Measuring Vacuum; Mechanical Engine Noise - Mercury MerCruiser Service Manual

Gauge Reading
17760
17761
17762
1
3
17763
2

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
Carefully follow the gauge manufacturer's instructions to ensure accurate vacuum measurement.

Mechanical Engine Noise

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.
90-8M0099748 eng DECEMBER 2015
Symptom
Indicator fluctuates rapidly at idle,
steadies as RPM is increased
Indicator continuously fluctuates
between low and normal reading at
regular intervals at idle RPM
Indicator drops to zero as engine
RPM is increased
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
Cause
Valve guides may be worn.
Burned or leaking valve.
Exhaust system is
restricted.
Possible piston ring leak
(check the compression).
91‑892651A01
WARNING
!
© 2016 Mercury Marine
Engine Troubleshooting
Action
Ream the valve guides and
install a valve with an oversized
stem or replace the cylinder
head.
Replace the valve.
Clear exhaust system.
Repair or replace as needed.
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