Cylinder Compression Test; Ignition System Service - GMC 4000 Series Service Manual

3. Stop engine, disconnect plug wires and re­
move spark plugs and gaskets.

CYLINDER COMPRESSION TEST

1. With carburetor choke and throttle in wide-
open position, operate starter with remote control
switch.
CAUTION: When using remote switch
to operate starter, the primary wire must
be disconnected from coil negative term­
inal and ignition switch must be turned on.
Failure to do this will result in damaged
grounding circuit in ignition switch.
2. S tarting w ith co m p r e ssio n gauge at z e r o ,
crank engine through at le a s t four c o m p r e ssio n
str o k e s to obtain h igh est p o s s ib le reading.
3. Make compression check at each cylinder
and record each reading.
4. If some cylinders have low compression,
inject about one tablespoon of engine oil into com­
bustion chamber through spark plug hole. Crank
engine to spread oil on cylinder walls, then re­
check compression with gauge.
a. If compression is higher but does not nec­
essarily reach normal pressure, worn piston rings
are indicated.
b. If compression is not improved by adding
oil to cylinder, it is probable that valves are not
properly seating possibly due to sticking in guides,
or burned valves or sea ts0
c. If two adjacent cylinders have lower than
normal compression, and injecting oil into cylin­
ders does not increase compression, the cause
may be a head gasket leak between the cylinders.
This condition could be cause of coolant leaking
into cylinders.
NOTE: An engine with low or uneven com­
pression cannot be tuned to givepeakperformance;
therefore, it is important to make necessary cor­
rections before proceeding with tune-up operations.
SERVICE AND INSTALL SPARK PLUGS
1. Inspect all sparkplugs carefully. Look for
glazed, broken, or blistered porcelains and burned
electrodes. Refer to ENGINE ELECTRICAL (SEC.
6Y) in this manual for correct type of spark plugs
for various engines.
2. If sparkplugs are serviceable, use an abra­
sive type cleaner such as sand blaster to thorough­
ly clean spark plugs. File end of center electrode
flat.
3. Adjust spark plug gaps to specifications.
4. Test spark plugs with spark plug tester.
5. Install spark plugs using new gasket. Use
torque wrench and special spark plug socket
wrench. Correct torque for spark plug installation
G M C S E R V I C E M A N U A L
is shown in respective engine tune-up specifica­
tions later in this section.

IGNITION SYSTEM SERVICE

access to ignition distributor, spark plug wires
and coil.
brittle, cracked, or otherwise damaged, replace
as necessary.
sockets, cracks, and for erosion at terminals in­
side cap. Replace the distributor cap if it is not in
good condition. When installing spark plug wires,
refer to figure 7 which shows cylinder numbering,
firing order and distributor cap wire socket num­
bering on V6 engines.
GASOLINE ENGINES
Figure 7— Cylinder Num bers and Spark Plug
Wire Locations on V6 Engine
1. Remove equipment as required to provide
2. Check condition of wiring. If any wires are
3. Inspect distributor cap for burned wire
On In-line, 6-cylinder engines the cylinders
Figure 8 — Cylinder Num bers and Spark Plug
Wire Locations on, In-Line Engine
Sec. 6A
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