So about a week ago I came home with a '94 Z28. I picked it up in a part of Ohio that does not have emission testing. I need to test it where I live and it failed because the Hydrocarbons and Carbon Monoxide levels were too high. Both of these are related to unburned fuel in the exhaust. The car has a tune from PCMforless because it has Kooks longtube headers and y pipe with a Flowmaster muffler and a K&N cold air intake. I believe my tune is too rich because I can smell a bit of fuel in the exhaust. I cant afford to send the PCM off to get it tuned again, and I dont feel comfortable doing myself yet. Is there another way I can reduce these numbers, or get around the car being rich, ie hotter plugs or something? Even something temporary that will work long enough to get it tested and pass. I was thinking about unplugging a couple injectors. This way the cylinders without injectors pulsing will have air moving through them still and that will add more air to the exhaust making seem less rich. Any thoughts?

 

If you pump air into the exhaust, it will show up as excess oxygen. That's why they measure the oxygen level. The figured that out a long time ago.

The emissions tests are run in the operating range where the PCM should be in closed loop A/F control. The target A/F (14.7:1) is hard coded, and generally not altered in a "tune". That would be particularly true with your very limited modifications.

You have to figure out why its running rich. One way would be to do a data log of the PCM, so you can see how the long term fuel corrections are responding to the rich exhaust. The rich exhaust might be the result of a "false" lean condition. That can result from exhaust leaks before the O2 sensors, misfires or faulty O2 sensors and wires. With a data scan you should be able to tell if its a condition that is affecting only one bank of the engine, or both (the two banks of the engine are independantly controlled).

Basic question... does it have a cat, and if it does, are you sure it is healthy?

Do they also test for oxides of nitrogen (NOx)? A faulty cat will show up high on HC, CO and NOx. On the other hand, a rich condition will show high HC and CO, and low NOx.

 

Also do they test at idle and at a higher rpm? Here they will ask you to rev the motor to 2500 rpm if it doesn't pass. That info may help some also if you have it.