Owners Tips

What Should You Do If You Use the Wrong Type of Fuel?

Some cars have a special diet, a specific fuel they must use. It may need diesel instead of gas, or a specific octane rating to operate properly. Generally speaking, you’ll find a sticker or other indication of what your vehicle needs on the fuel filler cap. If you’re still unsure about what type of fuel your car takes, be sure to read your owner’s manual.

Occasionally, however, things get mixed up and the wrong fuel ends up in your fuel tank. Here are a few potential fuel-up foul-ups and what you should do in such scenarios.

Wrong Octane Level

The most common mistake is using the wrong octane level of gas. Most vehicles with turbocharged or high-compression engines require high-octane gas – 91 octane or higher. It’s commonly called “premium” fuel since it’s more expensive. Unfortunately, not all gas stations sell the good stuff, or sometimes the premium fuel is sold out.

If some lower-octane gas finds its way into your tank, don’t fuss too much as most vehicles can handle it – though you may see significantly reduced performance or hear some odd noises out of your engine. This noise is called “knock,” explains Sean Cooney-Mann, technician and service manager at OK Tire in Etobicoke, Ont. 

Under normal operating conditions, your engine mixes fuel with air and this mixture is ignited by a spark plug, creating a controlled burn. Using lower-octane fuel in the high-pressure environment of a turbocharged engine can cause this air–fuel mixture to “detonate,” where it combusts independently of the flame ignited by the spark plug. As you might have deduced from the name, this spontaneous explosion is not the controlled burn that usually happens, and results in what we hear as a knocking or pinging.

Uncontrolled explosions are, in a word, bad; especially when they’re happening in a hunk of metal hurtling forward at highway speeds – with you along for the ride. Fortunately, most new vehicles can tell when engine knock occurs and adjust the timing of the combustion process to accommodate the lower-octane fuel. This compensation noticeably reduces performance, but perhaps more importantly, also affects fuel efficiency. If you’re looking to save a few bucks at the pumps by going for the cheap stuff – don’t. You’ll end up burning more fuel and risk ruining your engine – the math just doesn’t add up.

If you notice you’ve picked the wrong octane at the gas station, there’s no need to panic; you can drive your vehicle normally – albeit at reduced power – with lower-octane gas until you have to refuel.

What about using a higher-octane gas than required? You won’t do any harm to your engine, but you certainly won’t be doing your wallet any favours. The only difference between high-octane and low-octane fuels is in how resistant they are to detonation. High-octane fuels are intended to prevent engine knock – that’s it. Paying extra for gas won’t translate to extra performance unless your engine is designed to take advantage of it.

Gas in a Diesel Vehicle

You may have noticed that the diesel nozzle at the pump is different from the one for gasoline: It’s often bigger than the gas one, and some stations make the handle a different colour – yellow, green, etc. – to further distinguish the two fuel types. But a gas nozzle can still fit into the opening of a diesel tank, and you may inadvertently put the wrong fuel into your car’s tank.

Filling a diesel vehicle with gasoline can be a destructive mistake. “The burn rate on gasoline is much higher with a much lower vapour point than diesel,” says Cooney-Mann. “[This] can cause excessive damage within the block and/or cylinder head, and the repair will be costly as the entire fuel system must be flushed out from the tank to the filters to the injectors.”

Simply put, if you realize you’ve pumped gas into a diesel tank – stop.

If you haven’t turned the engine on, have your vehicle towed to a mechanic, who can flush the tank and change the filter, ensuring you don’t cause any damage to the other components in the rest of the fuel system. This is a straightforward procedure and relatively inexpensive.

If you have started it, however, you need to stop as soon as possible, and get it towed to your mechanic. Do not attempt to drive it there yourself. Just running the engine for a few minutes can cause thousands of dollars in damage to your vehicle.

Diesel in a Gas Vehicle

As mentioned above, the gas and diesel pumps at stations are usually well differentiated. However, mistakes still happen and diesel can make its way into your gas tank.

Relatively speaking, pumping diesel into a gas vehicle is less catastrophic than the opposite, but that doesn’t mean everything is going to be okay.

“[The fuel] can still be burnt and the engine will run,” says Cooney-Mann regarding diesel fuel in a gas vehicle. “But a lot of unburnt fuel will travel down the exhaust, causing blockage and smoke.”

Furthermore, he explains that “damage can still occur within the engine block and cylinder head,” and advises anyone who has pumped diesel into their gas vehicle to flush the fuel lines and replace the filter. The best plan of action is to stop driving as soon as you can, then get those parts looked at and flush the diesel fuel out.

You’ll quickly notice if your gas car is using the wrong fuel type as the engine will run roughly – misfiring and generally struggling to work properly. If you don’t flush those parts and continue to drive the vehicle this way, it can lead to a variety of serious issues, including a loss of compression, which may require a costly engine rebuild to address.

If the fuel is in the tank, but the engine hasn’t been started, you can get the vehicle towed to a mechanic to get the tank flushed and minimize any potential damage.


Everyone makes mistakes – yes, even the gas station sometimes. While car manufacturers have designed their cars to accommodate some variance in fuel grade, some of these mishaps can be pretty costly.

If you notice a mix-up with gasoline and diesel, remember: Stop the car and get it towed to a mechanic. You’ll be saved most of the headaches and costs associated with flushing out the fuel system and other repairs besides.