What do I do if I put diesel into my petrol car, or petrol in a diesel engine?

With a recent mixup seeing diesel being erroneously placed into a petrol tank, the AA has advice on what to do if you've misfuelled
What do I do if I put diesel into my petrol car, or petrol in a diesel engine?

Whether you've put petrol in a diesel car or vice versa, it's crucial not to turn on the ignition

Following the inadvertent delivery of a tank of diesel into underground storage intended for petrol at a Circle K station in Kildare, and warnings of up to 87 motorists being affected by the issue, the question of misfuelling cars has hit the news.

Stopping to fuel up in the middle of a long-haul journey is convenient, but it happens the odd time — whether distracted by rowdy passengers, or tired out by circumstances, you might pick out the wrong fuel pump and load your car up with a tank of the wrong stuff.

Whether you've put petrol into a diesel car, or vice versa, the AA has advice on what to do.

The organisation emphasises that it is so important, if at all possible to "not to turn on the ignition, or start the engine".

Effects of running a misfuelled engine include:

  • 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.
  • 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.

Damage to the engine can be very expensive for some vehicle owners, as manufacturers often:

  • Recommend change of parts (as high as €2,000 or more)
  • Garages often charge €440 or more for a fuel drain

The AA offers its own FuelAssist service to members (including passengers in affected cars) - ring 0818 667788.

Once called, the organisation will:

  • Drain, flush and refill your vehicle’s fuel system.
  • Ensure that the contaminated fuel is recycled.
  • Get you back on the road and on your way.

Failing an AA membership, a number of auto recovery specialists offer a fuel-draining service.

Pic: Sam Boal/Rollingnews.ie
Pic: Sam Boal/Rollingnews.ie

What should I know?

The million-dollar question: which is worse? Putting petrol into a diesel car or diesel into a petrol car?

No misfuel is good and the AA says it's crucial not to even turn on the engine if you put the wrong fuel in your car.

Petrol in a diesel engine

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.

Diesel in a petrol car 

Make sure that you don’t turn on the ignition or start the car — but putting diesel in a petrol car is a less common problem: you would generally have to be pretty determined and patient to misfuel with diesel as the standard diesel nozzle at garages is larger than the fuel filler neck on modern petrol cars.

Again, if the car is still under warranty, you should check with the franchised dealer regarding their advice and correct remedial action.

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