Accidentally Filling Your Petrol Car With Diesel - Fuel Disaster or Easy Fix?

What happens if you accidentally fill your petrol car with diesel? Is there a save or are you a goner?

We’ve all been at the filling station, perhaps one that we’re not used to, and accidentally reached for the wrong nozzle. We’re not talking about grabbing the one for RON95 with RON97 by mistake, but the 'other' one. 

‘Misfueling’ is one of the most embarrassing mistakes in your car ownership (or drivership?) life, but such an easy one to make. In fact, you might be surprised to find out how often it happens. Some mixing is fine, but not this kind.

While the rate at which this occurs in Malaysia is hard to pinpoint, statistics from the United Kingdom show that an average of 150,000 British motorists fall victim to this error each year. However, unlike us, diesel-powered cars are much more widespread there and are a popular choice for buyers of hatchbacks to sedans to crossovers.

We wonder if this will happen to cars once synthetic fuels reach the market....

Getting the pump nozzles wrong is therefore more likely, which is made worse by the fact that there are more filling station operators in the UK than in Malaysia. On the bright side for most of us, just know that pumping diesel into a petrol vehicle is much less mechanically catastrophic than the opposite: pumping petrol into a diesel vehicle.

Firstly, because diesel engines rely on the fuel itself as lubricating agent, and mixing in petrol would nullify this characteristic, it causes much more wear on the engine internals. Secondly, because petrol is much more combustible, it would ignite much sooner than diesel would. This premature ignition and added volatility could cause irreparable damage to the diesel engine and its components.

Why It Happens?

First, a little background on what spurred the writing of this article. This writer was aware that people have been known to mistakenly fill their petrol-powered cars with diesel and vice versa, but thought that it was a problem of the past. Savvy motorists in 2021 won’t fall for it, right?

That was, until I saw someone in a HR-V do exactly this over the past weekend. My mind doubted it at first, but was sure it was a black nozzle that I saw her return to the pump. Before she could pull away, I managed to alert her and asked that her engine be promptly turned off. We soon got the help of a mechanic next door to push the Honda into his garage to drain the tank.

The lesson here is to always be aware of what you’re doing, both when driving the vehicle and when nearby it. At the moment, Malaysia is going through its longest period of lockdown yet, turning us into homebound zombies, and it’s easy to be oblivious to mundane activities such as topping our cars off with fuel.

It’s when we get lost in our own thoughts that a careless and potentially very expensive (but easily avoidable) mistake like this can happen.

Fail Safe

Both car manufacturers, safety regulators, and fuel station operators have all contributed to measures that can prevent the wrong fuel being pumped into an incompatible engine.

For one, nozzles for diesel fuel are made slightly wider to make it more difficult to insert into the neck of your petrol tank, but that doesn’t seem to stop most people who are willing to simply use more force. Secondly, pump stations will always reserve the far-most (usually left) nozzle for diesel but will also tend to label it black and place a removable flap before it can be picked up.

Petrol and diesel also have distinctive smells which might finally provide a sensory alert to the more experienced driver that has become accustomed it.

What Happens?

It does really depend on how much diesel managed to get into the fuel system - meaning the ratio of petrol to diesel in your tank and beyond.

If you managed to put in only a few litres of diesel into your mostly petrol-filled tank before realising and stopping the pump, it might be as simple as completely draining the tank provided you did not turn your engine on. You should arrange to have this done as quickly as possible, too.

You will likely need a mechanic to do this properly. If there isn’t a servicing staff on hand that can help, you might want to whip your phone out and arrange for the vehicle to be towed to your preferred workshop.

Then again, if your tank was mostly empty and was filled with even a few litres of diesel, it might have already contaminated the fuel lines and injection system. However, this is still rectifiable if the car was left turned off.

If the diesel fuel was mixed and managed to get into the injection system and combustion chamber itself, your engine is still saveable but there’s likely a big invoice coming your way. Diesel will clog fuel injectors and spark plugs, leading to a barrage of warning lights and plenty of weird noises and vibrations before the engine stops running completely.

Worst Case Scenario

Assuming either that the engine was running with the diesel/ petrol mixture or that the driver has complacently even started driving off normally with a full tank of the wrong fuel, eventually there will be judders and shakes before the engine will stop working completely.

This is a dangerous combination as cars with hydraulically assisted steering and brakes rely on the engine to maintain hydraulic pressure, meaning your ability to steer and stop will be severely impaired with a nonfunctional engine.

While older cars might still be able to operate for extended periods on the wrong fuel, modern ones with high pressure fuel injection systems and computerised control modules are not as resilient in the same way.

The absolute worst place for such engine failure to occur is on the overtaking lane of a wide multi-lane highway, but regardless, do your best to get the vehicle safely onto the emergency lane or shoulder as quickly as possible.

To get the car back into working order in this case, the entire fuel system will probably need to either be serviced or replaced outright: fuel lines, fuel pump, injectors, and maybe even an engine overhaul and new catalytic converter if the damage is severe and far-reaching enough.

Unfortunately, because this does not fall under defects related to the manufacturer, your warranty won’t cover the cost of repairs. By the same logic, your insurance policy will likely not provide any relief either.

Tips If You Pumped Diesel Into Your Petrol Car:

• Don’t panic: Causing a commotion or acting rashly will not be beneficial to anyone, especially at a crowded filling station.
   
• Immediately stop fuelling: As soon as your realise this error, stop the pump. As more diesel enters the tank, the worse the outcome could be. You might even be able to drive the car normally if only a tiny amount of diesel entered the tank, though we do not recommend this.
   
• Inform the filling station attendants: If they’re somewhat experienced, they might have already seen this mistake happen a bunch of times and will know how to advise you correctly. Typically, while keeping the engine off and the transmission in neutral, they will help you push it to the side while you call for assistance.
   
• Getting professional help: Some filling stations have a servicing garage attached that can help drain your tank. If not, you will want to call your insurance provider to arrange for the vehicle to be towed to your preferred workshop. In an ideal case, your car will have a ‘droppable’ fuel tank, allowing it to be easily detached and cleaned of the unwanted fuel and any residue.