MerCruiser 13, Service Manual

Page: 313 / 348

CLOSED COOLED MODELS - 6B - 3 90-816462 2-695
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 explained
under “Changing Coolant.”
Pressure Testing System
! WARNING
Allow engine to cool before removing pressure
cap. 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.
If coolant section of closed cooling system is sus-
pected of leaking or not holding sufficient pressure,
and no visible signs of leakage can be found, perform
the following test:
1. Remove pressure cap from heat exchanger or
reservoir.
2. Clean, inspect and pressure test pressure cap, as
outlined under “Testing Pressure Cap,” to elimi-
nate the possibility that cap is not maintaining
proper pressure in system and is causing coolant
to boil over.
3. Clean inside of filler neck to remove any deposits
or debris. Examine lower inside sealing surface
for nicks or other damage. Surface must be per-
fectly smooth to achieve a good seal between it
and rubber seal on cap. Also check locking cams
on sides of filler neck to be sure that they are not
bent or damaged. If locking cams are bent or
damaged, pressure cap will not hold the proper
pressure.
4. Adjust coolant level in fresh water section to 1 in.
(25 mm) below filler neck.
5. Attach an automotive-type cooling system pres-
sure tester to filler neck and pressurize closed
cooling section to amount specified in following
chart, based on pressure cap rating for your en-
gine.
Pressure Cap Rating
Amount of Pressure
Applied to Closed
Cooling System
14 PSI (97 kPa) 17 PSI (117 kPa)
6. Observe gauge reading for approximately two
minutes; pressure should not drop during this
time. If pressure drops, proceed with the following
steps until leakage is found.
7. While maintaining specified pressure on closed
cooling section, visually inspect external portion
of cooling system (hoses, gaskets, drain plugs,
petcocks, core plugs, circulating pump seal, etc.)
for leakage. Also listen closely for bubbling or
hissing, as they usually are a sure indication of a
leak.
8. Refer to “Testing Heat Exchanger” in this section
and test as outlined.
9. If no leakage could be found in above steps, en-
gine is leaking internally, and it probably is due to
one or more of the following: (1) loose cylinder
head bolts or damaged gasket, (2) loose intake
manifold bolts or damaged gasket, (3) loose ex-
haust elbow or distribution block retaining nuts or
damaged gasket, (4) cracked or porous cylinder
head or block, or (5) cracked or porous exhaust
manifold. Proceed as follows until location of in-
ternal leak is found.