tion will decrease or stop when plug wire is re-
moved from leaking cylinder. Stop engine.
c. At Higher Speeds (4000+ RPM): Normal op-
eration is the same as described in “b” above.
A failed head gasket will cause the bubbles to
come faster and may be accompanied by vio-
lent, intermittent bursts of coolant.
b. Remove spark plugs (one at a time) from cyl-
inders and examine for presence of coolant.
A spark plug that is perfectly clean or milky
appearing is a sure indication of a leak.
It is important not to confuse normal warm-up expan-
sion with a failed head gasket. Normal warm-up pro-
duces an intermittent flow of coolant which 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 pas-
sage of air. This may be accompanied by violent, in-
termittent bursts of coolant leaving the reservoir. If
coolant continues to flow (not in violent, intermittent
bursts) from the reservoir at cruising speed, some-
thing else besides the head gasket is causing the en-
gine to overheat.
c. Drain oil from engine and examine for pres-
ence of coolant. Oil usually will be milky if
coolant is present. If coolant is present, re-
move engine from boat and drop the oil pan.
With engine in the upright position, re-pres-
surize closed cooling section to previously
specified amount and examine internal sur-
faces of engine to locate leak.
d. Ifnoleakagecanbefoundinabovesteps,en-
tire engine must be disassembled and in-
spected for leakage.
Testing Heat Exchanger
Testing for Cylinder Head Gasket
Leak
FOR INTERNAL LEAK: An internal leak will cause
coolant to go into the seawater circuit when pressure
is put on the closed cooling circuit.
A leaking head gasket will cause combustion gas to
be forced into the cooling system. The mixture of
coolant and tiny air bubbles is a poor heat conductor
and will overheat an engine quickly. Compression
tests or cooling system pressure check normally will
not detect the leak because the test pressure is far
below the combustion pressures which cause the
leak. An effective test is as follows:
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.
IMPORTANT: Run boat in lake for this test. It is
best to run the engine at or above cruising speed
during this test. Usually a failed head gasket will
not cause the engine to overheat below cruising
speed.
FOR BLOCKAGE:
IMPORTANT: Seawater flows THROUGH the
tubes in the exchanger. Closed cooling coolant
flows AROUND the tubes.
1.
Install a clear plastic hose between the reservoir
and coolant recovery bottle. Use a 2-3 ft.
1.
Remove end caps and inspect for any blockage
in the seawater circuit (broken impeller blades,
weeds, etc.).
(610-910
mm) long hose for this test.
2.
3.
Route this hose so a “U” is formed.
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.
Put enough coolant into hose to fill the center 4
or 5 inches (100-130 mm) of the “U.”
4.
Observe the “U” while the engine is running.
a. During Idle and Warm-Up: Some coolant
and/or air will leave the reservoir.
Testing Pressure Cap
b. During Cruising Speed (2500-3500 RPM):
Coolant and/or air leaving the reservoir
should stop after approximately five minutes
running at a given RPM. A leaking head gas-
ket will produce air bubbling through the “U,”
going to the coolant recovery bottle. The fre-
quency and size of the bubbles will depend
on the size of the leak.
Pressure cap is designed to maintain a pressure of
approximately its rated capacity (refer to “Specifica-
tions”) in closed cooling section once engine has at-
tained operating temperature. Cap should be
cleaned, inspected and pressure-tested at regular
tune-up intervals or whenever cap is suspected of
maintaining improper pressure as follows:
Index
90-823324--2 796
CLOSED COOLING SYSTEMS - 6B-5
| Categories | Mercury MerCruiser Manuals |
|---|---|
| Tags | Mercury MerCruiser 454 CID, Mercury MerCruiser 502 CID |
| Model Year | 1993, 1994, 1995, 1996, 1997 |
| Download File |
|
| Document File Type | |
| Copyright | Attribution Non-commercial |
(4 votes, average: 4.5 out of 5)
Marine readers have rated 1993 1997 Mercury-MerCruiser GM V8 454 CID 7.4L and 502 CID 8.2L Marine Engines Service Manual Number 16 4.5 out of 5.0 based on 4 product reviews. Hugely helpful service manual! Perfect. Thank you!
Manuals are all Important for Technician
This book is invaluable if you do your own repairs. Great book.
Could not find a copy until a search brought me here.
Excellent print.
Thank you