High Nox on 2001 Chevrolet Express 1500

I'm having problems to pass emissions. I already took my van to a three trained technicians and they couldn't find anything wrong. The level of NOx is a 2.5 were it should be a 2. I really would apreciate any help on this matter. There is no codes stores at the computer. Replace a spark plugs, did a good tune up still failing.

by in Longmont, CO on January 23, 2011
1 answer 1 comment
ANSWER by on January 23, 2011
These are the usual causes for high NOx: Misfire condition Malfunctioning or improperly adjusted EGR valve Failed oxygen sensor Leak in exhaust tubing upstream of converter Excessive carbon deposits in combustion chamber Improper spark advance Blocked coolant passage Overly lean air-fuel mixture Damaged cold air duct Failed or malfunctioning catalytic converter In my experience it's mostly a failing cat. converter and a bit higher than normal operating temperature. I'd check the entire cooling system, make sure the radiator fins are clean, the fan clutch works good, thermostat opens ( you can try a lower temp. thermostat ). If the cooling system is OK, then I'd go for the catalytic converter. On a 10 years old truck it could be the problem also. Zee
COMMENT by on January 23, 2011
The thermostat wasn't working I replaced it and NOx went a little bit higher when tested again, the van has two catalytic converters they look good visually but you are right there are 10 years old probably a good idea just to replace them. Thanks FIX IT Previus owner changed a wrong catalytic converter, I replaced with an obd II converter an it Pass. Thanks