GMC 4000 Series Service Manual page 549

Sec. 13
Page 538
RADIATOR A N D SURGE TANK
V a c u u m
V a lv e
P re ssu re V a lv e
Figure 4 — Pressure Relief Valve and Filler Cap
and a vacuum valve (fig. 4). When pressure in sy s­
tem reaches valve setting (see "Specifications" at
end of this section), pressure valve opens and
vapor is allowed to escape. As liquid in system
cools it contracts; this allows pressure valve to
close and also creates a partial vacuum in system .
Atmospheric pressure acting through overflow
tube unseats vacuum valve and allows air to enter
system . The overflow pipe connects to valve out­
side the valve seal; thus no liquid or air can e s ­
cape from the system when both valves are in the
closed position.
Radiator filler cap is constructed with a spring-
loaded rubber seal which is pressed firmly against
surface of filler neck seat when cap is installed.
Rubber seal must be in good condition and top of
radiator filler neck must be clean and smooth in
order to form an air-tight seal. Seal of filler cap
and operation of pressure relief valve can be
checked using a conventional cooling system test­
ing kit.
NOTE: When engine is at normal operating
temperature or above, the internal pressure built
up in the cooling system will blow out scalding
fluid and vapors if the radiator cap is suddenly re­
moved. To prevent loss of coolant and to avoid the
danger of being burned, the coolant level should be
checked or coolant added only when the engine is
cool. If the cap must be removed when the engine
is hot, place a cloth over the cap and rotate the cap
slowly counterclockwise to first stop and allow
pressure to escape completely. Then turn cap
again slowly counterclockwise to remove.
RADIATOR SURGE TANK
A separate radiator surge tank on all "L"
models, is mounted to rear of control island fram­
G M C S E R V I C E M A N U A L
C a p Unit
P re ssu re
V a lv e
Sp rin g
ing. Tank serves as a coolant reservoir for the
radiator core which is mounted at front of engine.
Hoses and pipes connect tank to radiator and water
pump. Surge tank incorporates the cooling system
filler cap which is accessible from inside cab
through door at top of seat riser. Tilt cab forward
to gain access to surge tank.
At regular intervals check tank and connecting
hoses for leaks. Keep hose clamps and mounting
bolts tightened securely.
ENGINE COOLANT
COOLANT RECOMMENDATIONS
The year-around engine coolant used to fill
the cooling system at the factory is a high quality
solution that meets General Motors Specification
1899-M. This factory-fill coolant solution is form­
ulated to withstand two full calendar years of norm­
al operation without draining or adding inhibitors,
provided the same concentration of coolant is added
if the system needs additional fluid between drain
periods. The original factory fill coolant provides
freezing protection to -20°F.
Every two years, the cooling system shouldbe
serviced as described in ENGINE COOLING (SEC.
6K) of this manual. A thorough description of in­
spection, draining, and cleaning of the cooling sy s­
tem is given.
IMPORTANT: Alcohol, methanol base cool­
ants, or plain water are not recommended. Only a
sufficient amount of Ethylene Glycol base coolant
meeting GM Specification 1899-M should be used.
DO NOT use glycol ether (methoxy propanal type)
base permanent type anti-freeze coolants inDH478
diesel engine as damage to cylinder head gasket
seals will occur.
COOLANT TESTING
Always test solution before adding water or
anti-freeze. Engine should be warmed up to oper­
ating temperature. Fill and empty tester several
tim es to warm tester before using. Keep tester
clean inside and out.
Some testers will indicate correct freezing
point only when test is made at a specific temper­
ature. Other testers are provided with thermom­
eters and tables and indicate freezing points cor­
responding to readings made at various temper­
atures. Disregarding temperatures of solution may
cause an error as large as 30°F. Read and be
guided by instructions furnished by tester manu­
facturer.
In the event coolant freezes solid in extreme
cold weather, place vehicle in warm building or
improvise some means of thawing coolant before
starting engine. Under no circumstances should
engine be operated when coolant is frozen solid.