Re: hissing noise on clutch pedal release
The noise - Hissing is while the clutch is still slipping or after it's hooked up?
If when slipping, probably it's just breaking in -- worry after 100 miles and it still makes the noise.
NOx - overheating can drive NOx up. Also over advanced timing, too retarded timing, too lean a mixture, not enough EGR, carbon in the chamber, etc.
Anything that drives the combustion temps up. Oxides of Nitrogen form when the combution temperature goes over a magic number just under 3000 degrees.
If it's running extremely hot, then he's on the right track.