Chevrolet 10 Series 1968, Service Manual

Page: 352 / 713

ENGINE 6-5
Service M anifold Heat V alve (Figs. 8 or 9)
Check manifold heat control valve for freedom of
operation. If shaft is sticking, free it up with GM Mani­
fold Heat Control Solvent or its equivalent.
NOTE: Tap shaft end to end to help free it up.
Tighten M anifold
Tighten intake manifold bolts to specifications in the
sequence outlined on Torque Sequence Chart* A slight
leak at the intake manifold destroys engine performance
and economy.
Service Fuel Lines and Fuel Filter
1. Inspect fuel lines for kinks, bends or leaks and cor­
rect any defects found.
2. Inspect filter and replace if plugged.
NOTE: If a complaint of poor high speed per­
formance exists on the vehicle, fuel pump tests
described in Section 6M should be performed.
Service Cooling System
1. Inspect cooling system for leaks, weak hoses, loose
hose clamps and correct coolant level, and service
as required.
NOTE: A cooling system pressure test, as de­
scribed in “ Additional Checks and Adjustments"
in this section, may be performed to detect
internal or external leaks within the cooling
system.
Check and Adjust Accelerator Linkage
1. Disconnect accelerator rod at carburetor throttle
lever.
2. Hold carburetor throttle lever in wide position.
3. Pull accelerator rod to wide open position.
(On
vehicles equipped with automatic transmission, pull
through detent).
4. Adjust accelerator rod to freely enter hole in car­
buretor throttle lever,
N OTE:
Accelerator linkage is outlined in detail
in Section 6M.
5. Connect accelerator rod at throttle lever.
Service Crankcase Ventilation (Fig. 10)
All engines have either "P o s itiv e ” or “ Closed Po­
s itiv e " ventilation systems utilizing manifold vacuum to
F ig . 8—M anifold Heat Control V a lv e (In Line)
F ig . 9—M anifold Heat Control V a lv e (V 8)
draw fumes and contaminating vapors into the combustion
chamber where they are burned. Since it affects every
part of the engine, crankcase ventilation is an important
function and should be understood and serviced properly.
In both "P o s itiv e " and “ Closed P ositive" ventilation,
air is drawn through the engine, (through a regulating
valve) into the manifold, drawing crankcase vapors and
fumes with it to be burned. “ Positive" ventilation uses
a vented-meshed cap for clean air intake to the engine,
while “ Closed P ositive" ventilation system draws the
clean air from the carburetor air cleaner and has a
nonvented oil filler cap.
1. Ventilation valve may be checked as outlined under
“ Additional Checks and Adjustments".
2. Inspect for deteriorated or plugged hoses.
3. Inspect all hose connections.
4. On closed positive ventilation systems, remove flame
arrestor and wash in solvent then dry with com­
pressed air.
Service A ir Injection Reactor System
Inspect air injection reactor ( A . I . R . ) system for evi­
dence of leaks, deteriorated hoses, cracked air manifolds
or tubes and loose hose clamps. Inspect air injection
pump belt condition and tension. Make all necessary
repairs as outlined in “ Section 6T ".
Because of the relationship between “ Engine Tune Up"
and “ Unburned Exhaust Gases", the condition of Engine
Tune Up should be checked whenever the A.I.R. System
seems to be malfunctioning. Particular care should be
taken in checking items that affect fuel-air ratio such as
the crankcase ventilation system, the carburetor and the
carburetor air cleaner. Carburetors and distributors
for engines with the A.I.R. System and Controlled Com­
bustion System are designed, particularly, for these
engines; therefore, they must not be interchanged with
or replaced by a carburetor or distributor designed for
different appreciations.
Choke Adjustment
1. Remove air cleaner.
CHEVROLET TRUCK SERVICE MANUAL