I'm getting closer. Here are my results:
15 mph
%CO2 = 13.3 %O2 = 1.9 HC = 99 CO(%) = 0.61 NO = 82 I PASSED @ THIS STAGE!
Maximum HC @ 25 mph is 108
Maximum CO(%) @ 25 mph is 0.71
25 mph
%CO2 = 13.3 %O2 = 1.7 HC = 93 CO(%) = 0.76 NO = 90
Failed at this stage
Maximum HC @ 25 mph is 83
Maximum CO(%) @ 25 mph is 0.58
1991 Z-28; 350 TPI
Since my last failed smog, I changed the O2 sensor. I also ran a bottle of guaranteed to pass stuff; I ran that through a full tank of gas. Right before the test, I drove it for about an hour, put in half a tank of gas with four bottles of HEET. Although I did do a little better, I still failed. Since my first failure months ago, I have replaced the spark plugs, wires, distributor cap, changed oil/filters, knock sensor, checked for vaccum leaks (none), O2 sensor, cleaned throttle body...what else can I do? The car has been running great. Service Engine light does not come on. Any advice would be greatly appreciated. Thank you.

 

Are you still running the A.I.R. pump? If so, check the diverter valve for it. That should help lower the CO numbers. Does it smell like it's running rich at idle? That affects the HC numbers. Still using the stock exhaust and one-wire O2 sensor? Does it burn oil? That can cause the high HC numbers.

 

I just checked the diverter valve...seems to work. I turned on the engine and pulled the hose that goes to the manifold and there was air blowing out of it. After the car warmed up, I pulled the one that goes to the cat. and it was blowing air. I don't know how strong the air is supposed to blow out, but there was air present. Here's a question. Do these things begin to fail or do they either work or don't work? How strong should the air be blowing out to the cat. when it is functioning properly? Thanks.

Does it smell like it's running rich at idle? No.
That affects the HC numbers. Still using the stock exhaust and one-wire O2 sensor? Stock exhaust...yes. I changed the O2 sensor recently. Does it burn oil? No.