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.

 

I have a 92 Z28 with the 305TPI and after tuning it up failed on the hydrocarbons. Tried a few of the tricks with no better results. I was close but no cigars. Took it to a shop that does emission repairs and said my diverter valve for the smog pump was stuck and not sending air to the converter. Somehow he unstuck it and I passed. He said I should replace it the next time.

 

Your Catalytic convertor is "worn out" The Chemicals inside have lost their reactivity. Good Luck.

 

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. 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.

 

How many miles are on the cat converter? Looks like it isn't doing much to the exhaust if anything.

 

I live in Northern VA, which is pretty **** about emissions (of course not as bad as CA). I failed twice (I have no EGR), but managed to get it to pass. I took out A LOT of timing and ran part alcohol in my gas and that did the trick.

 

The cat that I have on right now has a great number of miles on it...not sure how many though. What are other signs that a cat is bad? I had one that rattled like a SOB. Any suggestions on the type of cat that I should have installed? Thanks for your help guys!

 

If you have access to an infrared thermometer the outlet of the converter should be approx 100F higher than the inlet of the converter.

 

ok, here is a trick i have used to get my cars to pass and it might be a good trick for yall. the 93- 97 lt1 had a electric air pump, on my 89 iroc with 388 heads cam car i tried to get it passed and it flunked, then i thought about if im failing because of the NO% reading why couldnt i rig electric air pump to put fresh air into my exhaust using a bung on the y pipe before the cat. well i rigged it up and the car passed and also won a 50 dollar bet with my friend about it. it is gonna cost about 40 bux to rigg it up and make it look like it was from the factory but it is well worth it. maybe some of yall can take my idea and go with it, i dont know, its just a idea i have used before that worked. i dont have that iroc anymore or i would post pics for yall.