Failure fuel pump causes difference in amount of delivered fuel.
Demanded amount of fuel to the engine is controlled by the signal from ECU to the injector. If there is problem with the fuel pressure, amount of delivered fuel will be different when using same equipment for same period of time (High pressure - gain amount of fuel, Low pressure - reduce amount of fuel). Fuel pressure adjustment has to be done under the condition below.

Condition@
Standard value
Engine IdleiRegulator not operatedFDisconnect the hosej
Approx. 3.0kg/cm2

Since fuel injection is performed into the intake manifold, if fuel pressure is kept steady, there will be a difference in delivered fuel between the time of high vacuum and low vacuum. Therefore, regulator adjusts the fuel pressure according to the vacuum of intake manifold (Fuel pressure is kept approx. 3.0kg/†C higher than manifold vacuum). If the fuel pressure is once adjusted, regulator will keep it in optimized value.