GMC 1982 Light Duty Truck, Service Manual

Page: 761 / 1435

ENGINE COOLING 6B-7
DIAGNOSIS
If the cooling system requires frequent addition of
coolant in order to maintain the proper level, check all units
and connections in the cooling system for evidence of
leakage. Inspection should be made with cooling system
cold. Small leaks which may show dampness or dripping can
easily escape detection when the engine is hot, due to the
rapid evaporation of coolant. Tell-tale stains of grayish
white or rusty color, or dye stains from anti-freeze, at joints
in cooling system are almost always sure signs of small leaks
even though there appears to be no damage.
Air may be drawn into the cooling system through
leakage at the water pump seal or through leaks in the
coolant recovery system. Gas may be forced into the cooling
system through leakage at the cylinder head gasket(s) even
though the leakage is not sufficient to allow coolant to enter
the combustion chamber.
SY ST E M C H E C K S
E x h a u s t Leaks
To check for exhaust leaks into the cooling system,
drain the system until the coolant level stands just above the
top of the cylinder head(s), then disconnect the radiator
upper hose and remove the thermostat and fan belt(s). Start
the engine and quickly accelerate several times. At the same
time note any appreciable coolant rise or the appearance of
bubbles which are indicative of exhaust gases leaking into
the cooling system.
N O TIC E : A defective head gasket may allow exhaust
gases to leak into the cooling system . This is
particularly damaging to the cooling system as the
gases combine with the water to form acids which are
harmful to the radiator and engine.
W a te r P u m p
Water pump operation may be checked by running the
engine while squeezing the radiator upper hose (engine
warm A pressure surge should be felt. Check for a plugged
venthole in pump.
R a d ia to r
Test for restriction in the radiator, by warming the
engine up and then turning the engine off and feeling the
radiator. The radiator should be hot along the left side and
warm along the right side, with an even temperature rise
from right to left. Cold spots in the radiator indicate clogged
sections.
T h e rm o s ta t
An operational check of the thermostat can be made by
hanging the thermostat on a hook in a 33% glycol solution
25°F (4°C ) above the temperature stamped on the
therm ostat valve. Subm erge the valve com pletely and
agitate the solution thoroughly. Under this condition the
valve should open. Remove the thermostat and place in a
33% glycol solution 10°F (-12°C ) below temperature
indicated on the valve. With valve completely submerged
and coolant agitated thoroughly, the valve should close
completely.
O v e rh e a t a n d / o r N oise
Engine overheat and/or cooling system noise may be
caused by restrictions in the cooling system.
Components which may be prone to this condition are
cylinder head, water pump, block, thermostat housing and
inlet manifold. Symptoms of this condition are as follows:
• Engine may make snapping/cracking noises.
• Heater core may gurgle or surge.
• Radiator hoses may collapse and expand.
• Heater hoses may vibrate and thump.
• Overheat light may or may not come on.
Symptoms are the result of coolant boiling at some
localized area and may be noticed after extending idling
and/or while being driven. Determine which side of the
engine is involved and whether it is more at the front or rear
of engine.
D ia g n o s is /In s p e c tio n
1. Isolate area of engine the localized boiling is
originating from. This can be done by probing engine
with a sounding bar (large screw driver).
2. W ith radiator cap removed, observe water being
circulated in radiator. Feel the front area of radiator
for cold spots which indicate blockage. Blocked
radiators generally occur on units that have accrued
miles and not on new vehicles.
C A U T IO N : T h e ra d ia to r c ap sh ould b e re m o v e d
fro m a cool e n g in e o n ly . If th e ra d ia to r c ap is
re m o v e d fro m a h o t c o o lin g s y s te m , serio u s
perso n al in ju ry m a y re su lt.
3. Inspect thermostat to see if it opens completely.
4. Inspect therm ostat housing to make sure it is
completely free of obstructions.
5. Remove water pump from vehicle and remove the back
cover on the pump. All internal passages can be
inspected using a flash light.
6. Inspect cross over at the front of the inlet manifold.
This entire passage can be seen with only the
thermostat removed.
7. Remove heads, but lay them aside for now and check
the block first because the heads are the most complex
pieces as far as coolant passages are concerned.
8. With water pump and heads removed, ALL coolant
passages C A N be inspected by using a pen light flash
light. All water jacket areas can be seen directly and a
block should never be replaced as being suspect unless
the restricted area can be DIRECTLY SE E N .
9. If none of the above inspections reveal the problem
area, the heads must be considered prime suspect.
Heads with blocked coolant passages generally have
more than one area that is blocked. Inspect the heads
for signs of overheat discoloration (a dark blue or black
area). If none are found look in the coolant passages for
blockage and probe all passages that are accessible.
The head is very intricate and all passages cannot be
reached. Use a probe that is fairly substantial as a tag
wire may go through or around a partially blocked
area. If nothing is found by visual inspection and
probing, inspect the passages for a rough ragged
appearance. The roughest internal passages are
probably the ones that are blocked.
Replace a blocked or suspect head and inspect the
replacement head before installing it.