I need to pass smog for a 'new' car I got, and because it's been non-op for over 5 years, I want to do everything I can to try to ensure passing first time. So Ive been reading at various sites, not only Volvo related, and see that NOx can be a major issue. One of my current cars only just passed NOx the last time it was tested, and that is further impetus.
One of the issues seems to be that, as NOx goes down, HC can go up, so it's finding the right nexus I guess.
So here is a distillation of some things I found, one very good one right here on BB.
High NOX problem resolved

but here is the most important part of that I think.

"I put in a tank of Chevron 91 octane, a bottle of techron, drove it 40 miles at highway speed and brought it to the facility. The tech looked at the previous two test results and didn't give me too much hope. He placed it on the dyno, ran all the tests and came back saying that it had passed. NOX was now 30% of allowable having fallen from 738 to 179 with nothing being done.

"When the tech and I discussed this he asked how hot it had been in the other town on the day of the test. I looked it up and it was 108F. Today on the coast when the car was tested it was about 65F. He wondered if the difference in ambient temp could have made the difference."

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Here are some more tips;

Put the heater on, to lower head temps.

Do not use alcohol in the fuel; at every site I looked at, for every type of car, alcohol made NOx go up.

I don't know what to say about this bit, about whether it's at all possible or practical:
"I would recomend a catalyst rated for a much heavier vehicle to get a good life span and effective reduction."

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General tips/comments from various sites;

I'm also desperate to know of a solution to lower NOX emissions. I have made all the usual repairs and checks and am to the point of using perhaps a fuel additive like alcohol. Does anyone have any experience with any substance that can be added to fuel to reduce NOX emissions?
as well as post if you like. And Thanks much, Don ^
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Combustion chamber deposit buildup also adds to higher NOx output. High doses of some detergents such as Polyether-amine types can sometimes reduce these over the course of 300-600 miles. Some engine designs are much more prone

Some types of detergents used in pump gas may increase CCDs. Alcohols at higher levels can also increase CCDs, so if you ran alcohol routinely rather than just to pass the test then it might temporarily mask the problem while making it worse.
Retarding the timing will probably help too, but it is a cheat. EGR will also reduce NOx. So will running richer. The problem is, most of these "solutions" are set from the factory. Maybe the standards are too strict and the decision makers should be alerted and perhaps exceptions can be made. I'm assuming the cars are all fuel injected and have 3-way cats, which usually means you're S.O.L. A good tune-up and clean combustion chambers, and a functioning O2 sensor, cat, and EGR are about all you can do.
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Combustion chambers were cleaned, first using water through carb while engine was running, followed by GM's "top engine cleaner" again through carb (its carburated w/fixed jetting and a mixture solenoid as a booster), then removed plugs and poured directly in cylinders- two bottles / applications. Then Techroline fuel additive was added to the fuel and run through. The Cat converter is a week old and was removed when the above cleaning was performed and reinstalled after a couple hours running time. O2 sensor is also new. The EGR valve is 2 weeks old and the passage was cleaned with various chemicals and is open... proof of that is that the engine stalls when you manually squeeze open the EGR valve at idle. All the emissions solenoids are working and the vaccume going to the EGR valve appears to be sufficient. Spark plugs, wires, cap and rotor are all new (a couple weeks ago). The truck runs fine, no error codes are stored on the computer, and all emissions measurements except NOX are good. Timing looks good, although I need to check to see if the harmonic balancer has slipped. Other than that, I don't see what more I can do to fix it. I'm at 2200 for NOX, and I need to get down to 1500 or so at most, and the smog guy's printout says around 550 is the average. If the balancer has slipped, could those few degrees timing make the NOX that bad? I'm told that overly milled heads might do this, but it passed smog two years ago, although I'm also told that they changed the smog test for my area in S. California. Anybody have any ideas? They just gotta have a fuel additive out there for this... anybody?
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Have you tested pre-cat? A lot of aftermarket cats are not up to snuff when reducing NOX, for a commonly failing model such as yours, I would recomend a catalyst rated for a much heavier vehicle to get a good life span and effective reduction. That carb I believe is adjustable for dwell angle and the internal throttle position switch is also adjustable (has a big effect on timing). ??5

Update on the 1982 GMC S-15 with 2.6 V6. Because the timming mark measured off, I bought a new harmonic balancer (comparison also revealed it was off also- about 2 degrees)and I installed that and retimmed the engine. Just for sport, I threw in some off the shelf octane booster. Unfortunately, the NOx remained just over 2050 ppm, down from around 2200, but short of the acceptable 1550 (and for reference, the average vehicle like mine measures around 550 ppm according to smog database.) At this point, I had exhausted every possibility I (and many others) can think to check or repair, and without firm information on a fuel additive, a low tech solution was adopted. The smog guy, believe it or not, experienced what I can only describe as empathy for me after so many failed attempts. Didn't think it was possible?... Me neither, but it happened. Empathy... it had to be. Out of no where, a paper clip jumped into the diagnostics connector in the vehicle placing the computer in test mode and the NOx dropped like a rock. Down to about 435 or so and all other measures looking about as good. It passed like it was a new car, which isn't far from the truth anyway. So there you have it... the solution. That bit of wizzardry is sure to save you a ka-jillion dollars on your next smog test. This creative solution reminds me of when I used to work in aerospace, for many reasons that there is too little space here to share. Anyway, thanks everyone, and feel free to write if anyone does come up with an "easier to cheat" fuel additive- and consider marketing the stuff and making serious cash. Good luck DonClark@mchsi.com ??1
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I used water injection in my car, and it can be done simply for about $20.
My car was dieseling all the time. Now it runs smooth.
I live in the woods, I don't have to pass emissions, but I can tell you that the engine runs much better. Just for that, you can always try water injection and see if it helps for NOx (it should).

How I did it for $20:
A water bottle
A 1/8" check valve
Two 1/8" brass needle valves, a 1/8" brass tee.
A plastic Tee that will fit your PCV line with a 1/8" barb in the center.
A length of 1/8" ID plastic tubing.

The 1/8" tubing goes directly to the bottom of the water bottle, with the check valve at the end in the water.
I use 2 needle valves and a tee to mix water and air, it makes it easier to adjust the water flow (compared to just one needle valve). The water inlet (from the bottle) goes into 1 branch of the tee. A needle valve draws air in another branch, and the water+air mix exits through the second needle valve, to the tee connected to the PCV line.
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This is a $20 ghetto system. You can buy a system quite similar, professionally build, on line for $400 (check Aquatune.com).
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