Guess that yet another cause of pre-ignition would be lean mixture.
Unmetered air (better known as vacuum leak) could be the culprit. if it's
gross, you will hear hissing somewhere, perhaps around the intake
manifold.
If this is any kind of fuel-injected engine (as a 1986 should be), there
will be trouble codes stored in the engine computer; perhaps not very
specific, but worth taking a look, and it takes only a test light to read
them. Does this vehicle have a CEL (check engine light)? If so, is it on?
Again, what kind of mechanic was it that couldn't diagnose the source of
pre-ignition?
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