I just picked up a new to me 91 Firebird with the aero package and 305 TBI
/5 speed combo. The thing I can't figure out is when I shift or put the
Car in neutral the RPMs only drop to about 1300 and will stay that way till I come to a stop, then the idle settles down to about 700? What would cause this delayed drop to idle?

 

Dirty throttle body, dirty Iac, or a vacuum leak.

 

ya do know they are programed to drop RPMs slower, though dunno how much out of normal yours is without driving it.

video?

 

I don't have a video but I can try to describe. For example let's say I'm approaching a light. I press in the clutch, the RPMs drop from 2000 to about 1100. With clutch in I approach the light and the RPMs remain the same. I come to a complete stop. Maybe 5-10 seconds later the RPMs begin to drop till it settles at about 700.

I have used throttle body cleaner to clean up the throttle plate area of the TBI. What is the process for cleaning the IAC?

 

Pulled TBI off, cleaned it, cleaned IAC, new base gasket, cant find any vacuum leaks but still same issue. I have noticed I have to be completely stopped before RPMs start dropping from around 1200 to normal idle.

 

Was the car once a auto trans?

 

Is there something hanging in your throttle?

 

I have the same problem, I think it is in the program but
I haven't been able to find it yet.

 

RPO codes show M-5. It does have an open element air cleaner, but plenty of space under the bottom to clear throttle cable. At least no stalling like most TBIs but the idle after pressing in the clutch is driving me crazy

 

although i have yet to look through tbi code and i'm pretty new to tbi.....

i'd assume it's like most gm ecms, which have an iac adder based on vehicle speed that functions in open loop idle control, usually called an iac speed follower. it simply adds iac steps.

you gotta realize there's closed and open loop iac control (kinda like AFR)

open loop idle means it doesn't actually look for a target idle speed, it just has calcs that stack up a bunch of iac steps.

this usually kicks in while the vehicle is moving (you can change what it considers 'moving'), then when it drops below that speed, closed loop idle kicks back in, and it tries to attain target rpm.

in short this is normal, but you can likely tune it to your liking by lowering that speed follower.

also a vacuum leak or throttle plates too far open will result in the moving idle being high, but closed loop iac control will usually correct to to attain proper idle, so you should check for that stuff too.

 

sitting, stopped in the driveway i can rev the engine and the idle returns right back to normal. I think you might be on to something with the ECM programming...now how to fix it?

 

it also takes a long time to come down from a cold idle, like 10 min. I owned a 91 RS Camaro back in like 98 with the same engine. On that car cold idle would kick itself down in about a min of high idle. Could the two issues be connected? Maybe a bad coolant switch not letting the ECM know its warmed up?

 

that's pretty normal, i guess 10 minutes is a bit long though, usually a couple minutes and it'll kick down.

1300rpm while driving isnt a bad thing, thats the entire point of the iac speed follower -- any lower and the stupid thing would stall anyway, do you plan to cruise at 800rpm or something?

after all this, what's the problem exactly?

does a 1300rpm idle while cold bother you somehow?

you might just need to tune this out...

 

No but if your driving through a neighborhood and go to stop at a stop sign and press in the clutch and the idle raises to 1300 untill your completely stoped its kinda annoying. But again at least its not stalling like many TBI cars.

And the 1300 isnt just when its cold, it can be completely warmed up and it will idle there till stopped. Once stopped I can rev the Idle and it dropps right back to about 700. It only happens when driving