Idle Speed And Mixture; Crankcase Ventilation Systems - GMC 4000 Series Service Manual

4. While observing vacuum gauge and tach­
ometer, set engine idle speed and adjust carbur­
etor idle mixture screw s to provide steady running
engine.
NOTE: If difficulty is experienced in obtaining
satisfactory engine idle performance, the cause
may be due to malfunction of crankcase ventilation
valve(s). Inspection and service of ventilation
valves are covered later under appropriate headings.
5. Refer to ENGINE ELECTRICAL (SEC. 6Y)
of this manual for instructions for checking voltage
regulator settings, generator output, and specifi­
cations on various electrical equipment units.
CRANKCASE VENTILATION SYSTEM
(IN-LINE 6 CYLINDER ENGINE)
INSPECTION. SERVICE AND
UNIT REPLACEMENT
Figure 9 shows typical closed type crankcase
ventilation on In-line engine. The ventilation valve
(2) is inserted in hole in rocker arm cover and
flame arrester (3) is used in air hose (8) connect­
ing air cleaner to rocker arm cover.
TESTING CRANKCASE VENTILATION VALVE
Key number in text refer to figure 9.
1. Connect tachometer and vacuum gauge as
when setting idle speed and mixture.
2. Start engine and adjust idle mixture and
idle speed.
3. Remove ventilation valve (2) with hose (4)
attached. Block the inlet opening in ventilation
valve and note change in engine rpm.
4. If blocking the valve opening causes a
change of less than 50 rpm, a clogged, or partially
restricted ventilation valve is indicated.
5. Install new valve assembly; or if valve (fig.
10) is operative, reinstall in rocker arm cover.
IMPORTANT: At 12-month or 12,000 mile
intervals (whichever occurs first) the crankcase
ventilation valves must be replaced.
CRANKCASE VENTILATION SYSTEM
(V6 ENGINE)
INSPECTION, SERVICE, AND
UNIT REPLACEMENT
All V6 gasoline engines have two crankcase
ventilation valves, one valve is installed in cylin­
der head under each rocker arm cover (fig. 11).
Each valve assembly is screwed into a threaded
passage which leads into intake port in cylinder
head.
Ventilation valves (fig. 12) cannot be disas-
G M C S E R V I C E M A N U A L
1 Crankcase Ventilation
2 Carburetor Air Cleaner
3 Breather Cap
sembled but can be removed for inspection. A
crankcase ventilation valve which is not functioning
properly can affect engine idle and the inadequate
ventilation of crankcase may contribute to engine
oil contamination and sludge formation.
TESTING CRANKCASE VENTILATION
VALVE OPERATION
inder head.
GASOLINE ENGINES
4 Rocker Arm Cover
Valve Assembly 5 Cylinder Heads
6
Figure 11 — Schematic View of Crankcase
Ventilation System on V6 Engine
1. Remove rocker arm cover from each cyl­
Figure 12— Crankcase Ventilation Valve
Used in V6 Engine Cylinder Heads
Sec. 6A
Page 261
Ventilation Hose
T-4624