Chevrolet Light Duty Truck 1973 Service Manual page 419

4. Continue this flushing until the water from the
lead-away hose runs clear.
Heater Core
1. Remove water outlet hose from heater core pipe.
2. Remove inlet hose from engine connection.
3. Insert flushing gun and flush heater core. Care must
be taken when applying air pressure to prevent
damage to the core.
Fan Belt Adjustment
1. Loosen bolts at Delcotron mounting.
2. Pull Decotron away from engine until desired
tension reading is obtained with a strand tension
gauge. Refer to "Engine Tune Up Specifications" .
3. Tighten all Delcotron bolts securely.
Radiator Cap
The radiator cap should be washed with clean water and
pressure checked at regular tune-up intervals. Inspect
rubber seal on cap for tears or cracks. Install radiator
cap on tester (fig. 4). If the pressure cap will not hold
pressure or does not release at the proper pressure,
replace the cap.
Thermostat
The thermostat consists of a restriction valve actuated by
a thermostatic element. This is mounted in the housing
at the cylinder head water outlet above the water pump.
Thermostats are designed to open and close at
predetermined temperatures and if not operating
properly should be removed and tested as follows:
Replacement
1. Remove radiator to water outlet hose.
2. Remove thermostat housing bolts and remove water
outlet and gasket from thermostat housing (fig. 11).
Fig. 10—Pressure Checking Radiator Cap
E N G I N E C O O L I N G
3. Inspect thermostat valve to make sure it is in good
condition.
4. Place thermostat in a 33% glycol solution 25° above
the temperature stamped on the thermostat valve.
5. Submerge the valve completely and agitate the
water thoroughly. Under this condition the valve
should open fully.
6. Remove the thermostat and place in a 33% glycol
solution 10° below temperature indicated on the
valve.
7. With valve completely submerged and water
agitated thoroughly, the valve should close
completely.
8. If thermostat checks satisfactorily, re-install, using a
new housing gasket.
9. Refill cooling system.
Thermostatic Fan Clutch
Replacement
All mating surfaces (water pump hub and fan clutch
hub) should be inspected for smooth mating surfaces
and reworked as necessary to eliminate burrs or other
imperfections. Except for the fan belt, components
should be assembled to the engine (See Water Pump
Removal and Installation Procedures). Radial run-out
should be checked as follows:
1. Secure the fan blade to prevent rotation. (See
Figure 12.)
2. Mount a dial indicator (.001 graduations) to the
engine and place the indicator pointer on the fan
blade spider. Preferably on the longest band or
space on the spider. (See Figure 13.)
3. Rotate the water pump pulley in one direction and
note the total amount of indicator needle
Fig. 11—Replacing Thermostat
6 K -9
LIGHT DUTY TRUCK SERVICE MANUAL