Mercury 305 CID, Service Manual, Page: 811 / 967

CLOSED COOLING MODELS - 6B-37 90-823225--1 1096
Pressure Cap Maintenance
! WARNING
Allow engine to cool down before removing pres-
sure cap (in next step), as sudden loss of pres-
sure could cause hot coolant to boil and dis-
charge violently. After engine has cooled down,
turn cap 1/4-turn to allow any pressure to escape
slowly, then, push down and turn cap all-the-way
off.
Pressure cap is designed to maintain a pressure of
approximately 16 psi (110 kPa) in coolant section of
Closed Cooling System once the engine has attained
normal operating temperature. This raises the boiling
point of the coolant, thereby increasing the efficiency
of the cooling system. To help ensure proper opera-
tion, cap should be clean, inspected and pressure
tested (at intervals specified in chart), as follows:
1. Remove pressure cap from heat exchanger.
2. Wash cap with clean water to remove any depos-
its or debris from sealing surfaces.
3. Inspect rubber seal on cap for cuts, cracks or
other signs of deterioration. If seal is damaged,
cap MUST BE replaced.
70970
c b
a
c
a - Rubber Seal
b - Coolant Recovery Gasket
c - Locking Tabs (1 Hidden)
4. Inspect coolant recovery gasket for deterioration
and replace if bad.
5. Check condition of locking tabs on cap. Replace
cap, if tabs are bent or cracked.
6. Have pressure cap pressure-tested by your local
servicing dealer.
7. Clean sealing surfaces on heat exchanger filler
neck with a cloth. Inspect surfaces for any
damage or deposits that may prevent cap from
sealing properly.
8. Clean coolant recovery passage in heat ex-
changer filler neck with a wire and blow out with
compressed air.
9. Reinstall pressure cap, being sure to tighten until
its contacts stops on filler neck.
70260
a
b
a - Pressure Cap Sealing Surfaces
b - Coolant Recovery Passage