HT, you have a very good grasp on the subject of pre-ignition... A gold
star
for you 8^)
Anything that will cause the combustion event to occur early can give
pre-ignition... this can be as simple as improperly adjusted base timing,
inadequate fuel quality, any kind of "hot spot" in the combustion chamber
or even having the compression ratio being influenced by deposits....
Unmetered air might be a vacuum leak or, if it is upstream of the throttle
plates, it wouldn't be a vacuum leak and there would be subtle differences
in the affect. In 1986, however, I don't recall Ford having much to do
with
anything other than speed/density based systems.
For the OP.... it is normal for spark plug electrodes to suffer from
erosion... Pre-ignition will tend to leave deposits on the ground
electrode
if it is severe or has been left unattended for any length of time...
There
will be an accompanying , unmistakeable noise under acceleration. At this
point, I am left to doubt that pre-ignition or engine "ping" is part of
the
complaint. If your mechanic equates worn electrodes with pre-ignition, I
would get worried (Elmer Fudd told us to "Be afwaid... be vewy, vewy
afwaid").
If this vehicle is, indeed, a 1986.... IIRC, Ford used a "negative
feedback"
type EGR valve... There is a small ****t inside the valve that can carbon
over and this would have the effect of having the car "nose over" on light
acceleration... In effect, the EGR valve opens too far, too early. Going
to
WOT (wide open throttle) seems to "cure" the problem - this is because at
low RPM WOT, manifold vacuum drops to very close to atmospheric pressure.
Without sufficient vacuum, the EGR valve closes and the problem seems to
go
away.
If the car is an Explorer... one would need to know the year and,
possibly,
the engine "flavour" to say anything meaningful.
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