Gauge Reading Symptom Cause Action
17760
Indicator fluctuates rapidly at idle,
steadies as RPM is increased
Valve guides may be worn.
Ream the valve guides and
install a valve with an oversized
stem or replace the cylinder
head.
17761
Indicator continuously fluctuates
between low and normal reading at
regular intervals at idle RPM
Burned or leaking valve. Replace the valve.
17762
Indicator drops to zero as engine
RPM is increased
Exhaust system is
restricted.
Clear exhaust system.
17763
1
2
3
Indicator holds steady at 41–54 kPa
(12–16 in. Hg): (1) Drops back to
zero (2) and back to about 71 kPa
(21 in. Hg) (3) as the throttle is
engaged and released
Possible piston ring leak
(check the compression).
Repair or replace as needed.
Measuring Vacuum
IMPORTANT: Use an accurate digital vacuum gauge that reads in either inches of mercury (in. Hg) or kilopascals (kPa) to
check engine vacuum. Dial vacuum gauges are not accurate enough.
Digital Pressure Meter
91‑892651A01
Carefully follow the gauge manufacturer's instructions to ensure accurate vacuum measurement.
Mechanical Engine Noise
!
WARNING
Moving parts can cause serious injury or death. Wear eye protection and keep hands, hair, and clothing away from moving
parts when performing tests or checking adjustments on an operating engine.
No definite rule or test will positively determine a source of engine noise. Use the following information only as a general guide
to engine noise diagnosis.
• Use a timing light to determine if noise is timed with engine speed or half of the engine speed. Noises timed with engine
speed are related to crankshaft, rods, pistons, piston pins, and flywheel. Noises timed to half engine speed are valve train
related.
• If noise is suspected to be confined to one particular cylinder, ground spark plug leads, one at a time. If noise lessens
noticeably or disappears, the problem is isolated to that cylinder. Noise confined to one cylinder is normally related to a
problem with a wrist pin, rod bearing, or piston slap.
• Try to isolate the noise to one location in the engine. Identify an area front to back or top to bottom. This can help
determine which components are at fault.
Engine Troubleshooting
90-8M0099748 eng DECEMBER 2015 © 2016 Mercury Marine Page 3C-3
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