At 12:44 PM 10/16/2014, Stuart MacMillan wrote:
>I've checked the TPS with an ohm meter and it works fine, and the idle valve
>hums.  I can't find any vacuum leaks (propane torch probe), and it was
>recently tuned up.  So, is it possible for a bad Idle control unit to cause
>this?  It seems odd that an engine can run at 3000 with the throttle closed!

Stuart, the idle controller is subject to solder fatigue just like
the ECU, and it has a lot fewer joints to reflow so it's well worth
it to simply reflow everything rather than trying to find a 35X
stereo scope to really look hard at the joints.  You have to add a
bit of clean 60/40 or 63/37 rosin core solder to each joint, and if
you end up with too much use a soldersucker or braid to remove it and
redo the joint.

Even before doing that, remove and replace the controller.  You can
look for corrosion on the terminals, and replugging it will exercise
the connections nicely as the Faston type terminals have good scrubbing action.

Yours,
David
ps -- I have long ago seen a Jetta with a sticking idle valve cycle
up to 2500 rpm and back every few seconds.  I presume that its idle
controller kept advancing the power until the valve unstuck, at which
point it jumped up.  Cleaning with solvents cured it, at least for
the moment.  Your symptoms do not sound like this if you're
describing accurately, but rather a valve that's being actively
driven to stay open.