What Happens If You Put Diesel In a Gas Car?

My family has both a gasoline car and a diesel van, so sometimes, I accidentally drive up to the wrong pump. Luckily, I’ve always realized my mistake before reaching for the wrong nozzle, but I can see how one could make the mistake.

“Putting diesel in a gas car is a common mistake that most people make at least once in their lifetime,” says Ralph Robert, an automotive expert with Way.com. “If you are driving your car, it’s easier to avoid, but if you drive someone else’s car, it can be very easy to fill in the wrong fuel.”

If you’ve put diesel in a gas-powered car, here’s how to minimize the damage, plus how to avoid making this costly mistake in the first place.

What’s the Difference Between Diesel and Gas?

The difference between gas and diesel comes in the refining process. Both are made from crude oil, but gasoline is more heavily refined than diesel. In your car, that difference means that gasoline engines use spark to ignite their fuel, whereas diesel engines use compression.

“Diesel fuel is more efficient, makes more power and has better MPG,” says Chris “Moose” Pyle, an automotive expert with JustAnswer. “Diesel engines last longer because they run cooler and do not need high RPMs.”

What Happens If You Put Diesel in a Gas Car?

If it’s just a tiny amount, your car will run poorly until the diesel gets diluted from refueling. “But if it is a lot of diesel fuel, then the engine will make low power, make a lot of noises and then die,” says Pyle.

That’s because diesel is heavier than gasoline, so if you start or run your car with diesel in the tank, it will quickly clog up your fuel lines. “On the bright side, this is okay to some extent because you only end up damaging fuel lines and injectors,” says Robert. “It probably won’t make it inside the combustion chamber, which would cause costlier damage.”

But it will still be an expensive repair. To fix the problem, you’ll likely need to replace your fuel lines, plugs and fuel filter, have the injectors cleaned and get the tank drained. You may also need a new catalytic converter.

What To Do If You Put Diesel In a Gas Car

If you realize the mistake while you’re still at the pump and have only put a tiny amount in (less than 5% of your fuel tank capacity), you might still be okay to drive, though the car will perform poorly. “This is because gasoline will easily mix and convert 5% diesel into 100% gasoline,” says Robert.

“Anything more than 5% could cause issues like misfiring and clogged fuel lines,” he warns. If this is the case, make sure to fill up the rest of the tank with unleaded, and top it off regularly to keep diluting the diesel.

However, if you have put more than 5 percent of your tank’s capacity in, and are still at the pump, it’s vital not to start your car. Have it towed to a shop that can drain the fuel and replace it with new gas.

What to Do If You Drove After Putting Diesel In Your Gas Car

If you don’t realize the mistake until you’ve set off down the road, your car will stall within a mile or two once your fuel lines get clogged. When that happens, pull over to the side of the road where you are safe and turn off the ignition. Don’t try to start your car; just call a tow truck.

How To Avoid Putting Diesel In Your Gas Car

Most diesel nozzles are larger than gas ones, so chances are, if you’re having trouble getting a nozzle to fit into your car’s filler hole, you have grabbed the diesel nozzle.

But since that isn’t always the case, pay attention to the label by the handle and follow the hoses to make sure someone has not mixed up the nozzles. Once you choose your fuel type, also make sure the button that’s lit up shows a grade of unleaded gasoline.

Another clue is that diesel nozzles are usually green, though this isn’t universal. For example, at BP gas stations, unleaded nozzles and labels are also often green, too.

You might also notice that the fuel coming out smells differently. Diesel is similar to kerosene, while gasoline smells more like paint thinner, says Robert.

“If you own both gas and diesel vehicles, pay special attention at the pump because putting in the wrong fuel happens too often,” says Pyle. “If you put gas in a diesel that’s a 2008 or newer, you’re looking at a repair in the range of $3,000 to $12,000.”

FAQ

Does adding gasoline to a diesel car cause any problems?

Yes, it can cause severe damage. Gasoline and diesel are not interchangeable for many reasons, but especially because the high-pressure injection systems in diesel engines depend on the lubrication in diesel fuel. Gasoline in a diesel engine will also cause premature combustion and knocking.

“It results in more severe damage than the mistake of putting diesel fuel into gasoline-powered cars,” says Alan Gelfand, owner of German Car Depot. “I have witnessed this error result in total destruction. A Mercedes Sprinter customer accidentally poured five gallons of gasoline into his nearly full diesel tank, which led to a $7,500 repair bill that required new high-pressure fuel pump installation along with injector replacement and complete fuel system cleaning.”

How much diesel can a gasoline engine handle before it gets damaged?

Adding diesel fuel in amounts as low as 1 to 2% will contaminate gasoline and cause issues such as misfiring, rough idling and reduced engine power, says Meegan Hall, senior editor of Car Talk.

Once you get to 5%, or one gallon in 20, you’re likely to cause severe damage to fuel injectors, fuel pumps and your engine, says Gelfand. “The worst danger occurs when diesel fuel blocks fuel injectors and catalytic converters,” he says. “The precise injector spray patterns of modern gasoline direct injection systems make them highly susceptible to contamination.”

About the Experts

• Ralph Robert is an expert writer for the Way.com blog and auto app, and an auto enthusiast who has been writing about cars for seven years.
• Chris “Moose” Pyle is a master-certified technician with 20-plus years of automotive experience. He has also worked as an expert for JustAnswer since 2006.
• Meegan Hall is senior editor of Car Talk, the digital content iteration of the long-running NPR show about all things automotive.
• Alan Gelfand has been the owner of German Car Depot for more than 30 years. His business is an independent automotive service center in Hollywood, Florida, specializing in the service and repair of German vehicles.