Service manual

181 CID (3.0L) 4 CYLINDER GM ENGINE
SERVICE MANUAL NUMBER 26
90-861329--1 MARCH 1999 Page 3A-13
Detonation
Detonation, commonly called “fuel knock,” “spark knock” or “carbon knock,” is abnormal
combustion of the fuel which causes 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.
CAUSES
IMPORTANT: Use of improper fuels will cause engine damage and poor performance.
1. Use of low octane gasoline is one of the most common causes of detonation. Even with
high octane gasoline, detonation could occur if engine maintenance is neglected.
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 (results in
higher compression ratio).
6. Inadequate cooling of engine by deterioration of cooling system.
Engine failures that result from the foregoing conditions are beyond the control of
MerCruiser. No warranty will apply to failures which occur under these conditions.
72425
Detonation Damage
72315
a
b
c
d
a-Spark Occurs
b-Combustion Begins
c-Combustion Continues
d-Detonation Occurs
Index