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What Happens if You Put Petrol Instead of Unleaded?

Fueling your vehicle is a routine task that most drivers perform without much thought. However, accidents can happen, and one of the more common and potentially costly mistakes is putting the wrong type of fuel into your car. Specifically, using petrol instead of unleaded fuel can lead to a range of problems, from minor issues to severe engine damage. This article explores the differences between petrol and unleaded fuel, the consequences of using the wrong fuel, and steps to take if you accidentally fill your tank with the incorrect type.


Understanding Fuel Types

To comprehend the impact of using the wrong fuel, it’s essential to understand the different types of fuel available and their intended uses.


Petrol vs. Unleaded Fuel

The terms “petrol” and “unleaded” are often used interchangeably, especially in regions where “petrol” is the common term for gasoline. However, it is crucial to differentiate between various fuel types:


Petrol (Gasoline): In many parts of the world, “petrol” refers to gasoline, which is used in internal combustion engines. Gasoline comes in various octane ratings, such as 87, 89, and 93, indicating its resistance to knocking.

Unleaded Fuel: This term generally refers to gasoline that does not contain lead additives. Most modern vehicles are designed to run on unleaded gasoline, which is better for the environment and less harmful to catalytic converters in cars.

Diesel Fuel

Diesel fuel is distinctly different from petrol/gasoline and unleaded fuel. Diesel engines operate on compression ignition, requiring a different type of fuel. Putting petrol in a diesel engine or vice versa can cause significant damage.

Consequences of Using Petrol Instead of Unleaded Fuel

While the terms “petrol” and “unleaded” typically refer to the same fuel in many contexts, there are instances where using the wrong type of petrol or gasoline with the incorrect octane rating can lead to problems. Here, we explore the potential issues that arise from such mistakes.

Immediate Effects

1. Engine Knocking: If you use a lower-octane petrol than recommended for your vehicle, you may experience engine knocking or pinging. This knocking occurs because the fuel-air mixture in the cylinders detonates prematurely, causing a metallic knocking sound.

2. Poor Performance: The engine may not run as efficiently, leading to reduced power, sluggish acceleration, and overall poor performance.

3. Increased Emissions: Using the wrong fuel can increase emissions, making your vehicle less environmentally friendly and potentially causing it to fail emissions tests.

Long-Term Damage

1. Engine Damage: Prolonged use of incorrect fuel can cause significant damage to the engine components, such as pistons, cylinders, and spark plugs. The knocking caused by the wrong octane rating can lead to wear and tear, eventually resulting in costly repairs.

2. Catalytic Converter Damage: Using petrol with lead additives (if available) or incorrect additives can damage the catalytic converter, a critical component in reducing harmful emissions.

3. Fuel System Issues: The fuel system, including the fuel pump, injectors, and lines, may suffer damage from using incorrect fuel, leading to leaks, clogs, and overall system failure.

Safety Concerns

Using the wrong fuel can also pose safety risks:

1. Fire Hazard: Incorrect fuel can increase the risk of fire, especially if there are leaks or spills during refueling.

2. Stalling: The vehicle may stall unexpectedly, creating dangerous situations, particularly when driving at high speeds or in heavy traffic.

See Also: Can You Make Gasoline from Waste Oil?

Steps to Take If You Put Petrol Instead of Unleaded

Accidents happen, and if you realize you have put the wrong fuel in your vehicle, it’s crucial to take immediate action to minimize damage. Here are the steps to follow:

Do Not Start the Engine

If you realize the mistake before starting the engine, do not start the car. Starting the engine can circulate the incorrect fuel through the system, increasing the risk of damage.

Assess the Situation

Evaluate how much of the incorrect fuel has been added. If only a small amount has been added to a nearly full tank, the risk of damage may be lower. However, if a significant amount of wrong fuel has been added, more extensive measures will be needed.

Call for Professional Help

Contact a professional mechanic or a roadside assistance service. They have the tools and expertise to safely drain the incorrect fuel from the tank and flush the fuel system.

Towing the Vehicle

If you have already started the engine or driven the vehicle, it is best to have it towed to a repair shop. Continuing to drive can cause more severe damage to the engine and fuel system.

Draining and Flushing

At the repair shop, the mechanic will:

1. Drain the Fuel Tank: Remove the incorrect fuel from the tank to prevent further circulation.

2. Flush the Fuel System: Clean the fuel lines, injectors, and other components to remove any residual incorrect fuel.

3. Check for Damage: Inspect the engine and fuel system for any damage caused by the wrong fuel. This may include checking spark plugs, injectors, and the catalytic converter.

Refill with Correct Fuel

After draining and flushing, refill the tank with the correct unleaded fuel recommended for your vehicle.

Preventing Fueling Mistakes

To avoid the costly and inconvenient consequences of using the wrong fuel, consider the following preventive measures:

Double-Check Before Refueling

Always double-check the fuel type before refueling. Pay attention to the labels on the fuel pump and ensure you are selecting the correct fuel grade.

Read the Owner’s Manual

Consult your vehicle’s owner’s manual to understand the recommended fuel type and octane rating. Keep this information handy to avoid confusion.

Use Fuel Caps and Labels

Some vehicles come with fuel caps that clearly state the required fuel type. Additionally, you can place a label near the fuel cap as a reminder.

Educate All Drivers

Ensure that all drivers of your vehicle are aware of the correct fuel type and understand the importance of using it.

Be Cautious with Rental Vehicles

When driving a rental vehicle, pay extra attention to the fuel type requirements. Rental cars may have different fuel requirements than your personal vehicle.


Putting petrol instead of unleaded fuel in your vehicle can lead to a range of immediate and long-term problems, from engine knocking to severe engine damage. Understanding the differences between fuel types and the consequences of using the wrong fuel is essential for maintaining your vehicle’s performance and safety. If you make a mistake, taking prompt action to drain and flush the fuel system can minimize damage. By double-checking before refueling and following preventive measures, you can avoid the costly and inconvenient consequences of fueling mistakes. Remember, proper fuel management is key to ensuring your vehicle runs smoothly and efficiently.

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