What causes bad NOX readings in an emissions test?

- Peter.

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2021 Chevrolet Spark
Formerly...
2000 GMC Sonoma
1981 240D 4spd stick. 347000 miles. Deceased Feb 14 2021
2002 Kia Rio. Worst crap on four wheels
1981 240D 4spd stick. 389000 miles.
1984 123 200
1979 116 280S
1972 Cadillac Sedan DeVille
1971 108 280S

lean mixtures, timing off, egr valve stuck open

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have no worries.....President Obama swears "If you like your gun, you can keep it

Had that problem on my Landcruiser recently. Non-operating EGR. The EGR exists to reduce NOx emissions by lowering combustion temperatures. I troubleshot mine with a Mity Vac. What kind of car? Mine had a bad EGR valve, a bad modulator and a bad vacuum solenoid valve. You can pass emissions by forcing the EGR to be on all the time. I did that because the VSV which controls the on/off of a Landcruiser EGR had not arrived before I had to take my free follow up test.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

High NOX comes from higher combustion temps. It is tougher to clean up so automakers tend to run a bit rich (which lowers temps) and just clean up the HC in the cat. Egr is supposed to lower NOX by "poisoning" the mixture and making it burn at a lower temp. EGR could be stuck closed or just clogged. i think early timing can up the temps too. of course a bad OX sensor will screw it up too. If you are lucky, you have the cheaper of the two models; usually with just one wire.

If you have OBD, start with codes. If not, always start with the EGR when troubleshooting a NOx issue.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.

Thanks for the replies. It's a 1990 Suburban I am considering. It passed it's emissions recently in all areas but the NOX just squeaked by. So I'm wondering how much of an issue it could be in the future.

- Peter.

__________________
2021 Chevrolet Spark
Formerly...
2000 GMC Sonoma
1981 240D 4spd stick. 347000 miles. Deceased Feb 14 2021
2002 Kia Rio. Worst crap on four wheels
1981 240D 4spd stick. 389000 miles.
1984 123 200
1979 116 280S
1972 Cadillac Sedan DeVille
1971 108 280S

5.7L came with no air pump 89-91, so that might be the design value. Squeak = pass

You passed so you're good. If it doesn't pass next time, look at the EGR.

__________________
Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.

Interesting. I'm thinking the earlier, the simpler the better.

- Peter.

__________________
2021 Chevrolet Spark
Formerly...
2000 GMC Sonoma
1981 240D 4spd stick. 347000 miles. Deceased Feb 14 2021
2002 Kia Rio. Worst crap on four wheels
1981 240D 4spd stick. 389000 miles.
1984 123 200
1979 116 280S
1972 Cadillac Sedan DeVille
1971 108 280S