So I fought with the idle speed motor from day 1 with my Monty-
always seemed like it was sticking, not working, etc
even after 2 ecu swaps and umpteen motors.

<edit- and found that this is the USUAL cause of poor idle-
only about 1 in 4 junkyard idle motors are any good! So make
sure your motor and ecu are working before you go further...>

Finally dug into the TB with a vengeance, and found some
things:

there IS a manual idle speed bypass, even though the manual says
there is not. It's under a lead plug. It has a LOT of range.
It may solve your problems with #2-

the warmup regulator. It's a thermostat, wax and all, that
allows significant bypass air when the truck's cold, and meters it
down to a little bit after it warms up. It's plumbed into
the throttle body heater circuit.
It also has a HUGE range of 'normal' operation, and thus
the bypass screw. The one on my truck was almost closed
at cold, and would shut by 150f. That left all the idle
on the ECU, and it just couldn't handle it. Of 5 junkyard
bypasses, it was by far the 'coldest'. Normal seems to be
mostly open at 70f, and almost closed by 212f. Some did
completely close, others did not quite. Almost all, except
mine, were mostly open at 70f. Mine was probably at most
25% open.

Yeah, there's nothing in my FSM about this part. At all.

#3 was hilarious- the 'don't touch it' idle screw wears a divot
into its stop, allowing the throttle plate to close further.
When I reset it according to the FSM, I think I added at least
2 turns...

#4 was common sense- clean your throttle body occasionally.

Also, the '89 parts are unique- the 90 and 91 parts
seem to interchange. This is par for the course with an '89...

So if all these things aren't right, the ECU just can't cope
with the idle air by itself. The FSM says it can, but given the
huge amount of air you can add via the regulator and manual
bypass, I don't believe it.

All that said, I haven't had an idle problem since cleaning
everything and putting in a different warmup bypass...
I used one that was 'way open' at cold, and didn't find a need
to add any manual air bypass to that.

Ideally, I suppose, you'd check your IAC at normal hot
idle and make sure it had travel left, but mine seems to
have settled in just fine as is.

hope this helps someone,

t