I have done a search on high idle, but can't find an answer to my specific problem.

Here are the specifics:
Changed from an '89 throttle body (idled fine, will put it back on if I have to) back to my stock '85 tb that has been reconditioned (one thing that is different is on the same side as the TPS, after reconditioning, there is a screw that turns with the shaft. This screw just barely touches the TPS).

The reason for the change was to be able to use the cruise control again (the throttle cable connections are different). Performance was fine on the '89.

After switching, I can get the idle down to 750 only for a short time, after doing the minimum air set, and then disconnecting the battery, and restarting the car. Upon restart, the car races to 2000, then settles to 1100 for about 10-15 seconds, then goes to 750-800. I then drive a block or two, stop at a stop sign, and I'm back up to 1500-1700.

The IAC pintle has been cleaned. IAC worked fine while on the '89 tb.
The '85 tb is clean, and the spring works fine and butterflies come back to the stops.
When doing the minimum air set, the screw is set to allow for the lowest idle speed possible.
Also when doing the minimum air set, when grounding the A-B on the ALDL, the cooling fan is NOT coming on as it should. I can hear the IAC doing its thing. No codes are showing. The fan does work, since while doing some of this work my coolant got up to 238 and the fan kicked on.
While switching tb's, I did a tb coolant bypass. This is new to the situation.
TPS is set at .54, but has not been checked since reinstalling the tb.
T-fitting vacuum line under the tb is connected.

It does not seem to be temperature related, since I was able to get it to idle at 750 for a little while when it was warmed up (oil 190, temp 195), but then driving it a block or two returned it to 1500-1700.
Anybody have any ideas?
I thought there might be a vacuum leak, but it ran great before doing this, and I can get it to idle where it needs to for that short period of time.

 

Sounds like false air. Check all vacuum connections, be sure the gasket is installed under the TB top cover plate. Run a little oil around the TB seal lines to see if idle speed drops.

 

I thouroughly agree about the 'false' air. I'm going back out to the battle field (aka garage) to check. I also remember hearing a little 'hissing' so I'm going to look for a possible vacuum leak.

 

OK. I've isolated the problem, but am stuck as to a solution.
Started it up, high idle upon start, settles down to 900 after 5 seconds or so, then down to 700-750 (right where I want it).
Play with the accelerator, run it up a couple of times, settles back at 1200-1300.
Check the cable connections and the connector is not returning all the way to the stop/screw. It is about 1/16" away, causing the idle to be where it is.
The problem is now isolated.
How do I fix this? The spring seems to be fine, it snaps all the way back when I do it when the car is off. What could be the hangup?

 

mine was doing that after TB change but fixed itself before i could get around to it. i had already bought a carburetor return spring set from auto parts store and was going to put a spring from the cable bracket to the throttle.
i love when cars fix themselves... try driving it a while, my surging idle has almost fixed itself too, guess the car gets tired of waiting for me to get around to it... LOL

 

So, doesn't the IAC control the 'stop position' of the throttle plates? And if so, does that mean he's having a problem is with the IAC??? :confused

.54 is the base, but can that number be misleading if there's another problem like a leak in the vacuum system? A leak might cause the IAC to extend more than it needs to? I'd spray some Gumout Carb Cleaner around the vacuum lines, looking for a drop in rpm...

 

Wait - I don't know what I was thinking - the IAC doesn't touch the throttle plates. It controls the bypass air into the throttle body, right?

So what would stop or slow the return of the plates? :confused

 

i guess it could, dunno... can't hurt to check for vacuum leak.
TPS can be set as high as .99 at idle, and still get good idle quality, i know, i've tried it... SUPPOSEDLY the TPS "auto-zero's" itself at each start and as long as the value is under 1.00 it uses that as new start point... i know the book doesn't say that, but that's the rumor...
maybe my IAC finally adjusted itself and that's why my idle is smooth and the accellerator now returns fully... now i hate to take my IAC off and check it... even though that was the plan...