Petrol in Diesel Engine: What Happens if You Put the Wrong Fuel in Your Car?


If you’ve never done it yourself, it’s tempting to think that putting diesel fuel in a petrol car and vice-versa would be quite difficult. If you have done it yourself, you’ll know that’s not the case. Our American friends would probably refer to it as misfuelling, but for the rest of us, it’s simply wrong fuel in car syndrome.

Whether it’s inattention or a lack of signage or just a car with which you’re unfamiliar, it remains that every year, thousands of people in Australia get the refuelling process wrong and wind up with at least a partial tank of what is the wrong fuel. So what happens when that happens?

Probably the most common version of this error is putting petrol in diesel engine vehicles. And what happens when you put petrol in a diesel car can be catastrophic. Sadly, this is also the variation of the mistake that is more common. Why is it more common? Simply because a petrol fuel nozzle will fit in a diesel car’s filler neck, while the opposite is not the case.

The consequences are potentially worse because of the way a diesel engine works (igniting its fuel by compressing it) and petrol that finds its way into that engine will cause all sort so mayhem and, potentially, long-term and terminal, damage.

Introducing petrol into a diesel fuel system can really cause chaos in that system. That’s partly because petrol is a solvent, while diesel fuel is, itself, a lubricant which the fuel system relies on in a diesel car. The petrol can destroy fuel lines, injectors and even the tank itself. Modern turbo-diesel engines which use ultra-high injection pressures, direct-injection and common-rail tech are particularly at risk from fuel contamination of this type. Driving any distance after making a refuelling error can wreck an engine in no time flat.

Since 1986 in Australia, it’s been quite difficult to put diesel in a petrol car. That’s because all new cars after that date were required to run on unleaded petrol and were also fitted with a restricted filler neck that would only accept the unleaded nozzle. Which means that a typical diesel nozzle physically won’t fit.

That said, there appears to be no end to people’s ingenuity, as diesel in petrol engine cars is a mistake that still seems to occur at service stations all over the country.

Regardless of whether you’ve added petrol to a diesel or the other way around, the advice is the same once you’ve realised the error. Hopefully, the penny will drop before you get back in the car and hit the key. If that’s the case, for goodness’ sake, don’t start the engine.

Try to push the car to a safe spot out of the way. Other customers might help you with this (if they take pity on you) and to do it safely, you’ll need to unlock the steering column with the key. But make sure you only turn the key as far as it needs to to achieve this. Turning it the ignition position (even if you don’t engage the starter motor) will still operate the fuel pump, and that will start pumping the wrong fuel along the fuel system and into the engine.

From there, the best advice is to call your road service provider if you have one. That applies to new cars that still carry the factory roadside service provided as part of the warranty. Be sure to explain what’s happened, as there’s not much point sending just a mechanic as the car will need to be moved to a workshop where the fuel tank and filters can be removed, the lines flushed and the car reassembled with a new tank of the correct fuel.

Another of the wrong fuel solutions is to call a specialist company called Wrong Fuel Rescue which is an Australian owned operation that offers a round the clock rescue service for this problem.

If you don’t have roadside assistance, your can have the car towed to your usual mechanic, but the one thing you don’t want to do is try to start the car. Even if it does start, it won’t run properly and will probably stop somewhere inconvenient or unsafe. Not to mention that you’ll be in danger of destroying your whole engine in the process.

The cost of this mistake, even if you don’t damage the car or its engine, is going to easily run into the hundreds at a minimum and could cost thousands. Aside from the towing fee and the hourly rate for a mechanic to do the work, there’s also the cost of replacing the filters and disposing of what could easily be 70 or 80 litres of tainted fuel (you’ve just filled up, remember). Not all fuel tanks feature a fuel drain plug, either, and if yours doesn’t there’ll also be the labour cost in physically removing the tank, turning it upside-down and draining the fuel. Suddenly, the original tank of fuel that caused all this is starting to look like small change.

If you have somehow managed to start the engine with the wrong fuel on board (usually to the accompaniment of clouds of smoke and terrible noises from under the bonnet) the pain will continue to mount as an investigation into what internal damage (if any) you’ve managed to inflict.

Now you know why rental cars often have a sticker on the outside of the fuel door specifying what fuel to add to the tank.

By David Morley

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