5.0l / 305 cid / 5.7l / 350 cid ENGINES
SERVICE MANUAL NUMBER 24
90-861327--1 OCTOBER 1999
Page 3A-17
Detonation
Detonation, commonly called “fuel knock,” “spark knock” or “carbon knock,” is abnormal
combustion of the fuel causing the fuel to explode violently. The explosion causes
overheating or damage to the spark plugs, pistons, valves and, in severe cases, results in
pre-ignition.
DETONATION CAUSES
1. Use of low octane gasoline or neglecting engine maintenance.
IMPORTANT: Use of improper fuels will cause engine damage and poor performance.
2. Over-advanced ignition timing.
3. Lean fuel mixture at or near full throttle (could be caused by carburetor or leaking intake
manifold).
4. Cross-firing spark plugs.
5. Excess accumulation of deposits on piston and/or combustion chamber that result in
higher compression ratio.
6. Inadequate cooling of engine due to deterioration of cooling system.
NOTE: Engine failures, that result from the foregoing conditions are beyond the control of
Mercury MerCruiser. No warranty will apply to failures that occur under these conditions.
72425
Detonation Damage
72315
a
c
d
b
a -
Spark Occurs
b -
Combustion Begins
c -
Combustion Continues
d -
Detonation Occurs
Index
