All diesel engines in light vehicle applications have a priming lever on the lift pump, or a separate priming pump to allow for removing air from the fuel system. This is called bleeding, or priming.
Air can enter the system during filter replacement, or when a fuel line is disconnected.
Without a priming facility, the start motor would have to crank the engine over, to bleed and prime the system. Excessive use of the starter motor for this purpose would damage it, and it would soon discharge the battery.
This diaphragm lift pump 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 pumps use an internal vane pump, their fuel supply system incorporates a diaphragm type priming pump, usually located on top of the fuel filter.
This filter housing includes a diaphragm-type priming pump. The diaphragm is connected to an actuating button, and it’s 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.
A plunger-type priming pump is often used with plunger lift pumps that are mounted on in-line injector pumps.
It can also be mounted on top of fuel filter housings for distributor-type injector pump systems.
The plunger-type priming pump consists of a plunger, and a barrel assembly, mounted on the side of the plunger pump, or on a 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 that of atmospheric. 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.
When the priming pump is not in use, the hand knob must be screwed closed. This stops fuel leaking out past the plunger, and also to stop air leaking into the priming plunger barrel.
