Detonation; Elevation, Weather, And Climate; Propeller Selection; Water Absorption - Mercury 15 EFI FourStroke Service Manual

Detonation

Detonation in a 4‑cycle engine resembles the pinging heard in an automobile engine. It can be otherwise described as a tin‑like
rattling or plinking sound.
Detonation is the explosion of the unburned fuel/air charge after the spark plug has fired. Detonation creates severe shock
waves in the engine. These shock waves often find or create a weakness: the dome of a piston, cylinder head or gasket, piston
rings or piston ring lands, piston pin, and roller bearings.
A few of the most common causes of detonation in a marine 4‑cycle application are as follows:
• Over‑advanced ignition timing
• Use of low octane gasoline
• Propeller pitch too high: engine RPM below recommended maximum range
• Lean fuel mixture at, or near, wide‑open throttle
• Spark plugs: heat range too hot, incorrect reach, cross‑firing
• Deteriorated or inadequate engine cooling system
• Combustion chamber deposits: result in higher compression ratio
Detonation usually can be prevented if:
• The engine is correctly set up
• Regular maintenance is scheduled

Elevation, Weather, and Climate

Elevation, weather, and climate changes affect engine performance because an engine's power depends upon the density of
the air that it consumes. Air density, in turn, is dependent upon the ambient air temperature, the barometric pressure, and the
humidity (water vapor) content. Loss of performance can therefore be caused by:
• Higher elevations
• Higher temperatures
• Low barometric pressures
• High humidity
For optimum engine performance under changing conditions, it is essential that the engine be propped to allow the engine to
operate at or near the top end of the specified maximum RPM range with a normal boat load during your normal boating
weather conditions. In most cases, recommended RPM can be achieved by changing to a lower pitch propeller.

Propeller Selection

IMPORTANT: The engines covered in this manual are equipped with an RPM rev‑limiter that is set to an upper RPM limit. This
limit, which is slightly above the normal operating range of the engine, helps prevent damage from excessive engine RPM.
Once the RPM returns to the recommended operating RPM range, normal engine operation resumes.
The boat manufacturer and the selling dealer are responsible for equipping the power package with the correct propeller. Refer
to Mercury Marine's web page https://www.mercurymarine.com/en/us/propellers/selector/#/step‑one.
Select a propeller that will allow the engine power package to operate at or near the top end of the recommended WOT
operating RPM range with a normal load.
If full‑throttle operation is below the recommended range, the propeller must be changed to prevent loss of performance and
possible engine damage. On the other hand, operating an engine above the recommended operating RPM range will cause
higher than normal wear and damage.
After initial propeller selection, the following common problems may require that the propeller be changed to a lower pitch.
• Warmer weather and greater humidity cause a loss of RPM.
• Operating in a higher elevation causes a loss of RPM.
• Operating with a dirty boat bottom causes a loss of RPM.
• Operating with increased load (additional passengers, pulling skiers) causes a loss of RPM.
For better acceleration, such as is needed for waterskiing, use the next lower pitch propeller. When not pulling skiers, do not
operate at full throttle when using the lower pitch propeller.

Water Absorption

It is imperative that all through‑the‑hull fasteners be coated with a quality marine sealer at time of installation. Water intrusion
into the transom core and/or inner hull will result in additional boat weight (reduced boat performance), hull decay, and eventual
structural failure.

Weight Distribution (Passengers and Gear) Inside the Boat

Shifting weight to rear (stern):
90-8M0125265 eng NOVEMBER 2017 © 2018 Mercury Marine
General Troubleshooting
Page 1E-3