High NOx in smog test

[Gscott_80]

Just failed on the NOx portion of the Colorado Emissions test with a 95 mtx, unmodified.

HC, CO, CO2 are within spec. CO cut it close though.
NOx was at 3.278 gpm. Limit is 2.5

Car has been running and driving as it should.

Could it be from car sitting running for half an hour in line on a 98 degree day?

 

[Pro*banshee]

This website may help:

http://www.car-forums.com/s9/t2590.html

It says the main cause is typically a stuck EGR valve. Or you could get it by running lean

 

[frosho]

Read through this thread: http://www.shoforum.com/showthread.php?t=56278

 

[Eric VerValin]

there used to be some stuff you could buy at Autozone and add to a very small amount of fuel in your tank. I have heard many people pass using that stuff.

But that being said, none of those car's were SHO's.. mostly older V8's ...

 

[hawkeye18]

Pull your SPOUT. The car will run like absolute ***** as timing will default to 10 degrees BTDC but NoX levels will be lowered significantly. Look for a gray jumper plug looking thing under the EEC test connector.

 

[shoon]

Pull your SPOUT. The car will run like absolute ***** as timing will default to 10 degrees BTDC but NoX levels will be lowered significantly. Look for a gray jumper plug looking thing under the EEC test connector.

x2

had the same problem, pulling SPOUT dropped NOx enough to pass emissions.

 

[rubydist]

As others have said, pulling the SPOUT connector should get it to pass.

Since the mtx cars (except CA) do not have egr valves, that's not the culprit. The most likely causes are: 1. bad cats (the CO near the limit suggests this too), 2. lean mixture, 3. hot spots in combustion chambers from carbon buildup.

Since the car sat in line for 30 minutes on a hot day today, its possible that there were hot spots from carbon buildup. Normally, in the old days you would hear ping on occasion if that were the case, but a working knock sensor will pull out timing before you ever hear it, so its harder to tell on a efi engine. However, you say its been running fine, not down on power, to the hot spots are less likely than the cats, imho.

If the car is really running lean, you should get a check engine light complaining about fuel trims being outside normal limits, or o2 sensors not switching, but not always.

If the SPOUT doesn't do it, you will likely be looking at cats and/or 02 sensors.