86 Suburban failed emissions with high NOx

[hotrodpc]

So, your Hydrocarbons (HC) and passing farily, kinda close. The Carbon Monoxide (CO) is passing flying colors with room to blow. So need to figure out some kind of adjustment that will lower Oxides of Nitrogen (NOx) but ok if it raises the CO moderatly since you have plenty of room there. Is this computer controlled vehicle??? I am thinking it is. Even if its Federal, by 86, anything under 8600GVWR was comptuer controlled if I recall correctly. That is probably why the standards are where they are. If it were an 85, it probably would have passed. Let me know if you do or do not have a computer, and I'll see what can dig up. I am not even sure what computer system it will have in it if it does. Maybe CCC, or C4, but I think its to old for OBD 1 and I know for a fact to old for OBD 2.

[hotrodpc]

Founds something. Causes of High NOx. 1. Defective EGR System, 2. Faulty Cat, 3. Lean Fuel Mixture, 4. Engine Overheating or running very warm. 5. High Engine Mileage due to carbon build up in the combustion chambers. So here is what I would probably do. Clean my EGR valve, check my water temp, try to take in the inspection semi cooled off, like maybe 120 degrees or so. Surely its gonna get hot by the time they do both tests, so if you have to pull your tstat out so it runs cooler, then so be it. Pull it out and put it back in. The cooler the better, that way it will compensate for other failures and maybe the NOx won't show up. The 3rd thing, I'd probably do is run some Sea Foam treatment in the engine to clean off as much carbon deposits as possible. That in itself would probably make it pass. You are not failing so terribly bad, although it is failiing.

[hotrodpc]

Oh and yes, you mentioned way up there, you had a new carb put on, but it may be a bit leaned out. You also mentioned it barely passed NOx last time too and that was with the new carb??? I would almost bet it is the high mileage engine carbon build up getting you to fail. As above, clean your EGR or at least test it to be sure its working good, and a seafoam treatment. If your tstat is easy to take out and you are not far from the test station, take it down there. Without a Tstat maybe it won't even get warmed up by the time they test it.You must have heat for NOx to show up I do believe, and if they do not require a certain temp to do the test, take it in cold. Then you'll just have one more year left.

[bradrhale]

Using Gas Analyzer Readings

something that mud posted a while back about everything you need to know about emissions and failing the tests.

[dcp]

hotrodpc-Thanks much. Almost failed NOX at 15mph before carb and distributor and coil replacement. Now fails at both speeds. Problem with running too cold at inspection is that rings/cyllinders are worn so have to warm it up to seal rings.
Bradrhale-thanks for mud's info sheet. Very helpful as well.

[hotrodpc]

Here is the link I had found that is spedific to High NOx.

What causes high NOx. What causes high Nitrous Oxide - SmogTips.com. Why did my car fail the emission test for high NOx emissions. What is NOx. My vehicle failed the emission test for high Nitrous Oxides. Why my car produced high NOx. High NOx failur

[chengny]

Hey dcp, what is the registered GVW (gross vehicle weight) of your truck. If it is over 8500 you are exempt. Check the drivers side door post for a decal.

I believe that this is a common exemption in all states. Mine, for example, is 8600 lbs. Consequently it has never had an emissions test conducted on it.

BTW: Another exemption is for age. Depending on how you interpert the language, vehicles 24 years old or older are exempt. Regardless, this will be the last year you will have to deal with this issue.

Here is the link to Georgia's auto and light truck emissions regulations:

Georgia Clean Air Force: Rules and Regulations

I split it into two sections as below:

[bradrhale]

you are the man chengny.

[hotrodpc]

I already mentioned that part about the 8600. Well guess not, I was reffering to the comptuer. But his is a 1500, so its not going to be 8600 GVWR. He is not exempt. If he were 8600 or better, he would not even have to have Cats on the truck. I know that from my 86. Heavy Duty 3/4 does not have Cats, and does not have the Unleaded fuel filler necks. It was an original non computer, non cat vehicle. The light duty 3/4 and 1/2 tons that year had computers and Cats even on the Federal emmsions.

[dcp]

Just got your questions. Haven't looked but I believe it is six thousand something. Definitely not 8500. I don't know how they count the years for sure but the info they sent to me says it has to be inspected in 2010 also. I did look over the regulations you sent. I think they add 24 on to the original model year to get the last year that it is still covered by the inspection requirement. That gives 2010. Hotrodpc, haven't gotten info on the computer yet. Crazy week at work. I saw mention of the computer and certain codes when I scanned some of the 86 GMC truck manual I have for it. Did you say my truck does have a cat? BTW, I'm pretty sure it is a 3/4 ton.