Home » GM 6.5 » GM/Detroit 6.5L – Engine Management – FUEL CONTROL – Fuel Flow

GM/Detroit 6.5L – Engine Management – FUEL CONTROL – Fuel Flow

Similar to other fuel delivery systems on diesel engines, fuel delivery on the 6.5L EFI powerplant consists of four stages:
• Pressurization
• Distribution and Metering
• Lubrication
• Timing


Fuel is drawn from the tank by a lift pump (figure 5-12) mounted on the inside of the left frame rail. The lift pump pressurizes
the fuel at a rate of approximately .24 liter (1/2 pint) in 15 seconds (15 GPH). Pressurization rates are as follows:
• On model year 1988-1993 pumps (p/n 25115224), fuel is pressurized to between 40 and 60 kPa
(5.8 to 8.7 psi, tee in running at idle)
• On model year 1994 and later pumps (p/n 25117340), fuel is pressurized to a 3 psi minimum with
the line open (after the filter) to an approved canister and the engine cranking.

From the lift pump, fuel is carried to the transfer pump. The vane-type transfer pump varies the pressure of the fuel, depending on the speed of the engine. At idle, the transfer pump outlet pressure is approximately 20 to 30 psi. At full engine speed, the transfer pump outlet pressure may be over 100 psi, with a maximum of 125 psi.

A regulator valve controls transfer pump outlet pressure. The valve uses a viscosity-compensating orifice. This regulator valve is at the end of the hydraulic circuit, rather than directly at the transfer pump. As with other previous 6.5L diesel regulators, however, it spills fuel back to the transfer pump inlet.


Fuel pressurized at the transfer pump travels to the charging annulus. The annulus has eight slots. From the charging annulus, fuel is distributed to two places (figure 5-13):
• the pump housing
• the pumping plungers of the rotor

Fuel delivery to the plungers is most important for the understanding injection pump operation. Fuel reaches the plungers
from two sources.

The first is through two inlet ports in the rotor. As the rotor spins, these ports align with two of the eight annulus slots. Pressurized fuel passes from the slots through the rotor inlet ports. Passages in the rotor carry fuel from the ports to the plungers.

The second source of fuel distribution to the plungers occurs at the front of the rotor, in the fill/spill chamber. Fuel enters through the open control valve. The valve’s movement is controlled by the fuel solenoid. When the solenoid is off, the spool-type control valve unseats to allow fuel to pass through.

Fuel in the rotor is metered for delivery to the injection nozzles.


Metering is a four-stage process consisting of:
1. Fill
2. End of fill
3. Pumping
4. Spill

These stages are the result of changes in cam ring, plunger, and control valve positions (figure 5-14). The plungers are housed in the rotor. A cam ring surrounds the plunger area of the rotor. The inner surface of the cam has eight lobes (high points) and valleys (low points).

The plungers don’t directly contact the cam. Instead, there are four shoe/roller assemblies that ride on the cam’s inner surface. The flat shoe surfaces contact the plungers. As the rotor spins, the shoe/roller assemblies rise and fall on the cam surface. When they do, the plungers move inward and outward. The in-and-out movement of the plungers pushes fuel to the injectors.

Metering occurs eight times in one revolution of the rotor. Each pair of inlet ports goes through the complete metering process once per revolution. The pumping chamber is always completely filled with fuel. Injection quantity is controlled by fuel spill at the end of injection.

Cam ring, plunger, and control valve activity at each metering stage are described on the following pages.


During fill (figure 5-15):
• the two rotor inlet ports align with the charging annulus slots. This allows transfer pump pressure into the rotor passages.
• the fuel solenoid is off, releasing the control valve. Fuel passes around the valve head.
• the shoe/roller assemblies are entering the valleys of the cam, causing the plungers to move outward.

End of Fill

During end of fill (figure 5-16):
• the rotor inlet ports are out of alignment with the annulus slots. Fuel is prevented from entering
the rotor.
• the fuel solenoid energizes to close the control valve. Fuel cannot enter around the control valve.
• the shoe/roller assemblies are in the valleys of the cam. As a result, the plungers are in the
maximum outward position.

At the end of this stage, pressurized fuel is trapped in the rotor, waiting to be sent to the injection nozzles.


During pumping (figure 5-17):
• the shoe/roller assemblies begin up the next cam lobe ramp. This pushes the plungers inward,
creating high-injection pressure that flows to the discharge ports.
• the solenoid remains energized to prevent fuel from escaping at the control valve opening


Spill (figure 5-18) signals the end of injection:
• the fuel solenoid, as commanded by the PCM, de-energizes. This opens the control valve. Fuel
spills out the control valve opening and returns to the fill/spill chamber.
• fuel from the injection lines spills back into the rotor through the discharge ports. Near the end of
the spill cycle, the rotor spins to close the discharge ports.
• the shoe/roller assemblies are at the top of the cam lobe, putting the plungers in the maximum
inward position


Timing on the electronic pump is managed by the injection timing stepper motor. It uses a sliding piston that connects to the cam ring with a pin. As the piston slides in its housing bore, it causes the pin to rotate the cam ring to change injection timing.

There is an outlet passage from the charging annulus to the advance piston (figure 5-19). The pressurized fuel in this passage is used to move the advance piston. There are also drain passages in the piston housing.

Stepper Motor/Advance Piston Components

The mechanical advance/retard components of the stepper motor and piston assembly include (figure 5-20):
• Lever Spring
• Pivot Shaft
• Control Lever
• Servo Valve
• Cam Pin
• Advance Piston
• Servo Valve Return Spring

Advance Piston Operation

Advance and retard are accomplished in the following ways (figure 5-21).
• In advance mode, the stepper motor arm retracts in steps. This causes the pivot shaft to rotate.
As it does, it swings the paddle-like control lever away from the servo valve. Spring pressure pushes the valve off the advance passage. Pressurized fuel enters the advance passage and pushes the advance piston in the advance direction.
• In retard mode, the stepper motor arm extends downward. This rotates the pivot shaft in the other
direction, causing the control lever to press on the servo valve to overcome spring pressure. The valve opens the drain passages, but blocks the advance passage. The lack of pressure at the piston causes it to move in the retard position direction.


The outlet of the transfer pump connects to a threaded restrictor known as the vent wire assembly (figure 5-22). The assembly includes a wire with hooked ends. The vent wire assembly causes fuel under transfer pump pressure to undergo a volume decrease. It also vents the pump of air.

Fuel passing through the vent wire assembly flows inside the pump housing to cool and lubricate most of the injection pump internal components, similar to non-EFI pumps.



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