1979-1993 & 94 Conv [Subscribe to Daily Digest] |
high NOx usually means a lean mixture, so this could be 1)lean to the engine ie:fuel injection or an air leak problem, or 2)lean to the catalyst ie:air leak in exhaust system pre-catalytic converter. The cat needs a "reducing" atmosphere to strip the oxygen from the nitrogen (NOx to N2) and use it to combine with CO (CO to CO2). If there is excess oxygen in the cat from an air leak in the exhaust system, the NOx to N2 conversion does not take place.
If a real techie here or on brickboard agrees that you seem to have eliminated the AMM/O2 sensor combo as a possible culprit, & you have checked all the hoses for vac leaks, then to continue to investigate the pre-cat exhaust leaks would be my next step. How expensive/time consuming an R&R is the Volvo headpipe? Or like, could a muffler shop spot weld the small holes you described- it might be worth it to tell your friend to call around to check.
http://www.gatewaycleanair.com/mechanic/air4_3/nox.htm
or
http://www.interro.com/techgas.html
have some great info... but I don't know any gazillionaire car enthusiasts with their own Gas Analyzer so I took the Saab into a local indie saab only shop. The 88 900T's "hooter" valve was hooting on the drive down from BWI to ATL, so that was an obvious place to begin a vac-leak check for a "lean to the engine" culprit.
If you are really into this stuff, check out http://www.worldbank.org/wbi/cleanair/cailac/infopool/pdf/catalystbased_diesel.pdf
(page 13+ is really interesting) but short the next MIT grad who comes up with an aluminosilicate substrate "zeolite" cat or "NOx adsorber" cheap & effective enough to plug into existing technology...
posted by 172.157.16...
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