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What Happens if You Put Petrol in a Diesel Car


Mistakes happen, and one of the costly errors that can occur is putting petrol (gasoline) into a diesel car or vice versa. It’s a situation that can leave car owners feeling anxious and uncertain about the consequences. In this comprehensive article, we will explore what happens when you put petrol in a diesel car, the potential damage it can cause to your vehicle, and the steps to take to mitigate the impact and prevent future mishaps.

The Difference Between Petrol and Diesel Fuel

Before delving into the consequences of misfueling, it’s essential to understand the key differences between petrol and diesel fuel:

Chemical Composition:

Petrol is a highly flammable liquid fuel primarily composed of hydrocarbons derived from crude oil. It is designed for spark ignition engines, which ignite the fuel-air mixture with a spark plug.

Diesel fuel is a denser liquid derived from crude oil and contains a higher energy density. It is intended for compression-ignition engines, where the air in the cylinder is compressed to the point that it ignites the diesel fuel without the need for a spark plug.

Ignition Method:

Petrol engines rely on spark plugs to ignite the fuel-air mixture. These engines typically have a lower compression ratio.

Diesel engines use the heat generated by compressing air to ignite the diesel fuel. They have a higher compression ratio, which leads to greater fuel efficiency.

Consequences of Putting Petrol in a Diesel Car

Misfueling, or putting petrol into a diesel car, can have several negative consequences, primarily because the two fuels have different properties and ignition methods:

Altered Combustion Process:

Petrol has a lower flashpoint (temperature at which it ignites) compared to diesel fuel. When petrol is introduced into a diesel engine, it can disrupt the normal combustion process.

The spark plugs in a diesel engine designed for compression ignition will ignite the petrol prematurely, leading to knocking and inefficient combustion.

Reduced Lubrication:

Diesel fuel acts as a lubricant for the fuel pump and injectors in a diesel engine. Petrol, on the other hand, lacks these lubricating properties.

The absence of adequate lubrication can lead to increased wear and potential damage to critical components.

Incomplete Combustion:

Petrol may not burn completely in a diesel engine, leading to the formation of carbon deposits in the combustion chamber, exhaust system, and exhaust gas recirculation (EGR) system.

These carbon deposits can hinder engine performance and may require expensive repairs or cleaning.

Loss of Fuel Efficiency:

The altered combustion process and incomplete burning of petrol can significantly reduce the fuel efficiency of a diesel engine.

Drivers may experience decreased mileage and increased fuel consumption.

Potential Engine Damage:

In severe cases, misfueling can result in engine damage due to the strain on critical components, such as the fuel injection system, cylinders, and pistons.

Repairs can be costly, and in extreme cases, engine replacement may be necessary.

Immediate Steps to Take

If you realize that you’ve misfueled your diesel car with petrol, it’s crucial to take immediate action to minimize potential damage:

Do Not Start the Engine: Starting the engine can circulate the mixed fuel throughout the system, worsening the situation. If the engine is already running, turn it off immediately.

Avoid Keyless Entry: Keep your key fob away from the car, as the keyless entry system may inadvertently start the engine.

Call for Assistance: Contact a breakdown service or a professional mechanic to drain the incorrect fuel from the tank.

Do Not Attempt to Drive: Avoid driving the vehicle until it has been properly drained and refueled with the correct fuel.

Draining and Flushing the Fuel System

Once you’ve sought professional assistance, the process of draining and flushing the fuel system will be initiated. This typically involves the following steps:

Draining the Tank: The technician will drain the mixed fuel from the tank, ensuring that as much of the incorrect fuel is removed as possible.

Flushing the Fuel Lines: The fuel lines and system will be flushed with the correct type of fuel (diesel in this case) to remove any remaining traces of petrol.

Replacing the Fuel Filter: In many cases, the fuel filter will need to be replaced, as it may have been contaminated by the mixed fuel.

Inspecting and Cleaning Components: Critical components like injectors and fuel pumps may be inspected, cleaned, or replaced as needed.

Refueling with Diesel: Once the system is thoroughly flushed and cleaned, the vehicle will be refueled with diesel fuel.

Costs and Repairs

The cost of draining and flushing the fuel system, as well as any necessary repairs, can vary widely depending on several factors:

The amount of incorrect fuel added

The extent of damage (if any)

The make and model of the vehicle

The labor costs of the service provider

It’s important to note that comprehensive repairs can be expensive, especially if significant damage has occurred to the engine or fuel system. To avoid such costs, quick action is crucial when misfueling occurs.

Prevention: How to Avoid Misfueling

Preventing misfueling is the most effective approach. Here are some steps to help you avoid putting petrol in a diesel car or vice versa:

Stay Focused: Pay close attention when refueling your vehicle, especially if you drive both petrol and diesel cars. Take a moment to double-check the fuel type.

Use Fuel Labels: Familiarize yourself with the fuel labels at the petrol station. Diesel pumps are usually labeled with a green handle, while petrol pumps have a black handle.

Don’t Rush: Avoid refueling in a hurry or when you’re distracted. Take your time to ensure you’re using the correct fuel.

Use Fuel Caps: Some vehicles have different-sized fuel filler necks for petrol and diesel. If your car has this feature, use it to prevent misfueling.

Educate Others: If multiple people drive the same vehicle, make sure everyone is aware of the correct fuel type.

Consider Fueling Aids: Some misfueling prevention devices are available that can be fitted to your fuel filler neck to prevent the wrong nozzle from fitting.


Misfueling your diesel car with petrol is a situation that requires prompt action to prevent potential damage and costly repairs. Understanding the consequences of this error, as well as the immediate steps to take, is essential for any vehicle owner.

Prevention is always the best approach. Stay attentive, use fuel labels, and take your time when refueling to ensure that you’re using the correct fuel type for your vehicle. By following these precautions, you can avoid the headaches and expenses associated with misfueling and keep your diesel car running smoothly.

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