What to Do If You Accidentally Fill Your Petrol Tank Up With Diesel

What to Do If You Accidentally Fill Your Petrol Tank Up With Diesel
Photo: StockLeb, Shutterstock

As the price of petrol rises, everyone is thinking more about the fuel they put into their cars. The glorious electric car revolution may be coming, but it ain’t here yet, leaving us to somehow find five minutes in our busy day to visit the pump, avoid running out of petrol in the drive-through, and reduce the likelihood of being mocked on the Internet. As a result, you may be more aware than ever that there are usually two kinds of fuel available at your local petrol station: petrol and diesel.

Both are refined from crude oil, but they’re produced using vastly different processes and additives and are not in any way interchangeable. Many people will insist that misfuelling isn’t easy to do because the pumps are colour-coded (diesel pumps usually have green handles) or at least clearly labelled, and diesel nozzles are larger than petrol nozzles, making it difficult to fit into a gas car’s filler neck.

Ah, but note that the word “difficult” doesn’t mean “impossible.” Despite these challenges, people do manage to misfuel their cars every year. A combination of stress, distraction, and determination is all that’s required, though sometimes there are errors on the other end, as when a delivery truck pumps diesel into the underground petrol storage tanks by mistake. However it happens, what does it mean if you put diesel into your petrol-powered car?

Not all fuels are the same

Diesel and petrol are very different fuels. Diesel is heavier than petrol, has a much lower octane rating, and ignites in a totally different way. Petrol has a distinctive smell, and diesel is much thicker than gas, which is a thin, watery substance.

If you have any doubt about you just pumped into your car, don’t take any chances. Putting diesel into your gas tank is a very, very bad idea. Depending on how much diesel you pumped in there, you might drive the car for a short time before it starts to knock and sputter, but it will eventually stall. Driving the car after pumping in diesel is the worst idea because that distributes the diesel throughout your fuel line and engine, gumming everything up and requiring a serious effort to clean up.

What to do if you you just fuelled up with diesel

If you realise your mistake before trying to drive the vehicle, here’s what you should do:

  • Don’t drive it. Have we made this clear? Diesel in your petrol tank is a minor problem. Driving a petrol car with diesel fuel will be an enormous problem. It doesn’t matter if it’s just a small amount — if you got diesel in there, step away from the ignition.
  • Note how much diesel you just pumped in there. This doesn’t have to be exact, but the beleaguered (and likely amused) mechanic who will eventually help save your car will find it helpful to know whether there’s half a tank, a whole tank, or just a small amount of diesel mixed in there.
  • Call a mechanic. If you’re at a service station, the kindly folks who work there may be able to help you, especially if you didn’t drive the car. What needs to happen is this: Your petrol tank needs to be drained and cleaned out. It’s relatively easy, though it will make a crimp in your schedule and your wallet.

If you didn’t notice in time and realised your mistake a quarter km down the road when your car made some extremely worrying noises and stopped cold, you’ll need to call for a tow and get a mechanic to fix things up for you. Be warned that this is going to be expensive. Your best-case scenario is that the fuel system just needs draining and cleaning. The mechanic will need to drain the petrol tank, clear the fuel lines and the fuel injectors (which might require dismantling and removing them to clear clogs and will add to the expense), and inspect everything for damage. This can take a few hours and can run you as much as $1,000 depending on the car and the problems they encounter.

The worst case is severe damage to your car’s cylinders from trying to combust the wrong fuel. This can be incredibly expensive, and might even cost more than your car is worth to fix. That’s right: Buying $20 worth of diesel fuel by accident can cost you a whole new car.

Again, making this mistake isn’t easy — but it does happen. Always pay attention when handling dangerous stuff like fuel.