Chevrolet 1977 light duty truck, Руководство по обслуживанию

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close, allow ing a stro n g er vacuum signal. The system will
then stablize at a g re a te r E G R flow.
At m axim um engine load, w hen m an ifo ld vacuum is
nearly zero, m o m en tarily , th ere will be no E G R
o p eratio n . This is because o f insufficient vacuum to pull
the valve open, even though high exhaust back pressure
has closed the control valve.
Th erm al Vacuum Switch
A th e rm a l vacuum switch shuts off vacuum to the
E G R valve until coolant te m p eratu re is approxim ately
100 °F (38 “C ) on L6 or 120 °F (49 -C) on V8.
DIAG NO SIS
R e fe r to D iagnosis C h a rt fo r diagnosis o f exhaust
gas recirculation system.
ON-VEHICLE SERVICE
Functional T e s t (on vehicle) ■ EGR Valve
1. W ith the engine ru n n in g , the vehicle in " P a rk "
or " N e u tra l" , set the fast idle cam on second step to hold
throttle open (ap p ro x im ately 1400 - 1600 RPM ). E ngine
coolant tem p e ra tu re m ust be above 120°F (4 9 'C).
2. Place finger b en e ath E G R valve in a m a n n e r to
feel m ovem ent o f d iap h rag m .
3. D isconnect the vacuum hose and w atch for
m ovem ent o f the d ia p h ra g m dow nw ard (valve closed).
T h is should be accom panied by increase in engine speed.
4. R econnect the hose. D ia p h ra g m should move
upw ard (valve open). E ngine R PM should decrease.
N O T E : A slight v ib ra tio n o f the d ia p h ra g m plate
assem bly m ay be noticed in back pressure m odels.
T his is due to the control valve m o d u latin g u nder
light load and does not indicate an u ndesirable
co n d itio n n o r one re q u irin g correction.
Failure Diagnosis
Diaphragm Doesn’t Move:
1. V erify engine speed. Should be approxim ately
1400 - 1600 RPM .
2. V erify te m p e ratu re: should be above 120 °F
(49
XI).
3. Check fo r vacuum at hose. If no vacuum
p resen t, find cause fo r no vacuum . (Plugged o r leaking
hose o r c a rb u re to r port, defective E F E /E G R switch).
4. In back pressure m odels, check control valve
o p e ratio n .
Diaphragm Moves With No Change In Engine RPM
Check m an ifo ld E G R passages fo r blockage.
Function T est (off veh icle) C ontrol Valve
(B ack Pressure EGR M odel O nly)
1. R em ove vacuum hose.
2. R em ove two atta c h in g bolts an d E G R valve
assem bly fro m in tak e m an ifo ld , discard gasket.
3. A pply external vacuum [10" Hg. (34 kPa) or
m ore] to signal tube. A co n stan t vacuum supply m ust be
used.
4. V alve should not open. If it does, control valve
is stuck closed. C lean the valve. (See E G R valve
cleaning).
5. W ith the vacuum still applied, d irect a stream o f
air from a low pressure source into the valve exhaust gas
intake.
6. V alve should open com pletely. I f it does not
open at all, control valve is stuck open or exhaust
passages are plugged. C lean the valve (See E G R valve
cleaning).
7. If E G R valve and control valve are both
fu n ctio n in g properly, clean the m o u n tin g surfaces, then
using a new gasket, install the valve to the intake
m anifold. T orque the bolts to 25 ft. lbs. (34 N m).
8. C onnect vacuum hose.
EGR Valve Cleaning
CAUTION:
Do not wash valve assembly in
solvents or degreaser - permanent damage to
valve diaphragm may result. Also, sand
blasting o f the valve is not recommended since
this can affect the operation o f the valve.
1. Rem ove E G R valve and 2 attach in g bolts,
discarding the gasket.
2. H old the valve assem bly in h an d , then tap
lightly on the sides and end o f the valve, and on the
pintle itself using a small wood dowel, w ith a small
plastic h am m er to rem ove the exhaust deposits from the
valve seat. E m pty loose particles. D O N O T P U T IN A
VISE.
3. W ith a w ire wheel, buff the exhaust deposits
from the m o unting surface and aro u n d the valve.
4. D epress the valve d iap h rag m and look at the
valve seating area through the valve outlet for
cleanliness. If valve a n d /o r seat are not com pletely clean,
rep eat step 3.
5. Look for exhaust deposits in the valve outlet.
R em ove deposit build up w ith a screw driver.
6. C lean m o u n tin g surfaces o f in tak e m anifold
and valve assem bly, then using a new gasket install the
valve assem bly to the intake m anifold. T orque the bolts
to 25 ft. lbs. (34 N m).
7. C onnect vacuum hose.
EGR Passage
If inspection o f E G R passages in the inlet m anifold
indicates excessive build-up o f deposits, the passages
should be cleaned. C are should be taken to ensure that
all loose particles are com pletely rem oved to prevent
them from clogging the E G R valve o r from being
ingested into the engine.
Replacem ent
1. D isconnect E G R valve (Fig. 6E-22 and 23).
2. Rem ove bolts securing valve to m anifold.
3. Rem ove E G R valve from m anifold.
4. R eassem ble replacem ent valve to m anifold,
using a new gasket.