Chevrolet 70 1969 Series, Shop Manual

Page: 347 / 913

ENGINE FUEL SYSTEM 6M -20
Since the helix angle on the plunger determines
the output and operating characteristics of a par­
ticular type of injector, it is imperative that the
correct injectors are used for each engine appli­
cation. If injectors of different types are mixed,
erratic operation will result and may cause serious
damage to the engine and to the equipment which
it drives.
C A U T IO N : Do not intermix the
needle valve injectors with the other
types of injectors in an engine.
Each fuel injector has a circular disc pressed
into a recess at the front side of the injector body
for identification purposes (fig. 5). The identifica­
tion tag indicates the nominal output of the injector
in cubic millimeters. A horizontal bar on the in­
jector identification tag between the " G M ” and the
injector size identifies the current needle valve
injector. Former injectors did not have a bar on
the identification tag.
The fuel injector combines in a single unit all
the parts necessary to provide complete and inde­
pendent fuel injection at each cylinder.
Fuel, under pressure, enters the injector at
the inlet side through a filter cap andfilter element
(fig. 1). From the filter element, the fuel passes
through a drilled passage into the supply chamber,
that area between the plunger bushing and the spill
deflector, in addition to that area under the injec­
tor plunger within the bushing. The plunger oper­
ates up and down in the bushing, the bore of which
is open to the fuel supply in the annular chamber
by two funnel-shaped ports in the plunger bushing.
The motion of the injector rocker arm is
transmitted to the plunger by the follower which
bears against the follower spring (fig. 6). In addi­
tion to the reciprocating motion, the plunger can
be rotated, during operation, around its axis by the
gear which meshes with the rack. For metering the
fuel, an upper helix and a lower helix are machined
in the lower part of the plunger. The relation of the
helixes to the two ports changes with the rotation
of the plunger.
As the plunger moves downward, under force
of the injector rocker arm, a portion of that fuel
trapped under the plunger is displaced into the sup­
ply chamber through the lower port until the port
is closed off by the lower end of the plunger. A
portion of the fuel trapped below the plunger is
then forced up through a central passage in the
plunger into the recess and into the supply cham­
ber through the upper port until that port is closed
off by the upper helix of the plunger. With the up­
per and lower ports both closed off, the remaining
fuel under the plunger is subjected to increased
M etal Identification Tag Pressed
into Recess In Injector Body
Identification M ark on Plunger
INJECTOR
L40
N 40
N45
N50
Id entification M ark on End o f Sp ray Tip
8-.0055-165A
8-.00 5 5 -1 6 5 A
8-.0055-1 6 5A
8-.0055-1 6 5A
CURRENT
SPRAY TIP*
8-.0055-165A
6 -.0 0 6 -165A
6-.00 6 -165A
6 -.0 0 6 -165A
PLUNGER
4L
4N
5*
5N
First num eral indicates num be r of sp ra y holes, fo llow ed
b y sizes of h ole s a nd a n g le form ed b y sp ra y from holes.
Figure 5—Injector Identification
pressure by the continued downward movement of
the plunger.
When sufficient pressure is built up, it opens
the flat, non-return, check valve. The fuel in the
check valve cage and spring cage passages, tip
passages and tip fuel cavity is compressed until
the force acting upward on the needle valve is suf­
ficient to open the valve against the downward force
of the valve spring. As soon as the needle valve lifts
off its seat, the fuel is forced through the small
orifices in the spray tip and is atomized in the
combustion chamber.
When the lower land of the plunger uncovers
the lower port in the bushing, the fuel pressure
below the plunger is reduced, and the valve spring
closes the needle valve, ending injection.
A pressure relief passage has been provided
in the spring cage to permit bleed off of fuel leak­
ing past the needle pilot in the tip assembly.
A check valve, directly below the bushing,
prevents leakage from the combustion chamber
CHEVROLET SERIES 70-80 HEAVY DUTY TRUCK SHO P MANUAL