Mystical Misfire 164L

[C M Smith <cmsmith@domain.elided>]

Three times I have experienced an intermittent misfire at highway speed in 
top gear just loafing along.

Ambient conditions:

Temp about freezing point
Elevation about 4500 ft
Humidity about 80% plus to about 100% (light snow or sleet, or about to)

Car speed about 70 mph.


What it is not:
Not fuel filter (new clean one installed sort of yesterday)
Not temp sender temp gauge rock solid at normal (for my car in winter at 
about 70 C on the gauge)
Not water in the fuel (gas line anti freeze and a fuel drier injector 
cleaner recently used.
Not dirty injectors (fuel rail and injectors recently cleaned professionally)
Not bad plugs (new plus 4 Bosch)

What it might be

ice in the throttle housing?
bad density sender on the airflow meter?
very intermittent tracing on the distributor leaking spark power away due 
to humidity?( top candidate at present)
bad plug wire (second candidate)

Symptoms:

dead misfire on one or two consecutive ignition events then runs fine then 
dead misses again, steadily worsening until you stop, shut off, restart and 
drive on. Then same symptoms begin about 5 minutes later. Park the car for 
an hour or two and the problem disappears, for another 6 months and does 
not occur during all other driving.

And here's the piece de resistance that baffles me: the engine runs sweet 
as can be everywhere except this one stretch of road between Calgary and 
Banff, beginning as I enter the front ranges of the Rocky Mountains. And 
only very occasionally when the temperature is around freezing and the 
humidity high and not always when those ambient conditions exist.

Anyone else had similar symptoms? anyone else any idea what it might be?


Michael Smith
Calgary, Alberta
Canada
91 Alfa 164L, White, original owner

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