What should you do if you put the wrong fuel in your car

We all know it’s possible to be distracted in modern life, but what should you do if you’ve had a momentary brain freeze and put petrol into your diesel-powered car, or vice versa? Is the car automatically ruined forever? Or is there a way to rectify your mistake? Here’s our quick rundown on what you should do if you make a refuelling boo-boo.

WHY IS IT A PROBLEM TO REFUEL WITH THE WRONG FUEL TYPE?

The issue here is that while they are both referred to as ‘internal combustion engines’, petrol and diesel engines use an entirely different process to burn their fuel, in order to generate energy to propel the car forwards. A petrol engine uses spark ignition to start burning the fuel-air mix in each of its cylinders, whereas diesel power is done by sheer compression of the fuel-air mix alone.

To that end, while both petrol and diesel are refined from crude oil and are made up of long chains of hydrocarbons, their make-up as ‘finished’ fuels is different. Petrol is a significantly lighter, less dense, more flammable and more volatile substance than diesel. So putting the wrong fuel type into a petrol or diesel car can have significant and serious repercussions.

The fuel companies have tried to prevent this forecourt mishap from occurring in the simplest of ways – the size of the nozzles on the fuel pumps are different, with the diesel nozzle being larger. Car manufacturers responded in kind and have made the fuel-filler necks in their vehicles’ fuel tanks the corresponding size of their fuel type’s nozzle. Which is to say, you will struggle to get diesel fuel into a petrol car by accident, because the fuel-pump nozzle simply won’t fit into your car’s fuel-filler neck opening. And even if you do somehow manage to cram the diesel nozzle into your petrol car, if you run a petrol car on a small amount of diesel fuel it’s not likely to be terminal to its health. It won’t run very well on the fuel type and it could clog up a petrol engine’s injectors and/or spark plugs, or the engine simply won’t start at all because the compression ratio required to burn diesel fuel is far, far higher than that found in many petrol engines.

The really bad move to make is to put petrol into a diesel car, which is possible because the petrol nozzle is smaller than the diesel one, so a petrol pump will fit into a diesel car’s fuel tank filler neck all too easily. The problem with petrol in a diesel engine is that the diesel fuel also acts as a lubricant for the fuel system’s internal components. Petrol, when mixed with diesel, actually acts as a solvent and starts stripping diesel away from these components, increasing the friction that occurs within an internal combustion engine. Trying to run a diesel car on even a small amount of petrol will be pretty much fatal to the engine and fuel system, so DON’T do it!

WHAT TO DO IF I’VE MISFUELLED MY CAR

The key thing here is the point of realisation on your part. Ideally, you want this to happen before you’ve even gone in to pay for your fuel, or as you’re on your way back to your car. If you drive off in a diesel-fuelled petrol car (if it even starts), you’ll probably notice lots of excessive smoke coming out of the exhaust and likely a misfire. In a diesel, you’ll likely get a loud and worrying knocking noise from the engine, a load of warning lights illuminating on the dash and then failure of the engine within a matter of kilometres (depending on how much petrol you’ve put in and how empty the tank was when you refuelled).

Therefore, if you THINK you’ve put the wrong fuel in your car or you know you have because you have checked your receipt and can see you’ve bought the wrong fuel, here are the key steps to follow:

1. DO NOT start the car or even turn on the ignition. The minute you turn the ignition on, the electrics onboard prime the fuel pump, so the wrong fuel type will already be sucked into your vehicle’s deeper fuelling system, even if it doesn’t make it all the way into the engine in the process.
2. Go inside and tell the garage what has happened, then get someone at the garage or another customer on the forecourt to help you push your car (with the ignition OFF and the engine definitely not started) to a safe place on the forecourt.
3. Call a roadside recovery body immediately. Most breakdown firms are well-versed in what to do in the event of misfuelling and have the equipment necessary to drain your tank of the relevant wrong fuel type.
4. Call your car insurance provider. Members of a roadside recovery body might get favourable rates on calling out the relevant body to drain their car’s fuel tank, but the service usually incurs a cost. Now, the cost in question might seem steep (it will vary from company to company), but it is a fraction of the price you’ll have to pay to fix a car that has been run on the wrong fuel type and damaged its major components.
5. REMEMBER – DO NOT even turn on the ignition of the car if you think you have put the wrong fuel type in it. If you do and you cause damage to the fuelling system or the engine itself, you will be looking at a bill in the thousands of Euro to remedy the matter – or you might even be told your car is a write-off as a result.

ADDENDUM: since the advent of tougher emissions laws, a lot of modern diesels also require a secondary exhaust-gas-cleaning fluid called AdBlue to be replenished from time to time, usually every 10,000km or so. Your diesel car will have a separate AdBlue tank in it with a separate filler neck too, and most fuel station forecourts will have an AdBlue pump located somewhere on their premises.

If you somehow manage to put the AdBlue into the tank meant for your diesel fuel, you absolutely MUST NOT turn the ignition on or crank the engine over. AdBlue is a corrosive substance and it will cause significant, expensive damage if dumped straight into the fuelling system. In the event you do this, call both your breakdown provider and insurance company straight away, and do the same thing as above – push the car to a safe, out-of-the-way corner of the garage’s forecourt while you await professional assistance. AdBlue cannot be drained at the roadside due to its corrosive properties, so your car will have to be towed away to a specialist facility to have the liquid removed from your fuelling system.

If you go and put diesel into the AdBlue tank instead, you might have to have the entire Selective Catalytic Reduction and AdBlue injection system in the car replaced – not cheap, in the slightest. Again, push the unpowered car out of the way and call for help from the relevant bodies.