How to lower NOx emissions
June 6, 2006 6:10 AM   Subscribe

Is there anything I can do, without the help of a mechanic, to lower my car's NOx emissions by ~60ppm?

My '93 Cutlass Supreme only just failed the NOx part of its required emissions test. (Driveclean.) It passed the HC and CO no problem.

To get a pass for the NOx emissions as well, every mechanic I've talked to wants to start with a $100 diagnostic test. If the emissions were way over, I'd do the test and get whatever repairs are required (clean air, and all.) But the mitigating factors here are: 1) it failed by only 60ppm, and 2) I'll be selling the car before the end of the year. I don't want to spend a penny on it if I don't have to.

Is there anything I can do myself to reduce the NOx emissions? I'm pretty sure the Cutlass Supreme's timing is computer controlled, so that's probably not an option. What about the fuel/air mix? Is there any way I can adjust that myself? Or what about simpler options like fuel additives?

Some other quick notes:
- I had the air filter and oil changed last week.
- The catalytic converter was replaced 18 months ago, and has less than 20,000 miles on it.
- For all the good it did, I ran one of those "guaranteed to pass" products through a full tank of gas, and filled up with Sunoco 94 before the test.
posted by ParsonWreck to Travel & Transportation (8 answers total) 1 user marked this as a favorite
Did you drive around for half an hour before taking the test? I've heard that doing it on a warm engine helps.
posted by croutonsupafreak at 6:18 AM on June 6, 2006

Ya, I took it on the highway for half an hour, and left it idling at the testing center while I waited.
posted by ParsonWreck at 6:23 AM on June 6, 2006

NOx is generally the result of really high temperature exhaust. Being the son of a mechanic, it is difficult for me to say what is beyond the scope of most individuals. I would richen the mixture slightly to lower temperatures, or advance the timing slightly. I think that will cool the engine down enough. A 30 minute run on the highway straight onto the test dyno would likely help if you can arrange it.

On preview: try to book the test for first one of the day or slip the mechanic a $10er to get it right on there off the highway.
posted by KevCed at 6:50 AM on June 6, 2006

or advance the timing slightly.

You mean retard the timing?

Parson, just because the timing is electronically controlled doesn't mean you can't adjust it. There is usually a base timing setting that is used as a starting point, which is adjustable by turning the distributor. Of course, I don't know about your particular car, but this is what I know from my cars. Good luck trying to tune your emissions in your driveway without a sniffer test though.

Does your car have multiple cats? Usually there is a primary and a secondary, at least. If you've replaced one, consider replacing the other. Replacing cats is the only reliable way I've seen to lower NOx emissions.
posted by knave at 7:34 AM on June 6, 2006

Richening the mixture, even if it does lower NOX, may adversely affect the HC/CO numbers
posted by Pressed Rat at 9:27 AM on June 6, 2006

This car has closed loop fuel control - you can't richen the mixture without changing the calibration prom. Also, many cars in this time frame didn't have distributors, and do not have a way to adjust spark timing. You might check to make sure your EGR valve is working (if equipped).
posted by rfs at 10:20 AM on June 6, 2006

New plugs?
posted by Thorzdad at 12:36 PM on June 6, 2006

For future searchers, here's what I did to get a pass:

First, high-octance gas, at least the Sunoco stuff with ethanol added, was a poor choice to reduce my NOx levels. A couple of references:

  • "The downside of gasohol use can be increased emissions of nitrogen oxides (NOx)"

  • "The auto/oil study concluded that there was a statistically significant increase in NOx of about 5% with the addition of 10% ethanol."

  • Second, I found info how to clean the EGR valve myself. (Instructions are specific to my Cutlass, though:)

  • Autozone


  • Finally, I ran a can of carb and fuel line cleaner through the system, filled up with regular (87 octance) gas, and re-tested.

    I passed, just barely. But hey!
    posted by ParsonWreck at 6:48 AM on June 21, 2006

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