What Happens If You Put Gasoline in a Diesel Engine?

As gasoline and diesel fuel are both petroleum-based fuels, we will first start by asking ourselves “what’s the big difference?” It all boils down to the chemical makeup and characteristics of each fuel type that makes them appropriate to their respective engine choice and harmful to the opposite engine types.

Well, to really understand the potential threats these fuels can be, we need to take a look at the fuel's role in each engine type and how the fuel is used in each engine based on their properties. The rudimentary understanding of the 4-stroke engine cycle is generally associated with gasoline-powered engines, which makes it a good starting point for this conversation.

Gasoline is used in a combustion cycle where the fuel is mixed with the air prior to the compression stroke and is ignited with the use of a spark plug. This order of operations is critical to a proper fuel burn. Diesel, on the other hand, does not introduce the fuel until the air is already compressed. These engines do not use sparkplugs but instead, rely on the heat of the compressed air to ignite the fuel.

These cycle properties are used on account of the fuels’ burn properties. Gasoline ignites easily and burns very quickly. Diesel, by contrast, burns slowly. Because of this, an engine designed to run on either fuel type will not run properly on the opposite fuel type if at all.

A major difference in the fuel types is that diesel fuel is an oil and gasoline is a solvent. Gasoline is volatile and will wash away any traces of oil where it is present. That said, the oil qualities of diesel fuel are critical to the operation of many of the fuel system’s components.

Running gasoline in a diesel engine is not necessarily dangerous to the system due to how the fuel types burn differently. It’s mostly because diesel is used as a lubricant in the fuel system as much as it is as fuel. Because of this, parts in the system face imminent danger in the case that they aren’t properly lubricated.

Fuel pump: All of the danger starts with the fuel pump. This is because the pump's operation uses metal on metal contact that is otherwise lubricated when diesel fuel is present. If it is not, metal shavings can break off and be sent through the fuel system. Wherever specific tolerances are needed for operation, any sort of wear can be detrimental to the function. In many cases, the pump may not need to be replaced, but it will need to be torn down and inspected before this call is made.

Fuel Lines: The fuel lines may not be damaged by gasoline being introduced to the system, but fragments of metal sent through the pump should be cleaned out if you’re going to reuse the lines. Brake cleaner and compressed air are typically used to clean out the lines.

Fuel Rail: If metal shavings from the pump make their way to the fuel rail, the rail should be replaced. This is because metal shavings may not be visible which makes them a lingering threat if they have not been dealt with.

Injectors: The last part of the fuel system to look at when it comes to running gasoline in a diesel engine is the injectors. The lack of lubrication and any metal contaminants to the system are just as much of a threat here as they are anywhere else in the engine. The injectors will likely need to be replaced on account of they may see severe wear and tear.

Rolling smoke may be considered cool by many, but if that black smoke is being produced by gasoline it can be harmful to the engine. Incomplete combustion will be addressed by the engine's sensors. In an attempt to adjust, the engine will tamper with the air-fuel mixture and the engine will not produce much power at all. This burn will also produce a ton of soot that will likely block up the sensors further preventing them from performing their task.

So what if we put the shoes on the opposite feet and run diesel fuel in a gasoline engine? This is not as harmful as running gasoline in a diesel engine, but it is far from harmless. Diesel fuel is less viscous than gasoline and traces of the fuel in the system will affect delivery to the engine. If a mixture of the fuel types is present, the engine will smoke a lot and run very poorly. If the system were completely full of diesel fuel, the engine would simply not run. In any case, the entire fuel system should be drained and cleaned. The only part to consider replacing would be the fuel filter.

There are many reasons not to run any engine on mixed fuel types. Along with the dangers of damaging your fuel system, your gaskets, sensors, engine performance are all at risk. Beyond simply running the right type of fuel, octane ratings are also critical for engine performance. It seems like basic knowledge, but the fact of the matter is manufacturers don’t do a good job at stressing the fuel types that are safe to run in any vehicle, so it is on us, the vehicle owners, to read into and understand fuel types to make sure we are using the best fuel possible for our vehicles.

In the case that gasoline has been introduced to a diesel fuel system, you will need to perform repairs, even if the engine was not run with the fuel. At the very least, you will need to tear apart your entire fuel system and clean it thoroughly. This will be the case even if you left the engine dead and towed the vehicle home directly from the pump.

If you did run fuel through the system, even if briefly, you can expect to replace parts. The tank will need to be cleaned, along with the fuel lines. Parts that should be replaced are the pump, fuel rail, injectors, and filters. Sensors may also be replaced but will likely just need to be cleaned.