For maximum efficiency, modern import fuel injection systems require that fuel be delivered at a specified volume and pressure. Because the fuel injector's job is to mix fuel with the intake air charge, it must produce a finely misted, conical spray pattern through the fuel injector nozzle. Lower-than-specified fuel pressures not only fail to provide enough fuel for combustion at larger throttle openings, but may also introduce an uneven air/fuel charge distribution into the combustion chamber itself, which usually results in hard starting, hesitation and reduced power output.

Of course, fuel volume is required to maintain fuel injection pressure at the fuel injector nozzle. While a marginally performing fuel pump might maintain pressure at idle, its effective operating pressure will decrease as engine load increases. Outside factors such as higher ambient air temperatures, ethanol gasoline and inlet filter or screen restrictions may further reduce fuel volume by aggravating the tendency of any fuel pump to cavitate or form bubbles in the pump inlet assembly.

Last, it's important to recognize that excessively high or low fuel pressure limits the PCM's ability to maintain a chemically correct or stoichiometric air/fuel mixture ratio. If low fuel pressure causes a post-1996 model PCM to increase injector pulse width to maintain a chemically stoichiometric air/fuel ratio, a positive short- and long-term fuel trim number ...