There is a limit to how much the PCM can adjust the timing. In
response to "bad" gas or other factors, the PCM will retard the timing
(and in some cases adjust other parameters) to try to eliminate the
pre-ignition. But it the gas to really bad, or the other factors are
excessive, you can still get pre-ignition. I've had it happen myself.
Ed - Be carefull of your terminology. Pre-Ignition and Detonation
are two very different (although also closely connected) problems.
Spark timing has no effect on pre-ignition other than the fact that if
you advance the timing far enough, the "pre-ignition" will be after
the spark. Pretty hard to do that and still have the engine run
reasonably well though. By definition, pre-ignition occurs BEFORE the
Pre-ignition CAN cause detonation, and detonation CAN contribute to
pre-ignition, but both CAN occur separately from each other.
The PCM can NOT correct for pre-ignition - only detonation.
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