​Adding petrol to a diesel car can lead to several problems, but the extent of damage can vary based on factors like the quantity of petrol added and how quickly the error is corrected. Here's what typically occurs when petrol is mistakenly used in a diesel vehicle:

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1. Fuel Filter Blockage: Diesel engines have different fuel systems compared to petrol engines. Petrol can act as a solvent in diesel engines, causing the diesel fuel filter to clog rapidly. This can restrict fuel flow to the engine, resulting in performance issues and potentially causing the engine to stall.

2. Increased Smoke Emissions: Mixing petrol with diesel can cause abnormal combustion processes in the engine, resulting in higher smoke emissions from the exhaust. This may be noticeable while driving and could indicate an issue with the fuel mixture.

3. Engine Performance Problems: As the engine struggles to operate on the incorrect fuel mix, you may experience various performance issues such as rough idling, diminished power, and difficulty starting the engine.

4. Potential Long-Term Damage: While immediate damage from using petrol in a diesel car might not be severe, prolonged operation with the wrong fuel mix could lead to more significant and costly damage to engine components.

To mitigate damage, it's crucial to address the problem promptly by draining the fuel tank, flushing the fuel system, and replacing any damaged parts. If you suspect petrol has been added to a diesel vehicle, it's best to avoid starting the engine and seek professional assistance to rectify the mistake.