Interpreting Results - Mercury MerCruiser Product Applications Manual

Interpreting Results

Changes must be made to the fuel system or ventilation system if the readings fall outside
of the specifications or the engine bogs or stalls under hard acceleration. Potential causes
may be:
Hesitation
Stumble
Bog
Stall
Excessive engine compartment temperature or pressure differential
• Insufficient cross-sectional area of air vents and related hardware.
• Inlet and exhaust vents improperly located on boat (too close to one another, both in
high pressure or low pressure areas).
• Vent hoses too restrictive (excessive bends, too small, laying in water).
• Vent hoses not positioned to allow good circulation of air.
• A blower may be required to get proper ventilation.
Excessive Fuel Inlet Temperature
• Same as above.
• Too much of fuel tank, lines, and related hardware exposed to engine compartment–
reduce or insulate.
• Fuel line located too high in engine compartment (warmest air is at top of compartment).
• Fuel line too close to hot surface on engine.
• Refer to Section 5—Fuel System for more information.
Excessive Fuel Pressure Drop
• Fuel system excessively restrictive (lines too small, too many fittings).
• Auxiliary fuel filter utilized.
• Anti-siphon valve too restrictive.
• Refer to Section 5—Fuel System for more information.
Everything Within Specifications, But Engine Still Hesitates, Stumbles, Bogs, or
Stalls.
• RVP of fuel too high.
90-863758060 AUGUST 2006
Condition
The throttle is opened and there is a 1–2 second hesitation before the engine responds
and possibly a backfire. Some hesitation is acceptable in moderate to extreme
conditions.
The engine accelerates and drops 200–1000 RPM for a moment and then recovers.
Backfire may occur. Some stumble is acceptable in extreme conditions.
Much more severe than a stumble. The engine drops over 2000 RPM after acceleration
for 10 or more seconds. Backfire may occur. The engine barely recovers.
The engine ceases to run or fails to start.
Symptom
Air Intake System
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