hunting idle on 5ktq

[Ben Swann]

[I have a problem, the idle speed rapidly varies from 1200 - 1400 rpm and
repeats about 1 time per second on my 1987 5ktq.  I have checked and
eliminated all of these items, and the hunting idle continues.

No codes
replaced idle switch(and checked with dmm), adjustment is fine, and wiring
checks good
idle speed valve changed, and if unplugged, hunting idle does not change
no vacuum leaks, and car dies if oil filler cap removed.
climate control turned off(ac not affecting idle speed)

The car drives great once gas is pressed, but when idle switch closes the
problem persists.  Anyone have ideas about this?  The thing that baffles me
most is that unhooking the ISV does not settle the idle down, which
eliminates the control relay and wiring in my thoughts.
Thanks,
Brian]

Most likely culprits are:

1) Idle Stbilizer Valve - remove and make sure you can't blow through it.
 A slight bit of air passage then it is marginal and thus compounded by

2) Throttle body/idle screw - the o-rings on these get completely hardened
and you may even find it difficult to remove.

If those pass muster, the next major culprit would be the throttle body
switch for idle and wide open throttle (WOT) positions.  I had a problem
with the wire undr the boot there once, so don't overlook that.

Good luck - let us know what you find.

Ben