1991 6sp Convert Idle is fluxuating

[parkercoman]

I have my 1991 Convert dialed in for the moment. Im ean this car is just giddy up and go ( had to pull the Leather out of wifes B@## a couple of times. LOL ) I purrs I m mean no miss nothing but one thing. The idle keeps fluctuation from 800 rpms to maybe 1100 RPMs I have ran every hose and fitting and Vacume or other with nothing. I find nothing,.

Anyone have a sugesstion


Chris

 

[Schrade]

Vacuum leak.

Follow this test to a 'T':

No one got the proper technique for vacuum leak detection... :confused

FIRST (fully warmed up motor); block the pedal, or block the throttle linkage. YOU HAVE TO DO THIS TO GET A STEADY IDLE , around 1,250 rpms, or a little higher. If you don't, you can fish starting fluid, a propane hose, or WD40, or Krylon, or hair spray, or pee, anything, until you need a sweater in Hades, but if the motor is lopin' already from a vacuum leak, you ain't gonna' find any vacuum leak, unless it's so big that Little Jack Horner can stick his thumb in it and pull out a pineapple NOPE.

AFTER you get a steady idle, THEN, you can spray starting fluid everywhere - that's the best detector, because it's vapor pressure is very high - a LITTLE at a time, until you get the SURGE. Follow the vacuum lines to the EVAP cannister too. You might even have to follow the EVAP lines from the EVAP cannister to the gas tank too, but that vac leak will show up as a DTC 32 EGR fault, and won't normally show as loping idle, unless the EVAP cannister has been by-passed (been there, saw that uh-huh).

If you get a surge in a tight spot, and can't tell exactly where it is, light a book of matches, blow out the match heads while they're still burnin', and feed the smoke to the tight spot.

Fear not the starting fluid; you cannot put the can down fast enough and light the matches fast enough to catch the starting fluid, because the vapor pressure is so high.

Bookmark this post YUP.

Wrench Wizard OUT!

 

[Steve40th]

Change your IAC. Disconnect it first, see if it hunts around for its idle. You can run a digital meter and check the electrical condition of it.
http://shbox.com/1/iac2.jpg
You will get no code from your ECM for it, and any datalogging you do only shows what the ECM is telling it to do, not what the IAC counts actually are.

 

[Schrade]

Change your IAC. Disconnect it first, see if it hunts around for its idle. You can run a digital meter and check the electrical condition of it.
http://shbox.com/1/iac2.jpg
You will get no code from your ECM for it, and any datalogging you do only shows what the ECM is telling it to do, not what the IAC counts actually are.

I'm not understand this at all here...

It's ALREADY hunting [loping] idle.

If he blocks the TB linkage, or the pedal, to increase rpms OVER idle, and get a steady RPM, what then would IAC manipulation tell us?

Why change stuff without doing tests first?

 

[Steve40th]

Schrade, Bad idles can and will be attributued to the IAC. It will throw no codes, and he has none, not one code. It will cause erratic idle. I recommend taking it out, put a meter on it and check and see if it is good, using the picture I sent.. I bet its not. The IAC is the controlled vacuum leak to keep idle steady according to the ECM. He says his idles hunts 800-1100. That is way to high for a stock L98.
His idle should be around 650 rpm. PERIOD.
Dont start chasing vacuum leaks, spraying crap into the engine. DO what is a known issue with GM, IACS!. Check it electronically via meter.
I have been dealing with these L98's and LT1 since 1989.

 

[aklim]

I suggest you clean your TB first. Take out the IAC and the IAC housing and clean them throughly. Even if you throw a new IAC at it, you can't set anything if the housing is dirty. Clean everything out including the TB blades. If they stick open, it creates another vacuum leak. Uncontrollable but still a leak.

Once that is done, simply reassemble and start. Get the engine to operating specs and check timing. Set to stock timing. When that is done, adjust the set screw for the TB Blades to the IAC count. You want to see about 15-25 counts with all the accessories off. The ECM should set the idle speed by itself. The IAC is there to maintain it. Should it fail to do that, it will attempt to do it via the timing, IIRC. I know the last numbnut that worked on my car set it poorly and the timing was across the board.