Ok- 1991 Z28 with L98 350. Idle is causing me FITS! It started idling 1500-2000 RPMS. I replaced the IAC valve with a cheap one from JC Whitney but, although it seemed to stop the idle bouncing all over the place, it was still too high. I tried backing off the minimal air screw on the throttle body, but no luck. So I screwed up THAT setting!
I had cleaned and lubed up the old IAC valve, so just for SAGs, I swapped it back... Voila! Problem solved. I had to adjust the idle screw, but she would idle at 500-750 and was great!
Then, it went back to 1500-2000 RPMs! Then, it went back to 500-750. Now it is idling HIGH again!!!
If it was a vacuum leak, it should be constant- what am I missing here??

 

Check out this link it should help. http://www.chevythunder.com/do_it_yo...ation_page.htm

I had this same problem with my car. Your lucky because the speed density cars are easier to deal with after making mods than the mass flow cars.

Did you make sure and push the IAC plunger all the way back down before you installed it? Do you have any codes stored in the computer?

Make sure you don't have any leaks in the ducting from the air box to the throttle body either.

If your car is still running the factory tune after a cold start the motor should idle around 1,000rpm to 1,250rpm-1,500 rpm then should start dropping back the idle speed as the car warms up.

I've got full bolt on's in my car but the engine is still a stock L98 - including the heads and cam. I like the way my car idles at 750 rpm. Any lower than that and the car feels like its getting ready to cut off. I know the GM idling specs on these cars is blow 750 rpm but, I never set my idle that low.

 

May sound weird, check that your tps is functioning correctly. I had one sticking at .91v and the idle would surge around 1500 while I was coming home from a ride. Lowest idle it would do. Changed the tps and haven't seen that happen in yrs

 

I just went through this same issue with my car. I changed the TPS and this resolved the poor performance driving issues I was having. I also have replaced the IAC, O2 sensor and some other components recently as well. Sometimes it's a process of elimination to figure out what the root cause of the issue is.