CLOSED COOLING SYSTEM
SERVICE MANUAL NUMBER 22
Page 6B-14
90-860074--1 FEBRUARY 2002
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 follows.
IMPORTANT: Operate boat in lake for this test. It is best to operate 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
61-91 cm (2-3 ft) long hose for this test.
2. Route this hose so a U is formed.
3. Put enough coolant into hose to fill the center 10-13 cm (4 or 5 in.) of the U.
4. Start the engine and observe the U while the engine is operating.
a. During Idle and Warm-Up: Some coolant and/or air will leave the reservoir.
b. During Cruising Speed (2800 - 3200 rpm): Coolant and/or air leaving the reservoir
should stop after approximately five minutes of operation at a steady 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 (3200 - 3600 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.
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 steady 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 (not in violent,
intermittent bursts) from the reservoir at cruising speed, something other than the head
gasket is causing the engine to overheat.
