Troubleshooting With Vacuum Gauge; Engine Noise; Important Information - Mercury MerCruiser MCM 454 Mag MPI Service Manual

SERVICE MANUAL NUMBER 23

Troubleshooting with Vacuum Gauge

Steady reading between 15-21 inches at
idle rpm
Extremely low reading, but steady at idle
rpm
Fluctuates between high and low at idle
rpm
Fluctuates 4 or 5 inches very slowly at idle
rpm
Fluctuates rapidly at idle, steadies as rpm
is increased
Continuously fluctuates between low and
normal reading at regular intervals at idle
rpm

Engine Noise

Important Information

No definite rule or test will positively determine source of engine noise; therefore, use the
following information only as a general guide to engine noise diagnosis.
1. Use a timing light to determine if noise is timed with engine speed or one-half engine
2. The use of a stethoscope can aid in locating a noise source; however, because noise
3. If you believe noise is confined to one particular cylinder, ground spark plug leads, one
4. Try to isolate the noise to location in engine: front to back, top to bottom. This can help
5. Sometimes noises can be caused by moving parts coming in contact with other
6. When noise is isolated to a certain area and component, removal and inspection will be
7. If noise cannot be distinguished between engine and drive unit, remove drive from boat.
Index Index
90-861326--1 MARCH 1999
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Reading
speed. Noises timed with engine speed are related to crankshaft, rods, pistons, piston
pins, and flywheel. Noises timed to one-half engine speed are valve train related.
will travel to other metal parts not involved in the problem, caution must be exercised.
at a time. If noise lessens noticeably or disappears, it is isolated to that particular
cylinder.
determine which components are at fault.
components. Examples are: flywheel or coupler; exhaust flappers rattling against
exhaust pipe; crankshaft striking (pan, pan baffle, or dipstick tube); rocker arm striking
valve cover; and loose flywheel cover. In many cases if this is found to be the problem,
a complete engine teardown is not necessary.
required. Refer to proper sections of service manual for information required for service.
Run a water supply directly to engine. Run engine without the drive to determine if noise
is still there.
Cause
Normal
Vacuum leak; incorrect timing;
underpowered boat; faulty boat bottom
Blown head gasket between two adjacent
cylinders
Spark plug gap too narrow, valves are
sticking
Valve guides are worn
Burned or leaking valve
TROUBLESHOOTING
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