Switching Fuel Pump through MOSFETs instead of Relays
The on-resistance of the Vishay IRF9540 is 0.2 ohms: -

 

This means that when 20 A are being passed the power loss is \$20^2\$ x 0.2 = 80 watts. Choose a MOSFET with a much much lower on resistance.

Apart from that it's only rated at 19A (the Vishay part) so you are definitely asking for trouble.