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

By: Ryan Suckling

There was a time when diesel fuel pumps were over to one side of fuel retailer forecourts so trucks could easily pull up and refuel.

With the number of diesel cars now on our roads, diesel pumps sit alongside the fuel dispensers for regular and premium unleaded petrols.

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This response to the greater demand for diesel has increased the chances of motorists putting the wrong fuel in their vehicles.

While both diesel and petrol pumps are clearly labelled, motorists taking only a passing glance at the line-up of dispensers can still pick up the wrong one due to differences in the colours associated with each fuel type across different fuel companies.

For example, at BP, 91 RON fuel is green, while BP’s diesel is green and black and its 98 RON petrol is dark blue and black. Caltex/Ampol market their diesel as Vortex, but also market their 98 RON unleaded petrol as Vortex. It’s a similar situation with BP and its Ultimate fuels, and Shell/Coles Express and their V-Power petrol and diesel fuels.

Just how much damage a fuel mix up can cause depends on the fuels and vehicles involved, petrol into a diesel vehicle being the mistake that could potentially do the most damage.

The best-case scenario is to realise your mistake before the fuel is pumped from the tank to the engine. Worst case is replacing the entire fuel system.

Of course, the best policy is to prevent it from happening altogether. If you’re confused when trying to identify the right pump to use, ask the station attendant for clarification. It’s a small extra step compared to the damage that might be done if you make the wrong selection.

What happens if you put petrol into a diesel car?

Putting petrol into a diesel vehicle is, unfortunately, an easier mistake to make as the petrol nozzle fits into the filler neck of most modern diesel cars.

In diesel engines, diesel fuel is also a lubricant for the high-pressure fuel pump. If petrol, which is a solvent, enters the fuel system of a diesel engine, it significantly reduces the lubrication abilities of the diesel fuel and can damage the diesel fuel pump and other parts of the diesel’s fuel system.

The higher compression ratios in the combustion chambers of diesel engines also means that pre-ignition may occur, which can reduce engine power and cause damage.

Some old diesel vehicles and heavy-duty farm machinery are said to tolerate a small amount of petrol, but modern systems are far less tolerant. Ideally, you wouldn’t want a drop of petrol to trickle in.

If you realise your mistake before you start the engine, ask someone to help you push the car into a parking bay and call RAC Roadside Assistance to organise a tow to a service centre where the fuel system can be flushed. RAC Auto Services can perform this service.

It’s important that you don’t turn the ignition on as this can cause the petrol to start affecting the tank.

Don’t attempt to siphon the fuel out of the car yourself. Draining the fuel out of the car is best undertaken by a specialist to prevent harm to you and the environment.

If you do inadvertently start the ignition and drive the vehicle with petrol in it, depending on how much petrol is in it, the vehicle may begin to lose power and/or make a knocking sound.

The extent of damage and cost of repairs can vary considerably. It’s possible your vehicle may only need a fuel flush-out, but it’s also possible the damage can go as far as requiring a new fuel pump, new filters, injectors, and possibly even a new engine.

What happens if you put diesel into a petrol car?

To put diesel in a petrol car is far harder to do. The diesel nozzles at the fuel bowser are much larger than petrol nozzles so won’t easily fit into the filler neck. But that hasn’t stopped some from making the error.

Despite being far from ideal, it’s less risky and usually less costly, than putting petrol into a diesel tank.

If you start the engine, you might find it won’t run as smoothly and, more noticeably, there could be a significant amount of white smoke coming from the exhaust – especially if you add more than half a tank’s worth of diesel.

The real dangers to your car will set in the longer the diesel is left to run in the tank. Yet for the most part, you’ll only need to get the fuel tank drained, and perhaps the fuel filters replaced, before you can then refuel. If you’ve only pumped in a small amount of diesel, topping up the tank with petrol may be enough to avoid any performance issues.

As with putting petrol into a diesel tank, if you’ve put a large amount of diesel into the tank of your petrol vehicle, as soon as you’re aware of the mistake, pull up to a safe spot and contact RAC Roadside Assistance to help you.

Putting the wrong unleaded petrol in your car

If you’re driving a diesel vehicle, you’ve usually only got one choice at the pump. For petrol in Western Australia the three grades of petrol are primarily regular unleaded (91 RON), mid-grade premium (95 RON) and high-grade premium (98 RON). Ethanol blends are less common in WA. The numbers in these fuel grades refer to their octane rating, or Research Octane Number (RON). RON is a measure of the fuel’s resistance to ignition when under pressure. The benefits of fuels with higher RONs can be leveraged by engines with higher compression ratios, because higher octane fuels can be compressed more before they ignite, meaning more power or efficiency can be gained from the engine.

Both 95 RON and 98 RON have a higher resistance to ignition under pressure, which can marginally improve engine performance and fuel economy – but only if the engine is designed to run on these fuels.

So, while putting a higher-grade petrol in your regular unleaded tank doesn’t pose any danger to your engine it will take a needless toll on your wallet.

But if your car is designed to take premium petrol, and it is fed a lower grade petrol than recommended, that’s when issues can arise.

For example, putting regular unleaded 91 RON in a car designed for the higher-octane 95 RON or 98 RON could result in knocking in the engine. You may hear unusual sounds or experience a less smooth drive, and in the long run it could diminish fuel economy.

It all comes down to staying alert at the pump and knowing what’s right for your tank.

Last updated December 2021