GMC 4000 Series Service Manual page 322

CLEANING
1. Drain system , then close cocks and install
drain plugs.
2. Fill system with cleaning solution. Always
follow manufacturer's directions.
3. With radiator covered and radiator cap on
tight, run engine 15 to 20 minutes at fast idle speed.
Drain system completely.
4. If cleaning solution used requires a neu­
tra lize^ use as directed by manufacturer.
FLUSHING
Remove thermostats before pressure flushing
system . Avoid excessive pressure. When pressure
flushing, apply air gradually, as radiator will stand
only a limited pressure.
CORROSION PREVENTION
Use of water containing lim e, alkali, and other
impurities is a major cause of rust and scale form­
ation in cooling system . Air or exhaust gas leaking
into the system can also be cause of rust and cor­
rosion. A rust preventive, inhibitor, or water filter
should be used continuously. WHENEVER WATER
FILTER IS USED IN SYSTEM NO INHIBITOR OR
RUST PREVENTIVE IS REQUIRED AS FILTER
CONDITIONS THE COOLING SYSTEM.
Drain and flush cooling system every 24
months and add new anti-freeze solution.
INHIBITORS AND SEALERS
In general, inhibitors are not cleaners and
will not remove scale and rust already formed.
GM Cooling System Inhibitor and Sealer will re­
tard rust and scale formation and is compatible
with aluminum components. Inhibitor and sealer
should be used immediately after new anti-freeze
solution has been added to system and every fall
thereafter.
It is important not to use too much inhibitor
or to use two different types at the same time.
USE INHIBITOR AND SEALER ONLY AS
INSTRUCTED ON LABEL.
AIR SUCTION TEST
Air may be drawn into system due to low
liquid level in the radiator or surge tank, leaky
water pump, or loose hose connections. This action
will also cause corrosion.
1. Replace radiator cap having integral p res­
sure relief valve with a plain filler cap less the
relief valve.
2. Make sure radiator cap seal is in good con­
dition and will make an airtight seal. Adjust level
of cooling liquid in radiator, allowing ample room
for expansion to avoid any overflow loss during
test.
G M C S E R V I C E M A N U A L
of overflow tube. This connection must be air-tight.
Run engine with transmission in neutral at % throt­
tle speed until temperature gauge stops rising and
remains stationary.
rubber tube in a bottle of water, avoiding kinks
and sharp bends that might block flow of air. Watch
for bubbles in bottle of water. The continuous ap­
pearance of bubbles indicates that air is being
sucked into the cooling system .
and fitting connections. Also examine all hoses
carefully and if cracked, swollen, or deteriorated
in any way, replace with new hose.
EXHAUST GAS LEAKAGE TEST
system past cylinder head gasket or through cracks
in the cylinder head and block. This action will
also cause corrosion and possible damage to en­
gine combustion chamber components.
belt to prevent water pump operation.
liquid level is at top of thermostat well. Remove
thermostat(s) and insure that coolant level is at
top of thermostat well.
and accelerate it several tim es.
surface of coolant while accelerating engine. Also
watch when engine speed drops back to idle. The
appearance of bubbles or a sudden rise of cooling
liquid indicates exhaust gas leakage into cooling
system . Make test quickly before boiling starts as
steam bubbles will give misleading results.
cylinder head gasket or gaskets, then test again.
Tighten cylinder head bolts to torque specified in
ENGINES (SEC. 6A or 6B) of this manual. If leaks
are still evident, cylinder head or block may be
cracked. Correct cause of leakage, then install
thermostats and adjust drive belt. Fill cooling
system .
a cooling medium in clim ates where temperatures
do not reach below 32°F. In lower temperatures,
anti-freeze solutions must be used. Before install­
ing anti-freeze solution, cooling system should be
inspected and serviced for cold weather operation,
as previously described under "Cleaning System"
in this section.
tightness and gasket replaced if necessary, to
avoid possibility of anti-freeze solution leaking
ENGINE COOLING SYSTEM
3. Attach a length of rubber tube to lower end
4. Without changing engine speed, put end of
5. Correct condition by tightening hose clamps
Exhaust gas may be blown into the cooling
1. Start test with engine cold. Remove drive
2. Partially drain cooling system until cooling
3. With transm ission in neutral, start engine
4. Watch for bubbles in water or smoke at
5. If exhaust gas leakage is evident, replace
COLD WEATHER OPERATION
Water, with an inhibitor, can be safely used as
Cylinder head bolts should be checked for
Sec. 6K
Page 311