Fuel Quantity Control Valve (FQCV)

General description
The quantity control valve (also referred to as an inlet metering valve or flow regulator) is designed to control the quantity of diesel fuel flowing from the low pressure or lift pump to the pistons of the high pressure pump. Greater amounts of fuel allowed to the pistons of the high pressure pump result in a higher pressure generated and thus higher pressure in the common rail. Smaller amounts of fuel allowed to the pistons of the high pressure pump result in lower pressure generated and thus create lower pressure in the common rail. Controlling the quantity of fuel supplied to the high-pressure pump pistons results in lower fuel temperature (often eliminating the need for a fuel cooler) and reduces the load on the high-pressure pump. Excess fuel returns to the fuel tank. 
Appearance
Fig. 1 shows how the fuel quantity control valve looks like.

Fig. 1

Principle of operation of the Fuel Quantity Control Valve

The high-pressure pump is driven at one half-engine speed and coupled to the engine through a belt or gear drive mechanism.
Pressurizing fuel and returning a large portion of it back to the tank impacts negatively on engine power output and fuel economy. To minimize parasitic loss, that is power lost to operate components, newer pumps have inlet metering capabilities to increase engine power output and fuel economy. For example, Bosch provides the ability of one of the pumping elements to electrically switch off. A solenoid attached to a pin on one inlet valve can hold the inlet check valve open resulting in no compression of fuel during pump operation. The Delphi Multec CR system uses an inlet metering valve on the pump that provides a closer match to the amount of fuel being pressurized in the common rail to the quantity required by the injectors. Closed loop feedback from the rail pressure sensor controls an electro hydraulic pressure control valve located on the pumps inlet valve. Less energy is expended when lower injection pressures are required and minimal quantities of fuel pass through the pump leading to further fuel economy gains. Another benefit of reducing the quantity of fuel pressurized by the pump is eliminates unnecessary heating of fuel during pressurization. Delphi reports its system improves fuel economy by as much as 3% and eliminates the need for expensive fuel coolers.

Order to verify functionality of the fuel quantity control valve
• Testing the fuel quantity control valve by oscilloscope

Voltage vs Current

1. Set the first oscilloscope input to 20V
2. Connect the signal test lead (1:1) of the first oscilloscope channel, to one of the valve terminals.
   Then connect the ground lead to the chassis ground.
3. Connect an AC/DC current clamp to another oscilloscope channel.
   Set the AC/DC current clamp range to ±20A.
   Important note: Only one of the two wires should be clamped, and not both of them. It doesn’t matter which wire will be clipped with the current clamp: the positive or the negative one. This will only affect the polarity of the measured current.
4. Start the engine and leave it idling.
5. Watch the oscilloscope screen. Compare result with the waveform in fig.2.

Fig.2

Voltage

1. Set the first oscilloscope input to 20V
2. Connect the signal test lead (1:1) of the first oscilloscope channel, to one of the valve terminals.
   Then connect the ground lead to the chassis ground.
3. Start the engine and leave it idling
4. Watch oscilloscope screen. Compare result with the waveform in fig.3.

Fig. 3

• Possible failures in the fuel quantity control valve

• Mechanical fault
• Broken valve solenoid
• Missing control signal

• Other checks:

• Check for mechanical damaged parts of the engine, ignition system or in the fuel system.
• If a signal error is present, test the wiring from the engine control unit to the quantity control valve.
• Ensure that the engine control unit (ECU) has good power supplies and grounds where required.