Troubleshooting With A Vacuum Gauge; Vacuum Gauge Troubleshooting Guide - Mercury MerCruiser Service Manual

Engine Troubleshooting
Special Tools
Digital Pressure Meter

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
17756
17757
17758
17759
Page 3C-2
Connects to the fuel system and can be used in conjunction with Computer
Diagnostic System (CDS).
5786
Symptom
Steady reading 51–71 kPa
(15–21 in. Hg) at idle RPM
Extremely low reading, but indicator
steady at idle RPM
Indicator fluctuates between high
and low at idle RPM
Indicator fluctuates 13.5–17 kPa
(4–5 in. Hg) very slowly at idle RPM
© 2016 Mercury Marine
91‑892651A01
Cause
Normal
Vacuum leak at intake
manifold or incorrect timing,
or underpowered boat.
Blown cylinder head gasket
between two adjacent
cylinders. (Check with
compression test.)
Valves are sticking or spark
plug gap is too narrow.
90-8M0099748 eng DECEMBER 2015
Action
No action necessary.
Determine the source of the
vacuum leak and repair. Replace
the propeller. If the problem
persists, contact the
manufacturer about the correct
power package.
Determine the cause and
replace the cylinder head
gasket.
Inspect the spark plugs and
service or replace if necessary.
Correct sticking valve.