i have a 1991 honda accord i took it for emission testing and it failed for NO, the car does not overheat and i already cleaned the intake ports, any ideas?

You might try decarbonizing the upper engine. If there is a lot of carbon in the head combustion chamber, compression could go up and create excessive NO. Also a old O2 sensor could cause the engine to burn lean which would also increase excessive NO. When you find the problem, tell us. We would all liketo know.

Most common cause of NOx emissions is a bad EGR valve...NOx is produced when combustion temps are too high, usually occurs because of a lack of EGR on acceleration, or overly lean condition.

Did they give you an emissions trace report that shows at what point the reading spiked? That can help a lot...

Jeff, if the EGR valve or something in the vacuum circuit was bad, you most likely would get a EGR fault CEL light. As for burning lean, when the O2 sensor goes bad, it creates a lean burn because the O2 sensor thinks the exhaust gas is rich so the ECU leans out the engine.

yeah...wasn't sure how good the Honda's feedback on the EGR was in '91...I don't think it could sense position back then...

Yep, it can. The ECU sends a signal to open the EGR solenoid valve causing the EGR to open which them sends a signal back to the ECU that sez the valve opened.

You might try decarbonizing the upper engine. If there is a lot of carbon in the head combustion chamber, compression could go up and create excessive NO. Also a old O2 sensor could cause the engine to burn lean which would also increase excessive NO. When you find the problem, tell us. We would all liketo know.

How do i know how much carbon is build up in the combustion chamber, pull the head off? is that the only way

Well, you could bore scope the cylinders through the spark plug holes. However, most people recommend sucking a can of Mercury PowerTune through the throttle body with the engine running about 2000RPM for the first half of can, then suck the remainder into engine at idle until it stalls. Then let it sit for an hour. Then start the car and run it until the carbon blows out. This doesn't need to be done for engines under 60K miles. But I would really consider changing the O2 sensor if the engine has over 100K miles. That might be the smartest first thing to do. Sensors are about $70.