1988 Volvo 240 DL normally aspirated with odometer that stopped working at 284k miles. I bought the car a couple weeks ago. Smog test last month was abnormal only for an elevated NOx at 15mph of 637 ppm (max 689).

In other words, the rest of the smog test at 15mph and 25mph, ie CO2, O2, HC, CO% all well within normal and in fact below average. I can post numbers and averages if needed.

Spark plugs and oxygen sensor had black soot. The spark plugs' gap was high at 0.037". I changed the plugs and oxygen sensor.

Idle speed was high at around 1500 rpm. I cleaned the throttle body, flame trap system, and IAC valve and changed two cracked vacuum hoses. Now idle speed is down to about 900 rpm.

What's the cause(s) of the elevated NOx at 15mph, and what can I do to get the numbers lower for next June (2019) smog test? For example, would Seafoam and/or CRC's Guaranteed to Pass work?

Thanks