My 1992 Isuzu 2W PU failed the Emission test. The Max allowed for NOX is 4.0 (g/mile) and recorded at 4.08 after the EGR valve replaced. Any idea for me where to look at to fix the old P/U, thanks.

A lot more information would help, such as general engine condition, but you probably have a tired catalytic converter.

The cat converter’s primary stage reduces NOx by breaking the bonds between the oxygen and the nitrogen, converting them to…oxygen and nitrogen. But it does this only when the NOx molecules in the exhaust stream come directly into contact with the converter’s platinum-paladium coated honeycomb. In older vehcles the surface of the honeycomb can become coated with residue from things like burning oil, the NOx molecules no longer come in direct contact with the honeycomb surface, and the NOx does not get reduced…the molecules never get broken apart.

Newer vehicles have a second oxygen sensor that compares signals before and after the converter to tell if it’s working, but I think yours predates this.

Was the truck sufficiently warmed up before the emissions test? If not, take it for a drive (20 minutes or so) to warm it up and bring it back for another test. Since you’re just barely over the limit this may make the difference.

Let me provide more info about my P/U problem. The first Emission test result: Hydrocarbons (HC): 0.55 and limit is 2.00. Carbon Monoxide (CO) = 10.10 and limit is 20.00. The Oxides of Nitrogen (NOX) = 4.49 and limit is 4.00. Since the P/U just had the tune up service from the shop about 5 months ago, so I only replaced the EGR valve by myself and had the second try. Well, the result was not much different. The NOX now recorded at 4.08. I only need to bring down the NOX for the last two years of this P/U , I guess since it is old. Any ideas appreciated, thanks.

I’m going to sick to the cat converter sugestion.

The EGR prevents excessive NOx generation, in addition to preventing preignition, but the cat converter is the “scrubber”.

No way should your CO be 10.1 or the limit be 20.00, pre cat CO on a 91 should be about 1 plus/minus .5. Perhaps your figures are in grams per mile and I am thinking in parts per million?.

Directly related to your question, timing good, no overheat? Can you alter your idle mixture? if yes a little richer will bring that NOX down.

First, I would like to thank to all, especially to “the same mountainbike”, who helped me to solve this Emission problem. I brought my P/U to a cheap local muffler shop, Mesa Muffler, to have the CAT replaced. The new result is beautiful; NOX = 1.09 vs 4.09 (grams/mile - 4.00 to pass). The HC and OC are also lower. Also, for a rather aging vehicle, you do not have to choose a big and famous shops for service as MIDAS where is more expensive; $145 vs $225 and more. Again, thanks to all.

You’re very welcome. And my sincere thanks for posting back. So many folks don;t and we never know the results.