My 1991 Camaro RS 5.0 TBI has a bad idle problem.
On a cold start my RPMs will shoot to about 3k for a short moment then decend to about 2.6K and slowly makes its way down to 1-1.2K after that. Now that it's idling steady it sometimes it will fluctuate down to 1K back to 1.2K.
When this is happening I can give it a little gas and hold it at say 2.5K RPMs. It seems to start trying to bog out. The RPMs drop and if I let go of the gas it will sometimes go to about 800 RPMs then bog out and die. Sometimes it tries to bog out but then gasps for air and it'll sit back at its 1.2K idle.
Another thing, when driving around for awhile and then arriving somewhere where I have to say park my car it'll die in the process of slowing down and turning the wheel, or shifting into reverse.
I have checked for vacuum leaks around the throttle body with starter spray and there were none. My RPMs did not rise with the spray.
Now does this sound like a IAC problem or a TPS problem? I'll be trouble shooting tomorrow so any tips is appreciated. Could a dirty cat have anything to do with this as well? It hasn't passed smog and I belive it's because of a dirty cat (failed high speed hydrocarbon test then months later it failed for both that and the nox)

 

I'd remove the tbi unit to clean the whole unit, the vacuum ports can get plugged up. Clean the throttle plates good. Look closely at the base gasket as you remove it, look for thin spots.
I'd also remove the egr valve & check for plugged passages. Is the egr getting vacuum while driving it? Hook up a vacuum gauge to it's vacuum source & tape it to the windshield. This might help get the NOX down.

 

Emissions failure for HC is typically misfire, including lean misfire. Emissions failure for NOx is typically too-high combustion temperature, generally due to EGR failure or lean mixture.

Catalyst failure can make either or both worse. What were the actual emissions amounts?

High and unstable idle can also be due to "false air"; air leaks into the intake manifold that shouldn't exist. Torn/rotted vacuum hoses, failed intake gaskets, etc. can cause this, along with creating a lean condition that then leads to misfire and/or high NOx.

I'd put a scan tool on the thing, and look at the data stream--short- and long-term fuel trim, coolant temperature, O2 sensor voltage and cross-counts, commanded vs. actual idle speed, commanded EGR operation, and so forth.