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Discussion Starter #1
Hello All,

I have a stock 1994 Trans Am LT1 Automatic. I just replaced the wires and plugs. I replaced the ignition coil a few years back. Its an Accell. Has stock opti. Mechanic suggested it may be opti and/or 02 sensors. Readings are the following:

HC 15 MAX 58 Tested 128 Fail
HC 25 MAX 34 Tested 105 Fail
CO 15 MAX .34 Tested 0.27 Pass
CO 25 MAX .32 Tested 0.17 Pass
NO 15 MAX 483 Tested 193 Pass
NO 25 MAX 494 Tested 128 Pass

So hydrocarbons are through the roof. Would a bad CAT be the underlying cause? Or is he right and might be the opti or 02 sensors? Any insight would be appreciated
 

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Hydrocarbons are produced by over rich fuel mixtures. Try doing a running sensor scan with a obd 1 scanner. See what your fuel trims & o2 sensors are reading. As well as other sensors.

A trick many people do to reduce hc is to change the oil right before testing. But this only helps if hc failure is minor. You need to find out why fueling is rich.
 

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Hydrocarbons are unburned fuel. CO is partially burned fuel and from air/fuel ratio, dirty oil, etc... High HC‘s can be caused from lean misfires, misfires, engine wear, ignition problems, etc...

Your max allowed values seem quite low to me. What state are you in? Also, what are the two tests being performed? Is the “15“ test done at 1500rpm and the “25” test done at 2500?

You should pass without a cat for HC, that’s not root cause. The vehicle must be at full operating temperature- take a 15 minute drive and make sure your at operating temps- hot oil and hot exhaust.

Has your cap and rotor been changed?
 

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Discussion Starter #4
I am in CA so super strict. rpms were 1321 and 1296. I had changed to a chinese knock off (I was broke at the time) and when I was having issues with spark I replaced the original. I couldnt pass smog before and I decided the original might perform better than a new chinese knock off. So, I switched to old original stock opti. Also ran sea foam and always pass in the gas tank. Burned an entire fuel tank and put in fresh 91 octane right before testing. Drove on freeway for 30 minutes and left it running while I waited to get smoged.
 

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Sounds like you were doing all the right things. You may need a good tech to help diagnose how everything is working (compression test, leak down, power balance test, check temps pre/post cat, monitor O2 cross counts if lazy and staying a bit lean, etc..). There could be some underlying issues but sure you just want to get through the test. Air injection system helps light the cats and there are some things that can be done to help reduce emissions. Higher idle speeds (I’m still a bit confused on the difference between the two test speed ranges you failed ) and retarded ignition timing. This was easy to do on older carb’d stuff.
If your cat is shot, the HC’s will eventually kill a replacement unless cause is addressed.

We use to do idle and dyno testing here but light load steady state. Certain vehicles did idle (1100 rpm max) and 2500 rpm no load sampling.

HTH
 

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Discussion Starter #6
If this may help here is the scan log from 94/95 program. Mostly idle with periodic revving. Let me know if this may help at all. I see the egr doesn't seem to do anything as far as % in the grid. Is that normal? I waited until the car was warm before I started the program
 

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