Fuel supply line to the pump is too thin. This will cause the fuel pump to draw a very low pressure on the inlet. When the fuel becomes hotter it will percolate due to the low pressure. This gas will then enter the pump making the pressure erratic. The gas is then pressurized at the injector tip and will reduce petrol flow and cause lean mixtures. The gas bubbles can also damage the pump.

Fuel pump heats up. It could be mounted too close to the exhaust or the pump is too large for the fuel rail system. The fuel flowing through it is too little to cool it off. It could be a dirty fuel pressure filter down stream that causes the pump to pump at too high pressures. If the pump is hot it will also cause the fuel to percolate and make gas bubbles. Do not bring the return line to the front of the fuel pump. The fuel must be cooled off in the tank.

Mechanical fuel pressure failure. Inside is diaphragms that could rupture or become soft with heat. Then accuracy may be a factor in cold and hot conditions. The vacuum line to the intake manifold can leak causing pressure differences.

Fuel lines are too thin. If the line to the fuel rail is too thin, the fuel pressure will drop before it reaches the injectors. If the return line is too thin the fuel pressure regulator cannot regulate properly and the pressure will be too high.

Erratic fuel pressure. This is usually caused by batch fueling as all the injectors open and close all at once. This makes for differences in pressure and fuel supply between the cylinders upstream and downstream.

Fuel pump is noisy and makes uneven sounds. These pumps get damaged by dirt, water and air bubbles. It is good practice to put an inline filter in front of the pump to ensure that dirt does not go through the pump. This will also indicate if air is in the system as most filters are see-through. It may pull in air through the connections.