1986 Explorer? Explorer was introduced around 1991. Perhaps a Bronco?
Our very knowledgeable Jim Warman hopefully will provide a thorough
explanation. Just in case he is too busy at the moment, here is my humble
understanding:
Pre-ignition means that the mixture ignites too early in the cycle. It
could
be that the plugs fire too early, or that the mixture ignites before the
plugs even fire. That can be caused by glowing carbon deposits in the
cylinder head or by a defective (or disabled) EGR (exhaust gas
recirculation) system. Lack of EGR flow will raise the combustion
temperature, which could lead to pre-ignition. However, EGR only kicks-in
at
cruise conditions, and you are talking about low RPM. So probably not the
case. Any half-decent mechanic should be able to determine which it is.
If this is indeed a mid-80's vehicle, it will have adjustable 'base
timing',
which is done simply by turning the distributor body a few degrees here or
there. It's very easy to check with a simple timing light (your mechanic
will have to disable the advance mechanism by disconnecting a wire or a
vacuum tube, depending whether this is a computer-managed system or not).
However, something is telling me that the problem is not ignition timing,
because due to the advance mechanism, incorrect timing will ordinarily
manifest itself as pinging under heavy load.
In a vehicle that old, a jumped timing chain (there is no timing belt in a
1980's Ford SUV, regardless of model) could create some real havoc. Not
sure
it that would lead to pre-ignition, and the good compression (whatever
that
means) seems to tell that this is not the issue.
But I am going above my head here -- Jim will hopefully enlighten us.
"Scott C" <sns1820@[EMAIL PROTECTED]
> wrote in message
news:hY0Tj.13459$V14.5429@[EMAIL PROTECTED]
>A friend of mine has a 1986 Ford Explorer. It's runs very rough from 700
> (idle) to about 1500 RPM.
>
>
> Can someone explain pre-ignition in more detail than it fires early.
What
> is
> the cause of pre-ignition? A timing belt problem? Seems the timing is
> off,but checks OK.
>
> Thanks
>
> Scott
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