Pressure Cap Maintenance; Heat Exchanger Repair - MerCruiser GM4 Service Manual

IMPORTANT: ALCOHOL OR METHANOL BASE
ANTIFREEZE OR PLAIN WATER ARE NOT REC-
OMMENDED 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 in-
hibitor and pure, soft water (mixed to manufactur-
er's recommendations).
Occasionally, check to 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 (espe-
cially 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 in-
creasing the efficiency of the cooling system. To help
ensure proper operation, cap should be cleaned, in-
spected and pressure tested periodically as follows:
! WARNING
Allow engine to cool before removing pressure
cap (in next step), as sudden loss of pressure
could cause hot coolant to boil and discharge vi-
olently. 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 depos-
its or debris from sealing surfaces.
3. Inspect rubber seal on cap for cuts, cracks or oth-
er signs of deterioration. If seal is damaged, 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.
Index
6B - 2 - CLOSED COOLING MODELS
c
a - Rubber Seal (Cap MUST BE Replaced if Damaged)
b - Gasket (Look for Cracks under Gasket)
c - Locking Tabs (1 Hidden)
6. Refer to "Testing Pressure Cap" and test pressure
cap as outlined.
7. Clean sealing surfaces on heat exchanger filler
neck with a cloth. Inspect surfaces for any dam-
age or deposits that may prevent cap from sealing
properly.
8. Clean coolant recovery passage in heat exchang-
er filler neck with a wire and blow out with com-
pressed air.
b
a - Inspect for Damage
b - Clean Coolant Recovery Passages
9. Reinstall pressure cap, being sure to tighten until
it contacts stops on filler neck.

Heat Exchanger Repair

IMPORTANT: Braze with BCUP 2 rod or silver sol-
der. Care must be taken not to melt other joints
during repair.
1. Internal leaks can be repaired by brazing shut the
ends of the leaking tube. This is only a temporary
fix because usually another tube will start leaking
after a short period of time and this also causes
a reduction in cooling capacity. Do not close more
than three tubes.
2. Nipples and drains that have been broken off the
heat exchanger can be reattached by brazing.
a
b
72714
a
72715
90-816462 2-695