GMC 1976 ZEO 6083 Maintenance Manual page 371

SECTION 6T
EMISSION CONTROL SYSTEMS
DESCRIPTION (FIGURE 1)
At idle or normal road speeds, intake manifold
vacuum causes fresh air to be drawn through the
engine air filter, then to the left valve cover where it
joins with the crankcase vapors . This mixture is then
drawn through the P.C .V . valve to the base of the
carburetor where the vapors are mixed with normal
fuel air mixture and burned .
When the engine is running at idle or the vehicle
is traveling at normal speeds, intake manifold
vacuum is sufficient to draw crankcase vapors
caused by engine blow-by through the spring loaded
P.C .V . valve.
Contents of this section are listed below:
EMISSION CONTROL SYSTEMS 6T- 1
SUBJECT PAGE NO .
Positive Crankcase Ventilation (P.C .V .)
Description ... . . .. . . . .. . . ... . . ... . .... . . .. . . . .. . . . . . . . ... . .. . . .. . . .. . . . .. . . . .. . ... . . ... . . . .. . . ... . . ... . .... . . .. . . . . . . . ... . ... . . .. . .6T-1
P.C .V . System Testing . ... . .... . . .. . . . .. . . ... . .... . ... . .. . . ... . . .. . . ... . .... . ... . . . .. . . ... . . ... . .... . . .. . . . .. . . .. . . ... . . .. . .6T-2
Instruction For Testing P.C .V . Valve ... . . . . . . .. . . .. . . . .. . . ... . ... . . ... . . ... . . ... . . ... . .... .... . . .. . . . .. . . .. . . .. . . .6T-2
Instruction For Testing Complete System . .. . . .. . . .. . . . .. . . ... . . ... . . ... . . .. . . . ... . .... .... . . .. . . ... . . .. . . .. . . .6T-2
Controlled Combustion System (C .C .S .)
Description ... . ... . . ... . . ... . . ... . .... . ... . . ... . . ... . .... . .. . . .. . . .. . . . .. . . ... . .... . ... . . . .. . . ... . . ... . .... . . .. . . . . . . . .. . . .. . . .. . . .6T-3
Purpose . ... . . ... . . .. . . . .. . . ... . . ... . . ... . . .. . . . .. . . ... . .... . .. . . .. . . ... . . .. . . ... . .... . ... . . . .. . . .... . ... . . ... . . .. . . . .. . . ... . ... . . .. . .6T-3
Operation . .... . ... . . ... . . ... . . ... . .... . . .. . . . .. . . ... . .... . .. . . .. . . .. . . . .. . . ... . ... . . ... . . ... . . .. . . . ... . .... . ... . . .. . . . .. . . .. . . . . . . .6T-4
Diagnosis . ... . . ... . . .. . . . .. . . . .. . . ... . .... . . .. . . . .. . . .. . . .. . . ... . ... . . .. . . ... . . .. . . . ... . .... . ... . . .. . . . ... . .... . ... . .... . . .. . . .. . ..6T-4
Vacuum Motor Replacement . . ... . . ... . . .. . . .. . . . .. .... . ... . .... . . .. . . . . . . . ... . . ... . . .. . . . . . . . . ... . ... . .... . ... . ... . . .6T-5
Sensor Replacement .. . . .. . . . .. . . .... . ... . .... . ... . . .. . .... .... . ... . .... . ... . . . .. . . ... . . ... . . .. . . . .. . . .... . ... . .... . ... . ... . ..6T-5
Exhaust, Manifold Shroud .. . .... . ... . . ... . . .. . . .. . . ... . ... . . .. . . ... . . .. . . . .. . . .... . ... . . .. . . . ... . .... . ... . .... . . .. . . .. . . .6T-6
Evaporation Control System (E .C .S .) . . . ... . . .. . . .. . . ... . . .. . . .. . . . ... . .... . ... . . . .. . . .... . ... . . . .. . . ... . . .. . . .... . ... . .6T-6
Thermal Vacuum Switch (T .V .S .)
Description . . . . .. . . .... . ... . . .. . . . .. . . ... . . ... . .... .... . ... . .... ... . . ... . ... . .... . ... . . . .. . . .... . ... . . ... . . .. . . . .. . . .... .... . ... . . .6T-6
Operation . .. . . . .. . . ... . .... . ... . . ... . . ... . . ... . .... .... . . .. . ... . ... . . . . . . . . . . . ... . ... . . . . . . . . .. . . .. . . .... . . .. . . . .. . . .. . . . . . . . . .. . . .6T-6
Vacuum Hose Routing . . . .. . . .... . ... . .... . ... . ... . ... . ... . . ... . ... . .... . ... . . . .. . . .... . ... . . ... . . .. . . . .. . . ... . .... . ... . . .6T-7
Functional Check.. . . .. . . . .. . . ... . .... . ... . . ... . . .. . . .. . .... .... . ... . .... . ... . . .. . . . ... . .... . . .. . . ... . . .. . . . ... . ... . .... . ... . . .6T-7
Throttle Return Control (T .R .C .) ... . . ... . . .. . . .. . . ... . ... . ... . .... . ... . . ... . . .... .... . . .. . . . .. . . ... . . ... . .... . ... . ... . . .6T-7
POSITIVE CRANKCASE VENTILATION (P .C .V .)
At high road speeds or heavy acceleration, the
engine blow-by is increased and at the same time,
intake manifold vacuum decreased. When this oc
curs, there is a reverse action, crankcase vapors
released through the crankcase filter are returned
back into the intake manifold through the carbu-
retor. When operating the engine under zero vacuum
or a manifold pressure condition such as a backfire
or during engine cranking, the check valve is closed
by spring tension to prevent fuel vapor from entering
the crankcase. The valve is also closed under wide-
open throttle condition but since this is for a very
short duration of time, no irregularity will exist.
NOTE : If vehicle has encountered dusty condi-
tions, be sure PCV filter is cleaned immediately.