It happens more often than you might think, especially with a hire car or a car you have just bought: you fill the tank and then realise you used the wrong fuel. What matters most is what you do in the next few minutes.
If you notice at the filling station
- Do not start the engine. On many cars the fuel pump runs as soon as you turn the key or press the start button, so do not even turn the key.
- Put the car in neutral and, if needed, push it to a safe spot with help from the station staff.
- Call your insurer's roadside assistance or a garage. The tank needs to be drained and flushed.
If the engine has not been started at all, the repair is usually just draining and cleaning the system, which costs far less than engine damage.
Petrol in a diesel: usually the most expensive case
Diesel fuel lubricates the high-pressure pump and the injectors of a diesel engine. Petrol does the opposite: it acts as a solvent, washes the lubrication away and can wear the pump very quickly. Metal particles then travel through the whole system and the cost goes up sharply. That is why, with a modern diesel, you should not even drive it to the garage.
"A little petrol cleans the injectors": is that true?
In older diesels with a mechanical injection pump, a small amount of petrol was tolerated. Old lorry and taxi drivers used to add a little petrol to the diesel to "clean" the system, and some manufacturers said that with a small amount of petrol you could top up with diesel and carry on.
In modern common-rail diesels, which means most cars from recent years, this no longer applies. The pump works at very high pressure with very tight tolerances, and even a little petrol reduces lubrication. Filling up with diesel dilutes the petrol but does not remove it. If you do not know which system your car has, assume it is common rail and do not start the engine.
Diesel in a petrol car
This is less common, because the diesel nozzle usually does not fit the filler neck of a petrol car. If you noticed before starting the engine, draining and cleaning the tank is usually enough.
If the engine was started and ran, though, the damage has usually already begun. The engine misfires, smokes and stalls, while the diesel fouls the spark plugs, the injectors and the fuel filter. Unburnt fuel can destroy the catalytic converter and the lambda (oxygen) sensor, which are expensive parts. The longer the car is driven like this, the worse the damage, and the bill can become very high. Switch off straight away and do not start it again.
AdBlue in the fuel tank
AdBlue goes into a separate tank with a blue cap. If it gets into the fuel tank, it can seriously damage the pump and the injectors. Do not start the engine and get help straight away.
If you notice while driving
If the car starts to hesitate, lose power, smoke or the check engine light comes on shortly after refuelling, stop safely, switch off the engine and do not start it again.
How it is put right
The tank is drained, the fuel lines are flushed and, depending on the case, the fuel filter is replaced. If the car ran on the wrong fuel, the pump and the injectors are checked as well. Give us a call and we will tell you what your car needs.
Frequently asked questions
What happens if I put petrol in a diesel car?
Petrol does not lubricate the high-pressure pump and the injectors of a diesel engine and can destroy them. Do not start the engine and do not even turn the key. The tank needs to be drained and cleaned.
If I only put in a little wrong fuel, can I drive?
In an old diesel with a mechanical pump, a little petrol in a full tank of diesel usually did no harm. In modern common-rail diesels there is no safe amount: even a little petrol can damage the pump and the injectors. If you put diesel in a petrol car and the engine ran, the spark plugs, injectors and catalytic converter may already be affected. The safest thing is not to start the engine and to call for help.
These tips are general. Every car has its own manufacturer specifications, so ask us about yours.