1995 Ford E350 RV 460 Pre-detonation Problem

The truck has 15000 miles on it and has been thru 3 engines due to pre-detonation. Timing is correct but jumps around a bit, have verified
chain to be OK. has new distributer and pickup, ICM checks OK, Ive been
thru idle air , map & 02 sensors , any ideas what else could cause this
condition besides fuel quality???
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