why my petrol tank in not showing full although I think pumped enough 7.5 l in it

 Your fuel tank may not show full after adding fuel due to a malfunctioning fuel sending unit (the most common issue), a stuck float, a bad electrical connection, a faulty fuel gauge, or a calibration delay in the car's computer system. To rule out a simple delay, try turning the ignition on and off a few times, ensuring the car is on a level surface, which can allow the computer to process the fuel level. If the issue persists, it's likely a component in the fuel sending system or the gauge itself is at fault.  

Common Reasons for an Inaccurate Reading

Faulty Fuel Sending Unit:

This unit, often connected to the fuel pump, has a float that moves with the fuel level. Over time, the float can become stuck, detach from its rod, or the variable resistor inside can wear out or become contaminated with rust or dirt, causing incorrect readings. 

Stuck or Damaged Float:

The float's job is to rise and fall with the fuel level. If it gets stuck in a certain position due to contamination or damage, it will provide a fixed, inaccurate reading to the fuel gauge. 

Wiring Issues:

A bad ground wire or a shorted signal in the wiring between the fuel sending unit and the fuel gauge can also disrupt the electrical connection and lead to inaccurate readings. 

Faulty Fuel Gauge:

While less common, the fuel gauge itself could be defective and not accurately displaying the signal it receives from the sending unit. 

Anti-Slosh Technology:

Some modern vehicles have a computer system that averages readings over a few minutes to prevent the fuel gauge needle from constantly fluctuating. If you add only a small amount of fuel, the computer may not immediately recognize the change. 

What You Can Do

1. Turn the Ignition On and Off:

Turn your car's ignition on and off a few times, ensuring the vehicle is parked on a level surface. This can sometimes reset the anti-slosh system, allowing the gauge to show the correct level. 

2. Check for a Simple Fix:

If the issue just started, look for any obvious loose wires at the sending unit location if there is an access panel on top of the fuel tank. 

3. Consult a Mechanic:

If the above steps don't resolve the issue, it's best to have a qualified mechanic diagnose the problem. They can inspect the fuel sending unit for wear or damage, check the wiring, and test the fuel gauge to pinpoint the cause.