SERVICE MANUAL NUMBER 23
TROUBLESHOOTING
Troubleshooting with Vacuum Gauge
Reading
Cause
Steady reading between 15-21 inches at
idle rpm
Normal
Extremely low reading, but steady at idle
rpm
Vacuum leak; incorrect timing;
underpowered boat; faulty boat bottom
Fluctuates between high and low at idle
rpm
Blown head gasket between two adjacent
cylinders
Fluctuates 4 or 5 inches very slowly at idle Spark plug gap too narrow, valves are
rpm
sticking
Fluctuates rapidly at idle, steadies as rpm
is increased
Valve guides are worn
Continuously fluctuates between low and
normal reading at regular intervals at idle
rpm
Burned or leaking valve
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
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.
2.
3.
The use of a stethoscope can aid in locating a noise source; however, because noise
will travel to other metal parts not involved in the problem, caution must be exercised.
If you believe noise is confined to one particular cylinder, ground spark plug leads, one
at a time. If noise lessens noticeably or disappears, it is isolated to that particular
cylinder.
4.
5.
Try to isolate the noise to location in engine: front to back, top to bottom. This can help
determine which components are at fault.
Sometimes noises can be caused by moving parts coming in contact with other
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.
6.
7.
When noise is isolated to a certain area and component, removal and inspection will be
required. Refer to proper sections of service manual for information required for service.
If noise cannot be distinguished between engine and drive unit, remove drive from boat.
Run a water supply directly to engine. Run engine without the drive to determine if noise
is still there.
90-861326--1 MARCH 1999
Page 1C-15
| Categories | Mercury MerCruiser Manuals |
|---|---|
| Tags | Mercury MerCruiser 454 CID, Mercury MerCruiser 502 CID |
| Model Year | 1998, 1999, 2000, 2001 |
| Download File |
|
| Document File Type | |
| Copyright | Attribution Non-commercial |
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