Fuel Pump - GMC 4000 Series Service Manual

Sec. 6M
Page 320
FUEL SYSTEM— GASOLINE ENGINES
Fuel pumps (fig. 7) used on vehicles covered
by this manual cannot be overhauled. In the event
of failure the entire pump must be replaced.
Pumps are diaphragm type, operated from an
eccentric on the engine camshaft. The diaphragm
is made from a single layer of fabric treated to
make it impervious to gasoline and engine oil.
FUEL PUMP OPERATION
In operation, the diaphragm is pulled against
pressure of spring as rocker arm is moved by
engine camshaft. The partial vacuum caused in
pump chamber draws fuel from tank into pump
chamber through inlet valve. As camshaft rotates,
pressure on rocker arm is relieved and diaphragm
spring acts on diaphragm which in turn forces fuel
out of pump chamber through outlet valve and
through fuel line to carburetor. Each revolution of
camshaft repeats this cycle, drawing fuel from
tank and discharging it through line to carburetor.
When float rises in carburetor bowl and closes
valve so no more fuel can enter carburetor, fuel
Figure 5 — H eat Stove (U sed on M o d e ls
E q u ip p ed with In-Line E n g in e sI
G M C S E R V I C E M A N U A L

FUEL PUMP

cannot escape from fuel chamber in pump and dia­
phragm spring is held in compressed position.
Rocker arm then idles on camshaft eccentric and
diaphragm moves only a few thousandths of an inch
to replace fuel which enters carburetor between
pump strokes. Thus a constant pressure propor­
tional to force of diaphragm spring is maintained
on fuel in line to carburetor.
on the engine and be sure there is gasoline in the
tank.
side of system . The line from pump to carburetor
is the pressure side of system . A leak on the p res­
sure side of the system would be visible because
of dripping fuel. A leak on the suction side would
not be apparent except for its effect of reducing
the volume of fuel on the pressure side.
for bends or kinks in lines which could reduce flow
of fuel.
PRESSURE TESTING
line at the carburetor inlet and attach a pressure
gauge between the carburetor inlet and the discon­
nected fuel line. Take the pressure reading with
TESTING FUEL PUMP
Always check fuel pump while it is mounted
The line from tank to pump is the suction
Tighten any loose line connections and look
To make the pressure test, disconnect fuel
Figure 6 — Pleated Paper-Type Fuel Filter