Mercury 305 CID, Service Manual, Page: 792 / 967

6B-18 - CLOSED COOLING MODELS 90-823225--1 1096
5. Check condition of locking tabs on cap. Replace
cap if tabs are bent or cracked.
72714
b
c
a
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 pres-
sure 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 seal-
ing properly.
8. Clean coolant recovery passage in heat ex-
changer filler neck with a wire and blow out with
compressed air.
72715
b
a
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.
Testing Closed Cooling
System
Testing Coolant for Alkalinity
! WARNING
Allow engine to cool before removing pressure
cap as sudden loss of pressure could cause hot
coolant to boil and discharge violently. After en-
gine has cooled, turn cap 1/4 turn to allow any
pressure to escape slowly, then push down and
turn cap all the way off.
Coolant in fresh water section should be changed ev-
ery two years and should be checked for alkalinity at
least once between change intervals. To check cool-
ant for alkalinity, proceed as follows:
1. Obtain pink litmus paper from a local supplier
(drug store, pet shop, etc.).
2. Remove pressure cap from heat exchanger and
insert one end of litmus paper into coolant.
3. If pink litmus paper turns blue, coolant is alka-
line and need not be replaced.
4. If pink litmus paper remains pink, coolant is
not alkaline and MUST BE REPLACED, as ex-
plained under “Changing Coolant.”