454 CID (7.4L) / 502 CID (8.2L) SERVICE MANUAL NUMBER 23
Page 3A-22
90-861326--1 MARCH 1999
Piston Failures
Pre-Ignition
Pre-ignition is abnormal fuel ignition, caused by combustion chamber hot spots. Control of
the start of ignition is lost, as combustion pressure rises too early, causing power loss and
rough running. The upward motion on the piston is opposed by the pressure rise. This can
result in extensive damage to the internal parts from the high increase in combustion cham-
ber temperature.
72424
Pre-Ignition Damage
72314
a
b
c
d
Combustion Differences
a-Ignited By Hot Deposits
b-Regular Ignition Spark
c-Ignites Remaining Fuel
d-Flame Front Collide
PRE-IGNITION CAUSES
1. Hot spots in the combustion chamber from glowing deposits (due in turn to the use of
improper oils and/or fuels).
2. Overheated spark plug electrodes (improper heat range or defective plug).
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