What Happens if You Put Gas in a Diesel?

Stephen Fogel
June 23, 2019

gas in diesel

The two most common vehicle fuels are gasoline and diesel. While these two fuels are very different from each other, they are usually sold together at most fuel stations. In fact, gasoline and diesel are often dispensed from the very same pump.

To make things more complicated, many family and business fleets are made up of both gasoline and diesel vehicles. In these circumstances, it can be easy to forget which type of vehicle you are driving.

Every so often, a driver pulls in for fuel, gets distracted, and ends up putting the wrong kind of fuel into their vehicle. Gasoline could be pumped into a diesel-fueled vehicle, or diesel might be put into a gasoline-powered car. Either way, you now have a serious problem. These fuels are not interchangeable!

The differences between gasoline and diesel

While gasoline and diesel fuels are both refined from crude oil, there are many differences between them:

  • Diesel has a thicker consistency than gasoline
  • Diesel is heavier; it weighs one pound more per gallon than gasoline
  • Diesel does not evaporate, gasoline does
  • Diesel has lubricating qualities, gasoline does not
  • Diesel will self-ignite at a lower temperature than gasoline
  • Diesel engines do not need spark plugs to ignite their fuel, gasoline engines do
  • Diesel engines have much higher compression ratios than gasoline engines

These major differences in how gasoline and diesel act within an engine are the very things that can cause serious problems when the wrong fuel ends up your vehicle’s engine.

What happens if you put gas in a diesel car?

If you accidentally dispense gasoline into your diesel vehicle’s tank, and then drive it, you could experience a variety of engine problems. These include:

  • Premature ignition of the fuel in the cylinders, causing internal engine damage
  • Contamination of the fuel resulting in reduced lubrication, which can damage your injectors and fuel pump
  • Incomplete combustion, producing black smoke, reduced power output, and damage to engine sensors

If you messed up, act fast

If you realize that you have misfueled your car while it is still parked at the pump, that’s great. Do not drive it even a short distance, in fact do not even put the key in the ignition. Call your mechanic immediately, and then summon a tow truck to deliver your vehicle right to the mechanic’s shop. Let the fuel station attendants know about your situation, and why you can’t move your car away from the pumps until your tow arrives.

If the wrong fuel has remained in your tank, and it has not been able to get into your engine, your mechanic should be able to simply empty and clean your fuel tank, and then refuel it with some diesel.

Some of today’s diesel vehicles have added what they call “misfueling protection.” Several European luxury brands come with devices that will prevent the narrower unleaded fuel nozzles from being inserted into their diesel vehicles’ tanks.

If you leave the fuel station and drive with gasoline in your diesel car’s tank, you won’t get very far. Once that the gasoline replaces the diesel in your fuel lines, it will disrupt your engine’s operation and start causing damage. The sophisticated emissions control equipment in modern diesel vehicles can be severely affected, along with the major moving parts of the engine. Catastrophic engine damage can result, which will be very expensive to repair. And no, this will not be covered by your warranty.

What happens if you put diesel in a gas car?

Putting diesel fuel into a gasoline car creates a whole different set of issues, compared to putting gas in your diesel. The results are not pleasant:

  • The lower octane rating of the diesel will result in premature ignition and heavy knocking, which can cause engine damage
  • Partially-burned liquid diesel fuel can accumulate in your cylinders, causing hydro-lock and heavy carbon deposits on your valves, spark plugs, and pistons
  • The thick, oily diesel can ruin your fuel lines, fuel pump, fuel filter, and fuel injectors
  • Unburned diesel fuel can ignite inside your very hot catalytic converter, clogging its passages and requiring an expensive replacement converter
  • The remaining unburned diesel will come out of your exhaust pipe, going directly into the atmosphere, where it will react with sunlight and create smog
  • The engine will stop running

Catch it quickly to avoid disaster

Just as with the gas in a diesel situation mentioned above, you can minimize the damages if you do not operate your vehicle with diesel in the gas tank. Keep the key out of the ignition and have your car towed directly to your mechanic’s shop, where the tank can be drained of the incorrect fuel before it gets into your engine and wrecks it.

Putting diesel into a gas car is tougher

Thanks to changes made to gas nozzles and fuel tanks back when unleaded gasoline was first introduced, it is not easy to accidentally put diesel into most gas cars.

During the transition from leaded to unleaded gasoline that started in the 1970s, cars that required unleaded gas received narrower openings in their gas tanks. These tanks would only accept the narrower nozzles fitted to the unleaded fuel pumps. The older, wider leaded fuel nozzles would not fit in the narrower openings, preventing leaded fuel from being used (because lead would destroy the newly mandated catalytic converters).

Diesel fuel nozzles are the same size as those old leaded gasoline nozzles. As a result, they will not fit into an unleaded gas tank. This is your first clue that you are using the wrong fuel — stop before you squeeze the handle!

Stephen Fogel

About the Author

Stephen has been an automotive enthusiast since childhood, owning some of his vehicles for as long as 40 years, and has raced open-wheel formula cars. He follows and writes about the global automotive industry, with an eye on the latest vehicle technologies.

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