After 4 weeks of trouble shooting i finally gave in and took my 91 Z28 to a local shop. the problem was that my car was running extremely rich and stalling at idle, or no throttle.

after changing the temp. sending unit, oxygen sensor, fuel pressure regulator, testing EGR valve... we pulled vacum and it made the car stall out (complete vacum check), cleaning injectors, I changed IAC and still no luck.

having access to a scan tool, i hooked it up and found that target air/fuel ratio was 128, but was giving an instant reading of 105. with this info. i was convinced that the ECM was operating correctly (going to full rich).

After two days my mechanic took off the EGR and found that it was stuck wide open. He explained that this made the car run rich because with the EGR stuck open the oxygen sensor saw excess lean mixture and therefore dumping fuel to correct for the problem.

This was all caused by internal vacum, just because EGR checks good doesnt mean that it cannot get stuck intermitantly. By the way... that is how all of this started (intermitantly), and obviously got progressivly worse.

I am writing this because there was '88 TPI trans am in the shop previously with the same problem before the mechanic i had working on my car was there, and it was sent out with the problem unresolved. I hope this will help someone with the same problem.