapidly and sheds tiny metal particles. Those particles are then carried through the system and can damage the injectors, fuel rail, and filters. In a bad case, that can mean replacing the pump and injectors, a repair that can run into thousands of pounds, plus a full system flush.
This is why the no-start rule matters so much. With the engine off, the petrol is simply sitting in the tank doing nothing. The damage only begins when the system is pressurised and the contaminated fuel starts moving.
What if I have already started the engine or driven it?
If you have started the engine, or driven off before realising, stop as soon as it is safe to do so and switch off. The damage is cumulative: the further and longer you drive on contaminated fuel, the greater the wear. Stopping early still gives the best possible outcome.
In this situation the system will still be drained and flushed, but the technician may also recommend replacing the fuel filter and, depending on how far you drove, checking for signs of pump wear. Many cars driven only a short distance on a diluted mix recover completely once the system is cleaned out. The key is not to keep driving in the hope it clears, it will not.
What a fuel drain involves
A mobile fuel drain is a clean, controlled process that can be carried out on a forecourt, in a car park, or at the roadside:
- The technician connects a specialist extraction unit and removes all the contaminated fuel from the tank under controlled suction.
- The fuel lines, filter circuit, and on many vehicles the high-pressure rail are flushed to make sure no petrol residue remains, this is more than just emptying the tank.
- The contaminated fuel is stored in sealed, approved containers and taken away by a licensed waste fuel handler.
- The correct fuel is added, the system is primed, and the engine is started and run to confirm it is operating normally before you drive away.
How much does it cost, and is it covered?
A drain is a fixed, one-off job and it is a tiny fraction of what a damaged high-pressure pump and injectors would cost to replace, which is the whole reason for acting before the engine is run. We confirm the price before any work starts, so there are no surprises on the forecourt.
It is also worth checking your car insurance. Some comprehensive policies include misfuelling cover as standard or as an optional extra. We can provide a receipt and a short report of the work for any claim you want to make.
Diesel in a petrol car: is it as bad?
No, and this catches people out. Putting diesel in a petrol car is far less common, because a diesel pump nozzle is wider than a petrol filler neck and usually will not fit. It is also less damaging, diesel does not strip lubrication from petrol-engine components the way petrol attacks a diesel system.
That said, a significant amount of diesel in a petrol tank will still stop the engine running cleanly. It may refuse to start, run very roughly, produce clouds of white smoke, and can foul the spark plugs and catalytic converter if driven. The same advice applies: do not drive it, and have the system drained if there is more than a small splash. A tiny amount of diesel in a full tank of petrol will often just cause a rough running spell that clears, but if in doubt, drain it.