PRINCIPLES GASOLINE/DIESEL FUEL SYSTEMS - OD1620 - LESSON 1/TASK 2
(3) As the bellows are fully expanded, a pair of contact points are
open, switching off the electromagnet.
(4) The return spring pushes the armature back up contracting the
bellows. This action pushes the fuel out of the pump through the outlet
(5) The contact points are closed as the bellows are fully
contracted. This causes the electromagnet to pull the armature down and
repeat the pumping process.
(6) The pump will stop when the fuel pressure is high enough to hold
the bellows expanded against the return spring. The operating pressure of
the pump is determined by the return spring pressure.
Fuel Tank Ventilation Systems
a. Purpose. The fuel tank needs a ventilation system to keep the
pressure within it equal to atmospheric pressure. This is important for the
(1) Air must be allowed to enter the tank as the fuel is pumped out.
Without ventilation of the tank, the pressure in the tank would drop to the
point where the fuel pump would not be able to draw fuel from it. In some
cases, the higher pressure around the outside of the tank could cause it to
(2) Temperature changes cause the fuel in the tank to expand and
contract. Absence of a ventilation system could cause excessive or
insufficient fuel line pressure.
b. Configurations. The most common methods of venting a fuel tank are:
(1) By venting the fuel tank cap to the atmosphere. This method was
the most common on earlier passenger cars and trucks. It still is used on
vehicles not subject to emission control regulations or that are not used
(2) By providing a line to the fuel tank that vents the fuel tank at
a point high enough to prevent water from entering when fording water.