It's been a while since my last post. Just for a recap, my setup is a 91 305 TPI, automatic drive train in a hot rod. Vats, air, egr removed from the program. Car runs great except that the air flowing thru the filter is noisy, I mean noisy. Disconnect the brake booster line and the noise goes away. Plug the hose coming from the plenum with my finger and the engine stalls. Occasionally when the car comes to a stop at an intersection it stalls (with the hose attached to the booster).
No codes. Shouldn't disconnecting the hose work the opposite being a bad vacuum leak? And reconnecting it idles better? This is doing the opposite.
Your thoughts please.

 

Little disappointed no one replied to my post after one week, but that forced me to read numerous threads, some of which touched on the IAC resetting for minimum air. So I dove into trying to set that when the 1st thing I found was the adjusting screw was NOT touching the throttle arm. Yes the plug was missing. RPM was 725 +/-25 in park, before and after jumping ALDL terminals A&B, key on engine off for 30s, disconnecting IAC conn, removing the jumper and starting the engine. Therefore could not achieve 500rpm.
I repeated the process and raised the rpm to 1450 before connecting the IAC and TPS was at 0.43v. The annoying noise was reduced by 50% (guesstimate). This time when disconnecting the brake booster hose the engine picks up a lot of rpm indicating a major vacuum leak, DOH!!! Going the right direction. Got an emergency call and had to drop the project. Will continue today.

At the original 725rpm the throttle plates were closed with only the space around them to allow air to go through. All the air was going by the IAC. Initially the IAC steps were 120 which dropped to 77 after the adjustment. The factory service book shows 5-50 as a good target. Is 5 better than 50? Why?
Thanks for the patience.

 

Having the engine stall by closing off a large vacuum leak is normal. The IAC cannot respond quickly enough to prevent this. The general rule for GM OBD1 systems is 60/40 with 60% idle air coming through the throttle blades and %40 through the IAC. Less than 60% through the throttles will aggravate the tendency to stall or stumble with sudden increases in load and can cause inconsistent idle behavior. That's why we want to set minimum idle at 450rpm(about 60% of curb idle).

I don't know where you found the procedure for setting minimum idle but the idea is to completely close the IAC. The process as detailed in the factory service manual is intended to accomplish this. One thing that can cause issues when trying to set minimum idle is unmetered air leaks. Be sure that there are no air leaks in the intake or vacuum accessories. The fact that someone had the throttle stop screw all the way out is a sign that there may have been an issue with this in the past.

One question I have is why wasn't the throttle sticking? In almost every case I've seen where the throttle has been allowed to close past its intended limit, the throttle has had an issue sticking shut. Could the throttle cable be adjusted too tight, not allowing the throttles to close completely?

 

ASE doc - thank you for the reply. I'll try to address your unknowns.
"the procedure for setting minimum idle" - I used the one on this site "IAC & TPS Setting"
"vacuum leaks" - none. When I 1st started this task, disconnecting the brake booster hose did not effect the rpm, but on my adjustment per post #2, disconnecting the same hose made a major difference. The rpm climbed and plugging the hose with my finger almost stalled (as per your reference to IAC's slow reaction), then idled normal.
"throttle sticking" - Yes it did have resistance, but easily overcome. The cable was not a factor, I even removed it to ensure that.
Here are some findings:
............B4 adj........1st adj.....2nd adj
RPM.......725.............725..........700
TPS(v)...0.43............0.51.........0.57
IAC.......120..............88............51
Noise.....annoying.......50%.........normal
I even repeated the same procedure on another TPS setup. Both using AXXF base less vats and both yielded 700rpm min. NO LESS.

Observation:
Looking into the bin file, the first table following the FLAG: switches is
TABLE: BLM Cell Boundaries, which shows the rpm
Hi/2000
Mid/1200
Low/700

Does this table have anything to do with not being able to bring the rpm down to 450???

 

That table is the ECM's target, or desired idle speeds for different operating conditions, mainly startup and/or warm up from cold start. These idle speeds are to be achieved by the ECM, using the IAC steps. The minimum idle is the mechanical setting of the throttle position without input from the ECM and with the IAC completely closed.

I test the IAC for being open by placing my finger over the IAC air inlet, which is just in front of the throttle blades. If there is air being drawn through the inlet and idle speed drops with the hole blocked, the IAC is not fully closed.

Typical causes for not being able to achieve minimum idle of 450 are an intake air leak or throttle shaft bushings worn.

 

At WOT (engine not running), the shaft assembly moves about 0.008" with the indicator prong on the leg where the adjusting screw contacts the tab. Obviously 0.008" on a 1/4" shaft/bore is not optimal.
Is a TB replacement the only option? Who done what?

 

New TB is the only option. The linkage is staked in place on the shaft so it's not servicable.