Failed emission high NOx

The backpressure issue comes up many times on this group. Do you REALLY think you could have low back pressure in a vehicle this old? Low back pressure is not an item I worry about.

High Nox is usually caused by high combustion chamber temperatures at ignition, and/or bad catalytic convertor, or both. I suspect that, but have not researched the issue, certain gasoline additives might push the Nox over the limit.

This car is old enough to need a new catalytic convertor. And you already point out that the EGR is "spooky".

High temperatures are caused by a number of issues. Lean fuel conditions, poorly functioning EGR system, etc etc..

I cant tell you exactly where the problem lies, but I think the oversimplified comments above might be something to consider.

I am sure others here will hop in with more concise suggestions. Good luck

Thanks for the tips, I will replace the converter and transducer and give it another shot.

Thanks for the tips, I will replace the converter and transducer and give it another shot.

Changing out parts is an expensive way to troubleshoot. Unless you are in a hurry for the car, and have a lot of money to spend, you might want to spend more time in diagnosis.

I believe it is in the EGR system: forgot to mention this will be my fourth attempt to pass.

First fail, didn't really know what to try and replaced the O2. Numbers improved a little, but still failed when tested again.

Researched, did some testing and found EGR valve partially stuck open, but after cleaning and re-testing a third time, the reading went higher.

The EGR valve pulsed at 2000 rpm with the transducer connected, but when by-passed, the valve stayed open more evenly. But being the last "free" test before spending a third $40, I'm thinking about trying the converter as well. I was also thinking about getting it tested with the transducer by-passed. It is supposed to prevent unwanted EGR operartion at low engine load, but should still operate. Your thoughts on this?

I believe it is in the EGR system: forgot to mention this will be my fourth attempt to pass.

Double check that you dont have any air leaks, missing air ducts, etc before you go much further.

The EGR is a common problem when Nox increases. Have you cleaned out the EGR ports, etc? Partially or fully blocked ports can do a number on you.

Have you had this pickup on the highway much recently, enough to sort of de-coke it. If not, that might also help.

Just as a benchmark, Autozone shows that EGR should cost you about $100. The catalyic converter is about $130.

Depends on whether you want to replace them both and have another go at it, or whether you want to do a bit more diagnosis.

What did the shop that did the emission testing say, or did they tell you if they found any parameters out of specification?

I have checked for leaks, none found, and all ducts are in tact.

The EGR ports looked clear, and when testing the valve, the engine nearly stalled when opening the valve with a vacuum pump.

Nothing really different in driving the vehicle, my son drives it back and forth to school.

The shop that tested says could be EGR system or cat, all other sytems or "smells" ok.

At this point, I can only try the transducer or leave it by-passed and try it. If it fails again, try the converter.

Maybe time to look into getting a decent OBD vehicle, the truck is economical and has been a good vehicle, but this is a pain in the neck.

Thanks for your help, much appreciated.