Jeep CRD Fuel Pressure Control Items1. Fuel Control Actuator or Fuel Pressure Regulator: (Also called by some as: *Fuel Quality Solenoid) Located on back of CP3 fuel injection pump. Controls the amount of fuel being pumped to the fuel rail, excess is returned back to fuel tank and the flow also helps cool the CP3 internally. 05159962AA 0928400822 (item #4 in diagram)
2. Fuel Pressure Solenoid: Located at the end of the fuel rail. Controls fuel rail pressure by the ECM. 05159964AA (item #13 in diagram)
3. Cascade Overflow Valve: Located on the side of the CP3 injection pump. The cascade overflow valve's (COFV) sole purpose is to prioritize the fuel flow between the pump itself (i.e. feed the FCA), then flow excess fuel to the lubrication flows to cool and lubricate the pump and finally divert any excess flow above that to the return line back to the fuel tank. (item #17 in diagram)
4. Fuel Pressure Sensor: Located on side of fuel rail. Tells ECM the fuel rail pressure and ECM in turn controls the rail pressure using items 1 & 2. 68020556AA; 05159963AA (item #15 in diagram)
*Note: Sometimes you will see these items called by other names, but these are the correct nomenclatures used by Mopar.
Understanding the CP3. The CP3 is a 3-piston metered inlet pump. By metered inlet that means that the Fuel Control Actuator valve (also called Fuel Pressure Regulator or Flow Control Valve) controlled by the ECM meters fuel going into the pumping chambers to control how much fuel the CP3 pumps and thus the engine rail pressure. This is in sharp contrast to other pressure control systems that divert large amounts of PRESSURIZED flow OUT of the pump back to the tank. The FCA meters how much fuel actually gets pressurized in the first place. Very neat and efficient.
In order to meter the fuel into the chambers, the CP3 has an internal metering/lift pump built right into it. This pump takes the fuel at supply pressure and boosts it to pressures between 80 and 180 PSI depending on what is required. This fuel is fed to the FCA via the cascade overflow valve located on the side of the pump. The cascade overflow valve's (COFV) sole purpose is to prioritize the fuel flow between the pump itself (i.e. feed the FCA), then flow excess fuel to the lubrication flows to cool and lubricate the pump and finally divert any excess flow above that to the return line back to the fuel tank.
The COFV prioritizes the flow in that order. If the flow rate is low, only the FCA will get fed. No cooling or return flow will be provided. If the flow rate increases, the pump will get cooled. Any more flow than that and some fuel will get sent out the return line. Note that this prioritization happens AFTER the internal supply pump and there is NO passage from the CP3 inlet to the CP3 return line. The inlet feeds the internal supply pump. Unlike other pumps where the fuel supply pump inlet may feed the internal cooling passages and make its way back to the return line.
Fuel Control Actuator (FCA) also called Fuel Quality Solenoid or Fuel Pressure Regulator
The FCA is located on the back of the front cover of the high-pressure pump. The solenoid is pulse-width modulated valve controlled by the ECM and meters the amount of fuel that flows into the high-pressure elements inside the high-pressure pump.
This flow limiting valve is controlled by the ECM. It uses a variable pulse width 12v signal to charge an electromagnet. There is a small valve that is spring loaded to the open position inside this flow limiting valve. The electromagnet pulls the valve into the closed position to restrict fuel flow. The ECM constantly regulates this valve to achieve its desired fuel rail pressure.
The solenoid is inactive up to 30 seconds after the ignition switch is initially keyed to ON position to allow maximum fuel pressure to the fuel rail during cranking and start up. ECM assumes FCA valve control when CPS signal and rail pressure are within acceptable limits.
The FCA electromechanical device on the CP3 is also used by the ECM to limit fuel volume that the CP3 delivers. Notice that I said volume and not pressure
The ECM determines the fuel pressure set point based on engine sensor and rail-pressure inputs. If the actual fuel-rail pressure is too low, the ECM commands the solenoid to allow more fuel to flow to the high-pressure pump. This minimizes the difference between the actual fuel-rail pressure reading and the set point. The ECM will also operate the solenoid to delay fuel, reducing flow-rate, if the fuel-rail pressure becomes too high.
The FCA valve is commanded open by the ECM to allow the high-pressure pump to build maximum pressure (23,206 psi).
Thus, rail fuel-pressure can be increased or decreased independent of engine speed
BTW: Some shop manuals say the COFV isn't "field serviceable" they have been removed before, disassembled and re installed, or replaced with no problems.
So... the internal supply pump is a constant displacement gear pump. This pump is actually capable of developing enough suction to draw fuel through a filter right from the tank. It actually does this on some installations.
Being a constant displacement pump means that it you can't hook just any supply pump up to it. Whereas some injection pumps can tolerate a large difference in flow rates to/through it with relatively little pressure change, the CP3 cannot. If you try to push too much fuel through the CP3 (supply pressure), the discharge pressure will skyrocket to unsafe levels and the rail pressure will increase accordingly way beyond design pressure. There are no internal pressure relief valves in the CP3, only a direct feed to the internal supply pump. Conversely, if there isn't enough flow to it, the pressure will drop and the internal supply pump will provide vacuum in an attempt to supply itself enough volume of fuel.
Supplying the CP3 Users should never pump fuel to a CP3 at a high pressure. Because it has a built-in supply pump, the CP3 only needs to be supplied at a low pressure OR VACUUM and it will handle supplying the pumping chambers from there. Whereas previous pumps had to be somewhat pressurized, the CP3 handles its own pressurization.
Conversely, users do have to worry about getting enough fuel to the CP3. Remember the COFV? It prioritizes the flow and when there isn't enough fuel to the CP3, it shuts off internal lubrication and cooling flow. This is very bad news for the longevity of the CP3. Diesel fuel viscosity and lubricity falls dramatically with an increase in temperature. Hot fuel within the CP3 is bad.
Basically, one needs to get enough fuel from the tank to the CP3 inlet to keep the CP3 well fed, yet but never over pressure it when doing so.
