Overfilling a car's petrol tank before putting on the cap can have several potentially harmful consequences. Here are some points to consider:

Fuel Spillage:

Overfilling the tank may lead to fuel spillage, especially if the fuel pump does not automatically shut off when the tank is full.
Spilled fuel can be hazardous as it is flammable, posing a risk of fire or explosion.

Environmental Impact:

Fuel spills contribute to environmental pollution. Gasoline contains harmful chemicals, and even small amounts can contaminate soil and water sources.

Vapor Emissions:

Overfilling can cause the fuel vapour recovery system to be less effective, releasing more harmful vapours into the atmosphere.
Increased vapour emissions can contribute to air pollution and have negative health effects.

Damage to Charcoal Canister:

Modern vehicles have charcoal canisters designed to capture fuel vapours and prevent them from being released into the atmosphere.
Overfilling can saturate and damage the charcoal canister, leading to potential issues with the vehicle's emissions system.

Fuel System Damage:

Overfilling may cause fuel to enter components of the fuel system not designed to handle liquid fuel, potentially damaging the fuel pump, injectors, or other parts.

Check Engine Light Activation:

Overfilling may trigger the vehicle's onboard diagnostics system, causing the check engine light to illuminate.
This could result in unnecessary concern for the driver and may lead to additional diagnostic and repair costs.

Warranty Voidance:

Some vehicle warranties may be voided if damage is caused by overfilling the fuel tank. It's important to adhere to manufacturer guidelines to maintain warranty coverage.
Costly Repairs:

Repairing damage caused by overfilling, such as replacing a damaged charcoal canister or fuel system components, can be expensive.

Inefficient Fuel Use:

Overfilling the tank does not provide any additional mileage benefits. Excess fuel may be wasted or evaporated, resulting in inefficient fuel use.

In conclusion, it is crucial to follow manufacturer guidelines and use caution when refuelling to avoid overfilling the petrol tank. Doing so helps prevent potential safety hazards, environmental damage, and unnecessary repair costs.