Fuel Pump
TBI and MPFI fuel pumps have a check valve at the outlet end that consists of a steel ball held against a seat by force applied by a spring. When the fuel pump is operating, fuel pressure overcomes spring pressure and forces the ball off its seat, allowing fuel to flow. When the fuel pump is not operating, spring pressure forces the ball back against the seat preventing fuel backflow through the fuel pump.
When the fuel pump is operating and vacuum is supplied to the fuel pump pressure regulator, MPFI fuel system pressure is maintained at 31 psi (2.18 kg/cm2 ). If vacuum is not supplied to the fuel pump pressure regulator, because of a broken or clogged vacuum hose, fuel pressure will be 8-10 psi (.56-.70 kg/cm2 ) higher. When the fuel pump is not operating, fuel system pressure is maintained by fuel pump outlet check valve and vacuum assisted fuel pressure regulator at 19-39 psi (1.33-2.74 kg/cm2 ). TBI system fuel pressure is maintained at 14-15 psi (.98-1.05 kg/cm2 ) when fuel pump is operating and zero when the fuel pump is not operating.