Hi. This is my first post here.
My 94 z28 has failed the ohio emission test. I drove it 30 miles with 93 octane on my way there. Here are the numbers.

HC 79.1 ppm (limit 114.0) pass
CO 0.81% (limit 0.63) fail
NO 878.8 ppm (limit 796.0) fail
CO2 13.56% n/a result

Here's what I know about the car. it has a visible cat converter, 106,000 miles, msd opti, k&n air filter. I cleaned the air filter and re-oiled it, changed the engine oil and filter, I cleaned the maf sensor, no codes on the computer, rubber elbow was ripped so I replaced it. New muffler, Air pump is unplugged. The motor will have a bad stumble when I nail the gas and the tach gets to around 5000rpm. It does it when it is in closed or open loop. If I don't nail all the way, it won't stumble at 5000. It's hard to explain, but I'm hoping the emission readings will help me figure out what it is. egr? converter? 02 sensor? fuel pressure? msd timing? Any Ideas?

 

This is good summary of the emissions and what causes what and why. http://www.interro.com/techgas.html

I am not aware of one failure that would cause high CO and NO at same time, think you may have a couple of issues at the same time. Since part of the emissions (air pump) is disconnected, I would start by connecting that and make sure everything else emmission works as well.

Check the vaccum lines as well; number of fittings in the middle of the intake manifold on both sides as well as on and under the throttle body. Most of these are emissions controls which may be causing your problem.

The tach spike is wierd. I'll check the service manual after work but pretty sure the tach gets it signal through the PCM from the coil negative.

 

The AIR pump only affects cold start emissions. I doubt they even test that.

EGR helps reduce NOx. But when NOx is high, CO and HC are usually low. NOx results frow high combustion chamber temps, often caused by running lean. High CO and HC usually results from running rich. You have all 3 on the high side.

My guess would be the cat.

 

I was thinking preignition, maybe the msd is set totally wrong, or it needs a timing chain? Is there a way to check the timing chain without having to rip the whole front of the motor off? I do have an actron code reader that will give me some real time values.

 

pgerst - your link doesn't seem to work.

 

sorry, try this one. Using Gas Analyzer Readings

 

I found 2 problems, First someone punched out the converter. Any recommendations for a new one? Magnaflow?

I also found a weird problem with the egr valve. I put vacuum on it and it holds, but I start the motor and the vacuum goes away. I bench tested, put vacuum on it and blew in the hole with an air hose and the vacuum went down. That one makes no sense to me, so I ordered another one. Has anyone heard of that?

 

I passed the emission test after changing the catalytic converter. Here was the results.

HC 24.0 PPM (LIMIT 114.00)
CO O.OO% (LIMIT 0.63)
NO 712.7 PPM (LIMIT 796.0)
CO2 14.61%

Thanks for the help.

 

crazy i passed emissions with none installed on my car.. so a bad cat can make you fail but no cat at all doesnt hurt? weird