Pressure Cap Maintenance - Mercury MerCruiser MCM 454 Mag MPI Service Manual

CLOSED COOLED (FRESH WATER) MODELS
IMPORTANT: ALCOHOL OR METHANOL BASE ANTIFREEZE OR PLAIN WATER ARE
NOT RECOMMENDED FOR USE IN COOLING SYSTEM AT ANY TIME. In areas where
ethylene glycol is not available, and the possibility of freezing does not exist, it is
permissible to use a solution of rust inhibitor and pure, soft water (mixed to
manufacturer's recommendations).
Occasionally, ensure that coolant recovery system is functioning properly by removing
pressure cap from heat exchanger and checking level. Coolant level should be up to bottom
of heat exchanger filler neck. If low, examine entire fresh water section (especially coolant
recovery system) for leaks and repair, if necessary.
IMPORTANT: When reinstalling pressure cap, be sure to tighten it until it contacts
stops on filler neck.

Pressure Cap Maintenance

Pressure cap is designed to maintain pressure in fresh water 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 operation, cap should be cleaned, inspected and pressure tested periodically as
follows:
Allow engine to cool before removing pressure cap (in next step), as sudden loss
of pressure could cause hot coolant to boil and discharge violently. After engine
has cooled, turn cap 1/4 turn to allow any pressure to escape slowly, then push
down and turn cap all the way off.
1. Remove pressure cap from heat exchanger.
2. Wash cap with clean water to remove any deposits or debris from sealing surfaces.
3. Inspect rubber seal on cap for cuts, cracks or other signs of deterioration. If seal is dam-
aged, cap MUST BE replaced.
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. Refer to "Testing Pressure Cap" and test as outlined.
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.
Index
Page 6B-4
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c
a - Rubber Seal (Cap MUST BE Replaced If Damaged)
b - Gasket (Look For Cracks Under Gasket)
c - Locking Tabs (1 Hidden)
WARNING
a
b
SERVICE MANUAL NUMBER 23
72714
90-861326--1 MARCH 1999