Testing For Cylinder Head Gasket Leak - Mercury MerCruiser MCM 454 Mag MPI Service Manual

SERVICE MANUAL NUMBER 23

Testing for Cylinder Head Gasket Leak

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:
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.
1. Install a clear plastic hose between the reservoir and coolant recovery bottle. Use a
2. Route this hose so a "U" is formed.
3. 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.
It is important not to confuse normal warm-up expansion with a failed head gasket. Normal
warm-up produces 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 passage of air. This may be accompanied by violent,
intermittent bursts of coolant leaving the reservoir. If coolant continues to flow smoothly from
the reservoir at cruising speed, something other than the head gasket is causing the engine
to overheat.
Index
90-861326--1 MARCH 1999
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c. Drain oil from engine and examine for presence of coolant. Oil usually will be milky
if coolant is present. If coolant is present, remove engine from boat and drop the oil
pan. With engine in the upright position, re-pressurize closed cooling section to
previously specified amount and examine internal surfaces of engine to locate leak.
d. If no leakage can be found in above steps, entire engine must be disassembled and
inspected for leakage.
2-3 ft (610-910 mm) long hose for this test.
a. During Idle and Warm-Up: Some coolant and/or air will leave the reservoir.
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
gasket will produce air bubbling through the "U," going to the coolant recovery bottle.
The frequency and size of the bubbles will depend on the size of the leak.
c. At Higher Speeds (4000+ rpm): Normal operation is the same as described in "b"
above. A failed head gasket will cause the bubbles to come faster and may be
accompanied by violent, intermittent bursts of coolant.
CLOSED COOLED (FRESH WATER) MODELS
Page 6B-7