phatpharm85. Thanks for the reply. Basicaly HC is the issue but on three tests I also failed NOx at the same time. This makes no sense to me since as I understand it one is a rich indicator and the other is lean. MD standards are HC 2.6gpm, CO 80.0gpm, NOx 7.0gpm. Heres what I got: 1st failed HC at 3.53 (changed plugs), 2nd HC 6.81 NOx 7.52 wha?? (changed wires, cap, rotor, PCV), 3rd HC 4.72 NOx 7.42 (changed O2 sensor) 4th HC 4.28 NOx 8.01 (flushed throttle body with cleaner, ran a tank of gas with emisions cleaner, went to hotter plug), 5th HC 3.28 (took apart air valve to CAT hoses and found stuck valve, repaired and reinstalled), 6th HC 3.06 (went to mechanic, replaced EGR and flushed vaccum system), 7th HC 3.99 and now I am getting a check engine light almost every time I drive. I don't recall the code #'s but insufficant vaccum, and O2 sensor lean are what it says. That's when I read the O2 sensor voltage. A post here said it should be about 450mv, mine goes from 350-800mv at idle. I have not checked the MAP or air pump yet but they are next on my list. As far as the CAT goes it is not rusted out, the exaust is tight all the way out, but it is 17yrs old. Can a CAT go bad? I hope this is enough info for you. Other people here have given me some great input, but any new thoughts are always welcome. Thank you for your time, Karl....

 

Well a CAT very well can and at 17 years old there probally isnt a whole lot left to it. Some of those readings are really eractic. I could see some differences but those are pretty erratic changes. The O2 sensor readings should be fairly normal, if its flatlined at one voltage it would be bad.

I would switch to the stock plugs and do a compression test on the engine since your in there. If the compression test looks good I would check the map sensor and replace as need be, and check the air pump and all lines runing to the cat from the pump.

If all that checks out good I would suggest putting a new cat on your truck. Its amazing how much HC's a good cat can cover up, it may not like it but it will do it. We killed a whole bank on a car in school and the cat was able to bring the HC level down to a pretty good level. You'll self destruct the cat doing that tho.

The NOx could be from carbon build up in the engine and the HC could be a bad cat. I'm willing to bet you have a couple of issues combined.

Oh on the MAP sensor I'm not positive on how that engine is set up but alot of them have a hose running to the MAP rather then the MAP being mounted right on the intake manifold, make sure that line is clear of debris and that the intake is also clear of debris and that the hose is in good shape, its possible for that hose to collasp some during operating conditions and cause eracitc MAP readings.

I have access to all data online and I'm going to see if I cant find some compression specs and MAP readings for ya. I'm going to do some research and I'll update later.

 

phatpharm85, thank you for the input. I'm sorry I did not respond earlier but life had other things in mind. Once the weather breaks and I have a chance I will check out the things you recommended. One thing is for sure, the readings are bouncy, and that is urging me. I don't know how I could have gone from pass to fail so bad within two years. I'll post an update. Thank You for your time, Karl..........