MsAx,
Nice Subie! '02-'07 Imps are fabulous handling, albeit a bit brash
and noisy. I'm going to guess that the very fine gap between the
throttle body butterfly plate and its bore is gummed up over time.
Pretty easy service: remove the air hose/plenum in the rear, then
spray liberally the inlet with carb cleaner, wiping clean with paper
towel; THEN, IMPORTANTLY, stick your fingers in there and
force the butterfly open and try to hold it horizontally in place while
you spray the inner bore both top and bottom, wiping the sludge
out as best you can; repeat until you think it's pretty clean.
Note that restarting will be tough (and possibly smoky...sorry!), as
the motor chokes on all the cleaner, but after a couple of minutes
you'll probably have a much smoother idle.
The problem isn't that the gunk was CAUSING the rough idle, but
that it was blocking the small amount of air needed for the idle
circuitry (its sensor plugged in there) to maintain a smooth idle.
he gunk was acting to "choke" off the air supply.
This is a pretty good service to perform every 5-6 years, and costs
nothing. Do try to avoid breathing the spray carb cleaner.
If you have a CEL be sure the problem isn't just a water temp
sensor; as I recall these $15 sensors (which incidentally sit just
below the TB where you're working) can screw up low speed
running too, as the ECU doesn't know whether you're warmed up
or not, and messes up idle stability. If I remember this sensor
throws a CEL if bad.
Have fun, and wear rubber gloves.
Ern
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