What Happens if you put Diesel in a Gasoline Car?

Jessica Shea Choksey | Nov 19, 2020

Accidentally putting diesel fuel in a gasoline-powered vehicle is a more common mistake than one might think, especially since many fuel pumps often house the gas nozzle right next to the diesel nozzle. If a driver is not paying attention, they can potentially grab one over the other and attempt to pump the wrong kind of fuel into their tank.

What Happens if you put Diesel in a Gasoline Car?

However, this is not necessarily an easy mistake to make because diesel pumps are generally labeled in a vibrant green to differentiate themselves. Also, a vehicle's gasoline filler neck and the diesel fuel nozzle are purposely designed to be incompatible. Meaning, the diesel dispenser is too large to fit into a gasoline filler neck easily. That being the case, people still somehow manage to pour diesel fuel into their gasoline tank.

When diesel contamination occurs, it can have severe consequences on a conventional gasoline-powered vehicle's health and operation.

Gasoline vs. Diesel

In terms of application, diesel is most often used by heavy-duty trucks, semis, buses, boats, and vehicles which may require a higher torque rating and more low-end pulling power. Gasoline powertrains are most often associated with passenger cars, SUVs, and light-duty trucks.

Although they are both derived from crude oil, gasoline and diesel have different physical properties. Gasoline is much thinner and has a distinct odor. Diesel fuel has a thicker state of fluidity, almost like a lightweight oil. These physical differences come into play when diesel attempts to make its way through a gasoline vehicle's fuel system and engine components.

Diesel is also not as combustible as gasoline. Since each fuel has its own autoignition temperature, diesel and gasoline engines work differently. A gasoline engine utilizes spark plugs to ignite the fuel, while a diesel engine uses the pressure created by compression within the engine to ignite fuel (though a part called a glow plug might help when the engine is cold). In other words, diesel is heated up by being squeezed, while gasoline is lit with fire. Furthermore, gasoline is often mixed with as much as 10-percent ethanol, a highly flammable organic compound used as a biofuel additive. Ethanol makes gasoline even more combustible than it already is.

In short, gasoline and diesel engines are designed to operate using only their specific fuel-type and not the other.

What Happens when you put Diesel in a Gas Vehicle?

Since diesel fuel is thicker and denser than gasoline, the fuel pump will struggle to move the diesel/gasoline mixture through the system. Also, the diesel will not be able to easily pass through the fuel filter. Instead, it will clog up the fuel filter. And whatever amount of diesel that then makes its way to the engine will clog the fuel injectors, making them inoperable. This will result in the engine gumming up and seizing. The gasoline engine may run for a little while after diesel has been put in the tank, but that is only because it is still running on the fuel line's remaining gasoline.

As bad as that situation may be, the opposite problem – pouring gasoline into a diesel tank – would be far worse. Because of gasoline's high combustion tendencies, it would ignite much sooner than diesel fuel would. This early ignition and volatility could cause catastrophic damage to the diesel engine and its components.

What Should You Do if You Put Diesel in Your Car?

When you realize that you have accidentally put diesel fuel into your gas tank, immediate action is necessary. Leaving diesel in a gas tank for any length of time is not advisable. Above all, don't start the vehicle under any circumstance. You will want to have your vehicle towed to a garage for drainage right away. Attempting to operate the car could cause the diesel fuel to enter the fuel line and the engine system, which will make the repair process far more complicated and costly.

If the vehicle has a removable drain on the gas tank, this would be the ideal situation. The mechanic will simply open the drain and empty the entire amount of the gasoline/diesel mixture. The tank will then be filled with gasoline and drained again to remove all remaining diesel. This process may require repetition to clear the tank of all diesel contamination.

If there is no removable drain on the gas tank, the tank will have to be removed from the vehicle and drained. This is called "dropping the tank." The mechanic will then rinse the tank repeatedly with fresh gasoline until all the diesel fuel is washed out.

Draining the tank could cost anywhere from $200-$500 depending on whether the tank needs to be dropped and how much diesel is present. If diesel fuel has entered the fuel line or engine, the repair job can easily climb into the $1,500-$2,000 range.

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