Fuel Selector Valve - Cessna 1969-1976 172 Series Service Manual

a. Attach a rubber tube to the end of vent line be-
b.
Blow into tube to slightly pressurize tank. If
c. After tank is slightly pressurized, insert end of
neath the wing.
air can be blown into tank, vent line is open.
rubber tube into a container of water and watch for
a continuous s t r e a m of bubbles, which indicates the
bleed hole in valve assembly
is
open and relieving
p r e s s u r e
.
d. After completion of step "c", blow into tube
again to slightly p r e s s u r i z e the
tank,
and loosen,
but do not remove filler cap on opposite wing to
check tank crossover line. If p r e s s u r e escapes
f r o m filler cap, crossover line is open.
NOTE
Remember that a plugged vent line or bleed
hole can cause either fuel starvation and
collapsing of fuel tanks or the p r e s s u r i z a -
tion of tanks by fuel expansion.
e. Any fuel vent found plugged o r r e s t r i c t e d must
be corrected p r i o r to returning a i r c r a f t to service.
NOTE
The fuel vent line protruding beneath the wing
near the wing strut must
be
correctly aligned
to avoid possible icing of the vent tube. Di-
mensions are shown in figure 12-4.
12-12. FUEL SELECTOR VALVE.
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1 1 2 - 1 3 . DESCRIPTION. A four position fuel selector
valve is located between the pilot and copilot positions
on the pedestal. The positions on the valve a r e label-
ed "OFF, LEFT, BOTH ON and RIGHT.
''
Valve re-
pair consists of replacement of seals, springs, balls
and other detail parts. Figure 12-5 illustrates the
proper relationship of p a r t s and may be used
as a
guide during disassembly and assembly.
12-14. REMOVAL AND INSTALLATION. (See fig-
u r e 12-2.)
a. Completely drain all fuel f r o m win: tanks, fuel
s t r a i n e r , lines and selector valve. (Observe precau-
tions i n paragraph 12-3. )
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b. Remove selector valve handle.
c.
Remove pedestal cover.
d. Remove carpeting as necessary to gain access
e. Disconnect handle drive shaft f r o m valve.
f . Disconnect and cap or plug all fuel lines
at
valve.
g. Remove s c r e w s attaching valve to bracket and
t o plates
aft
of pedestal.
remove valve.
h. Reverse the preceding steps for installation.
P r i o r t o installing a c c e s s plates, service fuel tanks
and check for leaks.
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12-15. FUEL STRAINER. (See figure 12-6.)
12-16. DESCRIPTION. The fuel s t r a i n e r
is
mounted
a t the firewall in the lower engine compartment. The
s t r a i n e r i s equipped with
a
quick-drain valve which
provides a means of draining trapped water and sedi-
ment from the fuel system. The quick-drain control
i s
located adjacent to the oil dipstick and is accessable
through the oil dipstick door i n the upper engine cowl,
NOTE
The fuel s t r a i n e r can be disassembled, cleaned
and reassembled without removing the assembly
f r o m the aircraft. (Refer to paragraph 12-18, )
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12-17. REMOVAL AND INSTALLATION. (See fig-
u r e 12-6.)
a. Remove cowling
as
necessary t o gain a c c e s s to
strainer.
b. With selector valve in "OFF" position, drain
fuel f r o m s t r a i n e r and lines with s t r a i n e r quick-
drain control.
c. Disconnect and cap o r plug all fuel lines and
controls f r o m strainer. (Observe precautions i n pa-
ragraph 12-3. )
d. Remove bolts attaching assembly t o firewall and
remove strainer.
e. Reverse the preceding steps for installation.
With selector valve i n "ON" position check for leaks
and proper operation of quick-drain valve.
12-18. DISASSEMBLY AND ASSEMBLY. (See fig-
u r e 12-6.)
a.
With selector valve i n "OFF" position, drain
fuel f r o m bowl and lines with quick-drain control.
b. Remove drain tube, safety wire, nut and wash-
er at bottom of filter bowl and remove bowl.
c.
Carefully unscrew standpipe and remove.
d. Remove filter screen and gasket. Wash filter
s c r e e n and bowl with solvent (Federal Specification
P-S-661, or equivalent) and d r y with compressed air.
top assembly, install s c r e e n and standpipe. Tighten
standpipe only finger tight.
f. Using all new O-rings, install bowl. Note that
step-washer
at bottom of bowl is installed so
that
step
seats
against O-ring. Connect drain tube.
leaks and proper operation of quick-drain valve.
must have right hand wrap, a t
least
45 degrees.
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e. Using a new gasket between filter s c r e e n and
g. With selector valve i n "ON" position, check for
h. Safety wire bottom nut to top Lssembly. Wire
SHOP N O T E S :
Change 1 12-9