Damn, I had something similar a while back on one of my vehicals and it
drove me crazy for a while. I even took it in to a local repair shop and
they put it on the computer and couldn't get a diagnosis. Turned out to be
the idle stepper motor/idle solenoid/idel speed control motor (different
names for the same thing). It can be hard to diagnose this, since it often
doesn't show up when it's sitting idling like it would do in a shop
connected to a computer, but will show up when driving. Another possibility
is a vacuum leak. I'd check to make sure everything's bolted down to proper
torque specs and check for vacuum leaks as well. In my case, I tore apart
the fuel enjection system and all sorts of wonderful things before it was
If you've ruled out vacuum leaks and such, and it's a rhythmic surging
or loping (not erratic), it's almost certainly a dirty or broken idle
air control valve (large cylindrical object bolted to the upper intake
near the throttle plate). Take it off, blast it out with lots of
throttle body cleaner, let it dry, and put it back on. If that doesn't
help, it needs replacing.
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