What Happens If You Put Diesel In A Gas Engine?

In theory, it should not be possible to put diesel into a gasoline tank. A diesel fuel pump's nozzle and the filler necks of gasoline tanks are purposely designed to make it infeasible to fit one into the other. However, despite this built-in precaution, there is still potential to put diesel into a gasoline tank. So, what happens when you mistakenly pump the wrong fuel into your vehicle? There is no single answer to this question, but let us explore the possible effects of such a mistake.

Will The Car Still Run If I Put Diesel In it? 

The short answer to this question is that it depends on the amount of diesel in the tank relative to the amount of gasoline. In most cases, the vehicle will typically still run if the diesel volume in the tank is less than about 10% of the total volume of fuel. 

However, the vehicle will not operate normally for various reasons, one of which is that adding diesel to gasoline increases the viscosity of the resulting fuel mixture. Viscosity is a measure of a liquid's resistance to flow. In this instance, the increased viscosity will cause the contaminated fuel to not vaporize completely during fuel injection events. 

As a practical matter, gasoline must be at a specific viscosity to vaporize properly during injection events in order to fully combust. Moreover, due to the specific viscosity range of gasoline, direct injection systems require pressures of only about 1500 PSI to 4500 PSI to vaporize the fuel (and considerably less for port injection engines). In contrast, common-rail diesel injection systems require injection pressures of as much as 29,000 PSI to 32,000 PSI to vaporize the fuel. So in practice, it may be impossible for a gasoline injection system to vaporize the contaminated fuel.

Nonetheless, while a gasoline vehicle might still run on contaminated fuel containing about 10% of diesel by volume, the higher viscosity of the contaminated gasoline and the unique combustion characteristics of the fuel mixture’s diesel component will produce varying effects on different engines. There are many reasons why engines may react differently to diesel-contaminated gasoline, but the principal causes involve the differences between the ways the two fuel types combust.

Gasoline Combustion

In modern gasoline-driven engines, the bulk of fuel injected during an injection event occurs shortly before the intake valve opens to allow air to enter the cylinder. The air and fuel mix during the compression stroke (when the piston moves upwards) assists in heating the air/fuel mixture to make it easier for the mixture to ignite. 

During a critical moment of the compression stroke, the ignition system delivers a spark via the spark plug, which serves as a heat source to ignite the air/fuel mixture. However, due to the design of both gasoline engines' combustion chambers and combustion characteristics, the process starts at the tip of the spark plug. It then propagates outward from that point through the air/fuel mixture. 

In a fully functional gasoline engine in closed-loop operation, all of the available air in the cylinder is used to combust the fuel in the air/fuel mixture, which is not the case with diesel engines.

Diesel Combustion

While diesel injection systems also vaporize the fuel during injection events, diesel engines are not throttled like gasoline engines are - meaning that how well (or otherwise) the fuel combusts is not dependent upon the air and fuel being mixed in a fixed ratio. In fact, under some operating conditions, the ratio of diesel fuel to air can be as high a 60 parts of air to one part of fuel; whereas with gasoline engines, the ideal ratio of air to fuel is 14.7 parts of air, to one part of fuel. 

Since diesel fuel has a significantly lower ignition point than gasoline, diesel engines do not need a heat source to ignite the compressed air/fuel mixture. In diesel engines, the pistons’ upward motion on their compression strokes heats the air/fuel mixture adiabatically* to the point where the mix is hot enough to ignite spontaneously. This causes the ignition to occur simultaneously at multiple points throughout the air/fuel mixture. 

*This means that the air/fuel mixture is heated by the pressure the compression stroke generates.

However, because it is difficult to initiate and sustain the combustion process in a cold diesel engine, diesel engines are fitted with electronically controlled heating devices known as "glow plugs" to heat the air/fuel mixture during cold start-ups. 

Some Of The Effects Of Putting Diesel Into A Gas Engine

As referenced above, whether a gasoline vehicle will run with diesel-contaminated fuel depends entirely on the amount of diesel in the gasoline. Assuming that:

a) the fuel pump can pump the contaminated fuel, and 

b) that the fuel injectors can at least partially vaporize the contaminated fuel, the following is what is most likely to happen

The Vehicle May Be Hard To Start

The addition of diesel will increase the fuel's viscosity, and therefore, the "thicker" fuel will likely not vaporize sufficiently to form a homogenous mixture with the intake air. Essentially, either the contaminated fuel will remain concentrated in a single jet, or it will break up into droplets that are too large to sustain even combustion - if it actually ignites in the first place.

The Vehicle Will Smoke Heavily

Diesel fuel requires exceptionally high compression pressures to ignite; meaning that in diesel engines lacking sufficient cylinder compression pressure to ignite or combust the fuel completely, the result is clouds of white or black smoke being exhausted from the tailpipe. 

Since engines cannot generate sufficient compression pressure to ignite diesel, some of it may nevertheless combust due to gasoline combusting. In this situation, neither the gas nor the diesel in the fuel will fully combust, which will generate the thick clouds of smoke.

The Engine Will Lose Power

The power output of an engine is a function of how well the fuel in that engine combusts. Thus, reducing gasoline combustibility by contaminating it with diesel will lead to poor combustion and a loss of power. If the engine is even capable of still running,, you are all but assured to experience serious misfires, poor idling quality, and significantly increased fuel consumption.

Catalytic Converters May Fail

Catalytic converters on gasoline-fueled vehicles are built to cope with limited amounts of unburned hydrocarbons without suffering adverse effects. Therefore, gasoline that is contaminated by even trace amounts of diesel could increase the unburned hydrocarbon count in the exhaust stream to the point where the catalytic converter becomes clogged. 

How soon this happens is dependent upon the concentration of diesel in the gasoline. Still, when the catalytic converter becomes clogged, the exhaust backpressure can rise above the maximum threshold for the vehicle. This condition will not only further reduce engine performance but could also cause it to overheat. The only reliable remedy for this situation is to remove all diesel traces from the fuel system and replace the catalytic converter, which could run you several thousand dollars even if you have an average family sedan. 

Summary

For most gasoline vehicle owners, the question of what happens when you put diesel into the fuel tank will hopefully remain a hypothetical one. However, if it does happen, taking immediate corrective action is critically important to prevent potentially fatal damage from occurring to expensive fuel and exhaust system components. 

The best thing to do is not add gasoline to cancel out the diesel’s effects or attempt to run the tank dry to “flush” the diesel-contaminated fuel from the system. In the first approach, the contaminated gasoline will be distributed throughout the fuel system, which may immobilize the vehicle and damage the fuel pump and fuel injectors while also clogging the fuel filters. In the second case, running the tank dry will almost certainly destroy the fuel pump, which will also immobilize the vehicle.

If you realize that you have mistakenly pumped diesel into the fuel tank, the best course of action is to seek professional assistance to remove all diesel traces from the fuel system. Removing these traces will keep the contaminated fuel from ever entering the rest of the system. It goes without saying that prevention is always optimal, so pay proper attention to what type of fuel you put into your gas tank.