CLOSED COOLED MODELS
SERVICE MANUAL NUMBER 24
Do not confuse normal warm-up expansion with a failed head gasket. Normal warm-up
produces an intermittent flow of coolant that will stop within approximately five minutes at
a given rpm. A head gasket leak will not stop because the one thing that marks a failed head
gasket is the continued passage of air. This may be accompanied by violent, intermittent
bursts of coolant leaving the reservoir. If coolant flows evenly from the reservoir at cruising
speed, something other than the head gasket is causing the engine to overheat.
Testing Heat Exchanger
FOR INTERNAL LEAK: An internal leak will cause coolant to go into the seawater circuit
when pressure is put on the closed cooling circuit.
1.
2.
3.
Remove a seawater hose from the exchanger. Do not drain the exchanger.
Pressurize the closed cooling circuit to 16-20 psi (110-138 kPa) with a radiator tester.
If seawater begins to flow from the nipple there is a leak.
FOR BLOCKAGE:
IMPORTANT: Seawater flows THROUGH the tubes in the exchanger. Closed cooling
coolant flows AROUND the tubes.
1.
Remove end caps and inspect for any blockage in the seawater circuit (broken impeller
blades, weeds, etc.).
2.
Remove closed cooling circuit hoses and inspect the tubes just inside the nipples.
Because the complete exchanger cannot be inspected, the heat exchanger should be
replaced if blockage is suspected.
Testing Pressure Cap
Pressure cap is designed to maintain a pressure of approximately its rated capacity (refer
to “Specifications”) in closed cooling section once engine has attained operating tempera-
ture. Cap should be cleaned, inspected and pressure-tested at regular tune-up intervals or
whenever cap is suspected of maintaining improper pressure as follows:
WARNING
Allow engine to cool before removing pressure cap. 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.
Carefully remove pressure cap from reservoir or heat exchanger.
2.
3.
Wash cap with clean water to remove any deposits or debris from sealing surfaces.
Inspect gasket (if used) and rubber seal on cap for tears, cuts, cracks or other signs of
deterioration. Replace gasket, if damaged, or entire cap if rubber seal is damaged.
4.
Check condition of locking tabs on cap. Replace cap if tabs are bent or cracked.
Index
Page 6B-8
90-861327--1 OCTOBER 1999
| Categories | Mercury MerCruiser Manuals |
|---|---|
| Tags | Mercury MerCruiser 305 CID, Mercury MerCruiser 350 CID |
| Model Year | 1998, 1999, 2000, 2001 |
| Download File |
|
| Document File Type | |
| Copyright | Attribution Non-commercial |
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Marine readers have rated Mercury MerCruiser Engines Service Manual Number 24 GM V-8 305 CID 350 CID 4.0 out of 4.0 based on 1 product reviews. Great performance engine.
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