Special Tools
Digital Pressure Meter 91‑892651A01
Connects to the fuel system and can be used in conjunction with Computer
Diagnostic System (CDS).
Troubleshooting With a Vacuum Gauge
Most engines have a normal gauge reading of 51–71 kPa (15–21 in. Hg) vacuum. Before using the vacuum gauge the engine
must be at normal operating temperature. Use a tachometer to be certain that the engine is running at the specified RPM. The
vacuum gauge must be connected to the intake manifold at a manifold vacuum source.
The following table indicates possible malfunctions of various vacuum readings.
Vacuum Gauge Troubleshooting Guide
Gauge Reading
Symptom Cause Action
17756
Steady reading 51–71 kPa
(15–21 in. Hg) at idle RPM
Normal No action necessary.
17757
Extremely low reading, but indicator
steady at idle RPM
Vacuum leak at intake
manifold or incorrect timing,
or underpowered boat.
Determine the source of the
vacuum leak and repair. Replace
the propeller. If the problem
persists, contact the
manufacturer about the correct
power package.
17758
Indicator fluctuates between high
and low at idle RPM
Blown cylinder head gasket
between two adjacent
cylinders. (Check with
compression test.)
Determine the cause and
replace the cylinder head
gasket.
17759
Indicator fluctuates 13.5–17 kPa
(4–5 in. Hg) very slowly at idle RPM
Valves are sticking or spark
plug gap is too narrow.
Inspect the spark plugs and
service or replace if necessary.
Correct sticking valve.
Engine Troubleshooting
Page 3C-2 © 2016 Mercury Marine 90-8M0099748 eng DECEMBER 2015
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