High smog NO numbers at low speed only

[netarc]

Before I take my 92 Cabriolet into the shop to get this resolved, I wonder if anyone has any ideas re: why the NO/ppm measurement would be so dramatically different at low v. high speeds?

The full results follow, but the NO emission @ 25mph came out at a very low 50 ppm, while that at 15mph was 1157 ppm!? That seemed to be the only issue in my check, all the other numbers looked pretty stellar

fwiw, this car has not been driven at all for the past few years; been sitting in the garage, and only fired up once every few months when I need to move it out for access. I'm now in the process of reregistering the car, though, hence the smog check. again fwiw, the last smog check I had (back in '08) was successful.

Any advice would be much appreciated, thanks!

[SpclAgentD]

When was the last time you did a tune-up on the usual stuff? O2 sensor? Oil? Plugs?

Also, if it's been sitting in the garage a few years, I imagine you're running the same gas too.

What about your cat converter? Sometimes, high NO readings are from a bad cat converter. Speaking of, lean fuel mixtures can ruin a cat converter as well, so something else to check into.

Good luck!

[netarc]

It's been some time since the last maintenance, so yes you're right, best place to start would be to change oil, check plugs, cat, etc.

[vonfulk]

From my experience, numbers way out of tolerance usually has meant the cat is bad.