Priming Pump

Some diesel engines in light vehicle applications have a priming lever, plunger, or diaphragm button on the lift pump. Separating the priming pump for the removal of air from the fuel system is called bleeding, or priming. Air can enter the system during filter replacement or when a fuel line is disconnected. Without a priming pump, the engine would require excessive cranking, which could damage the starter motor and/or discharge the battery.

The diaphragm lift pump often has a lever that acts on the diaphragm rocker arm. Moving the priming lever moves the diaphragm down. Releasing the lever allows the diaphragm return spring to force the diaphragm up. The action of the diaphragm and valves during bleeding is the same for normal operation of the pump.

Since distributor-type injection pump systems use an internal vane pump, their fuel supply system incorporates a diaphragm-type priming pump, usually located on top of the fuel filter. The diaphragm is connected to an actuating button Figure 54-17 and is held in its uppermost position by a diaphragm spring. Reed valves connect the priming pump housing to the filter. Pressing down on the actuating button reduces the volume in the pumping chamber, which forces fuel into the filter element. Releasing the button lets the spring lift the diaphragm, which increases the volume in the pumping chamber. Air pressure in the fuel tank then forces fuel into the pumping chamber.

FIGURE 54-17
Diaphragm-type priming pump installed on a filter housing.

A plunger-type priming pump is often used with plunger lift pumps, which are mounted on in-line fuel injection pumps. It can also be mounted on top of fuel filter housings for distributor-type injector pump systems. The plunger priming pump consists of a plunger and a barrel assembly, mounted on the side of the plunger pump or filter housing. This plunger priming pump uses the valves of the plunger lift pump to direct fuel flow.

Plunger pumps mounted on the top of filter housings contain inlet and outlet check valves. Pulling the primer plunger increases chamber volume, which decreases the pressure in the chamber below atmospheric pressure. Fuel in the tank is forced along the fuel lines, through the inlet check valve, and into the pumping chamber. Pushing the plunger into the barrel decreases the volume in the chamber, which forces fuel out through the outlet check valve.

Technician Tip
When the plunger-type priming pump is not in use, the hand knob must be screwed closed. This stops fuel from leaking out past the plunger. It also prevents air from leaking into the priming plunger barrel.