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On 9 Jul 97, Woolston Craig wrote:
> Lately my '71 FI Fastback as been continuing to run after I turn the
> key off. It seems to last longer the warmer the engine is. I know
> that the timing is correct, the valves adjusted and the car runs
> fine the rest of the time (slightly rich, which is why I suspect
> fouled plugs). Could foul plugs cause the this condition? I'm
> assuming that the fouled plugs get hot and continue to provide
> enough heat ("spark") after the ignition is turn off.
Your car must have been converted to carbs, with the idle shutoff
valves leaking or disabled or the carbs misadjusted. Your car is
dieseling; that is why all the OE carb setups came with idle shutoff
valves.
Fouled plugs are plugs which have run too cool to burn off the lead
deposits that formed on their insulators when burning leaded gas. I
suspect that it is hard to foul plugs with today's gasoline. Those
of you old enough to remember will recall the grey crusty stuff that
used to adhere to the ground electrode.
This lead compound had the interesting property of being an insulator
at room temperature, but turning conductive as it approached the
spark plug's operating temperature. The symptom is: engine will
start when cold and run a mile down the highway, sputter a bit then
die as all the sparks are shunted to ground along the coated surface
of the insulator. Warm engine won't restart. Allowed to cool it
will restart and run for a period of time that depends on how long it
was allowed to cool.
Problem is completely cured by replacing or abrasive blast cleaning
the spark plugs. Interesting note: The lead layer is not thick
enough to make the plugs LOOK bad.
Jim
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Melissa Kepner Jim Adney
jadney@vwtype3.org jadney@vwtype3.org
Laura Kepner-Adney
Madison, Wisconsin
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