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Jaguar
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Posted 4 Years ago #1
Had my 1991 Chevy Caprice (155,000 miles) undergo emissions testing today, for Texas vehicle inspection. Results:

High Speed Emission Results (25 MPH) Low Speed Emission Results (15mph)

Standard Reading Result Standard Reading Result
HC (ppm): 95 317 FAIL 98 307 FAIL
CO (%): 0.53 8.70 FAIL 0.54 8.40 FAIL
CO2 (%): 9.3 9.5
O2 (%): 0.1 0.1
Nox (ppm): 645 197 PASS 711 211 PASS
DILUTION: >6.0 18 PASS >6.0 17.9 PASS

It badly failed the HC and CO tests, but easily passed the Nox test. What is the likely culprit here?
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EmeraldEyes
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Posted 4 Years ago #2
Could be a bad CAT or O2,could be quite a few things for that matter.
But hey,how about checking the basics first?When was the last time you had it tuned up?Does the car run ok?Give a little history of the vehicle.Sometimes that will make a world of difference in finding out whats wrong with your vehicle.
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syecorge
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Posted 4 Years ago #3
Plugged up air filter or bad O2 sensor?
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BillyD
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Posted 4 Years ago #4
Don't worry about the NOX. Oxides of Nitrogen only form when the combustion temp is too high, say over 2500 degrees.

Your HC (hydrocarbons) is unburned fuel, and your CO (Carbon Monoxide)
is partially burned fuel. Also notice the O2 is minimal, which suggests it does not have a misfire. It's running too rich for some reason.
You can start back at the basics: Plugs, contaminated oil, dirty air filter, bad O2 sensor (or just a slow one), incorrect coolent temp sensor, incorrect IAT sensor, bad/plugged PCV system.... The list goes on.
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34candyman
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Posted 4 Years ago #5
Oxygen sensor would be the first suspect, assuming no trouble codes are stored for anything else (you should be able to retrieve them if there are any with a jumper wire on this vehicle, my site below has instructions).
It's clearly running way too rich, 8% carbon monoxide is exceedingly high.
Quite likely you will notice the gas mileage improve as well..
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