Chevrolet 1977 light duty truck, Service Manual

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vacuum in 2 m inutes is allow able and does not
constitute a defective switch.
7. If o p e ra tio n is satisfactory, re-install switch in
th erm o stat housing. If switch is defective, replace w ith
new part.
EGR SYSTEM - IN S P E C T IO N A N D
C LE A N IN G
The follow ing p rocedure is to be used for the
inspection and clean in g o f the E G R valve and passages.
Inspection
1. R em ove a ir cleaner.
2. Rem ove E G R valve from intake m anifold.
3. Look for deposits on the valve pintle.
4. D epress the valve d ia p h ra g m a n d inspect fo r
deposits aro u n d the valve seatin g area thru the valve
outlet.
5. W ith the d ia p h ra g m still depressed, plug the
vacuum tube then release the d ia p h ra g m . If the pintle
has not closed in 20 seconds, the E G R valve is OK. If the
p in tle is closed w ith in 20 seconds, the valve m ust be
replaced.
6. T he valve req u ires clean in g if deposits exist.
C leaning
1. Hold the valve assem bly in h an d . T hen, using a
light sn ap p in g action w ith a plastic h am m er, tap on the
end o f the ro u n d pintle to rem ove the exhaust deposits
from the valve seat. E m pty loose particles.
2. C lean the m o u n tin g surface o f the E G R valve
w ith a w ire w heel or w ire brush, an d the pintle w ith a
w ire brush.
3. D epress the valve d iap h rag m and check the
seating area for cleanliness by looking thru the valve
outlet. If pintle or seat are not com pletely clean, rep eat
step 1.
4. Inspect the valve outlet fo r deposits. Rem ove
any deposit build-up w ith a screw driver o r o th er suitable
sh arp tool.
5. C lean m o u n tin g surface on the intake m anifold
w ith a w ire w heel o r wire brush.
N O TE: Do n o t use solvents fo r cleaning.
Intake M anifold Passages
1. Rem ove carb u reto r.
2. Rem ove the deposits from the E G R ports by
h an d tu rn in g a drill into the passage. F in ish cleaning
w ith a sm all screw driver.
CAUTION: Do not use a power drill or file.
3. Brush any sm all particles down the E G R port
and blow com pressed air through the port.
N O TE: Do not use solvents fo r cleaning.
Reassembly
1. R einstall ca rb u re to r using a new gasket and
torque hold-dow n bolts to 10-15 ft. lbs.
2. R einstall the E G R valve assem bly to the intake
m an ifo ld using a new gasket and torque bolt to 12-17 ft.
lbs.
3. C onnect fuel line and all vacuum hoses.
4. W arm up engine an d reset idle rpm to
specification if necessary p er E m ission C ontrol In fo rm a ­
tion label.
5. P erfo rm fun ctio n al check o f E G R system to
ensure correct operation.
POSITIVE CRANKCASE VENTILATION SYSTEM (PCV)
GENERAL DESCRIPTION
V en tilatio n a ir is draw n through a filter assem bly
located in the a ir cleaner, thro u g h a hose, down into the
crankcase, up through the v en tilato r valve, through a
hose a n d into the intake m anifold. Intake m an ifo ld
vacuum draw s an y fum es from the crankcase to be
b u rn ed in the engine.
W h en air flow th ro u g h the ca rb u re to r is high, added
air from the Positive C rankcase V en tilatio n System has
no n o ticeable effect on engine o p eratio n ; however, at idle
speed, a ir flow th rough the c a rb u re to r is so low th a t any
large a m o u n t ad d ed by the ven tilatin g system would
upset the a ir-fu el m ixture, causing rough idle.
F or this reason, a flow control valve is used w hich
restricts the ven tilatin g system flow w henever intake
m anifold vacuum is high.
ON-VEHICLE SERVICE
1. Rem ove PCV valve from intake m an ifo ld or
rocker arm sh aft cover.
2. R un the engine at idle.
3. Place your thum b over en d o f valve to check fo r
vacuum . If there is no vacuum at valve, check fo r
plugged hoses or valve. Hoses m ay be cleared w ith
com pressed air.
4. Shut off" the engine and rem ove PCV valve.
Shake valve and listen for the rattle o f check needle in
valve. If valve does not rattle, replace valve.