The background description provided herein is for the purpose of generally presenting the context of the disclosure.  Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
Referring now to FIG.  1, a functional block diagram of an engine system 100 is presented.  Air is drawn into an engine 102 through an intake manifold 104.  A throttle valve 106 is actuated by an electronic throttle control (ETC) motor 108 to vary the volume of air drawn into the engine 102.  The air mixes with fuel from one or more fuel injectors 110 to form an air-fuel mixture.  The air-fuel mixture is combusted within one or more cylinders 112 of the engine 102.  Resulting exhaust gas is expelled from the cylinders to an exhaust system 113. 
Fuel is supplied to the engine 102 by a fuel system.  For example only, the fuel system may include the fuel injectors 110, a fuel tank 114, a low pressure pump 115, a high pressure pump 116, and a fuel rail 118.  Fuel is stored within the fuel tank 114.  A low pressure pump 115 draws fuel from the fuel tank 114 and provides fuel to the high pressure pump 116.  The high pressure pump 116 provides pressurized fuel to the fuel injectors 110 via the fuel rail 118.  The fuel injectors 110, the high pressure pump 116, and the fuel rail 118 will be collectively referred to as the high pressure side of the fuel system. 
An engine control module (ECM) 120 receives a rail pressure signal from a rail pressure sensor 122.  The rail pressure signal indicates the pressure of the fuel within the fuel rail 118 (i.e., rail pressure).  The ECM 120 controls the amount and the timing of the fuel injected by the fuel injectors 110.  The rail pressure decreases each time fuel is injected by one or more of the fuel injectors 110.  The ECM 120 maintains the rail pressure via the high pressure pump 116.