Used Spark Plug Analysis; Normal Condition - Mercury MerCruiser Service Manual

Engine Troubleshooting
• 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, exercise caution.
Description
Stethoscope
• 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, the crankshaft striking the pan, a rocker arm striking valve cover, or
a loose flywheel cover. In many cases if this is found to be the problem, a complete engine tear‑down is not necessary.
• When noise is isolated to a certain area and component, removal and inspection is required. Refer to proper sections of
the Service Manual for information required for service.
• If noise cannot be isolated to the engine or the drive unit, remove the drive from the boat. Bring a water supply directly to
the engine. Operate the engine without the drive to determine if noise is still present.

Used Spark Plug Analysis

Spark plug condition can suggest a variety of possible engine malfunctions and indicate needed engine repairs. When the old
plugs are replaced, replace the entire set. Perform plug service only on those plugs suitable for additional service using the
following procedures:
1. Remove any oil deposits with solvent and dry plugs thoroughly.
2. Open the electrode gap wide enough to permit cleaning and filing.
3. Remove the combustion deposits from the firing end of the spark plug with a plug cleaner. Use compressed air to remove
abrasives.
4. File the electrode surfaces to restore clean, sharp edges. Remove filings with compressed air.
5. Reset the gap to specifications by bending only the side electrode with the proper tool.

Normal Condition

Few deposits are present and probably will be light tan or gray in color. This plug shows that the plug heat range is compatible
with the engine, and the engine is electrically and mechanically in good running condition. With proper plug servicing (clean,
file, and regap), this plug can be reinstalled with good results.
Page 3C-4 © 2016 Mercury Marine
27643
90-8M0099748 eng DECEMBER 2015
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