High NO on smog test

[Noah930]

I have an '87 Turbo. It's stock. I recently took it for its bienniel California smog test, and it passed. Motor was hot, full tank of 91 octane, O2 sensor is hooked up, etc.

But looking at the values, I noticed that NO is at the higher end of the allowable spectrum. HC and CO are well at the bottom of their scales, though. For my own education, what is the cause for this? Is it as simple as the motor's running lean? Please teach me something.

[RNajarian]

I’d be curious to know too, but at least you passed.

[Joe Bob]

My past experience is that when HC and CO are low, then the NOx is high. Fuel mix can be adjusted to raise the HC and CO and bring down NOx. I assume your Turbo has CIS. You can hook up to an exhaust analyzer and adjust the values.

[yelcab1]

Your engine is running too hot. Probably a little lean.

[Quicksilver]

NO is a product of higher pressures during combustion. You only knock it down by reducing power or by using a three way catalytic converter. (The original 2 way catalysts didn't deal with NO and NO2.)

In the 80s Motronic Carreras they have a brown jumper wire that made the system retard the ignition by 4° to reduce the combustion pressure specifically to reduce NO for California's emissions. (So yeah, we disconnect the wire when it was time for a smog check.)

[proporsche]

Originally Posted by yelcab1

Your engine is running too hot. Probably a little lean.

^^^^^^this^^^^^^i`d say the same^^^^

Ivan

[FloydPink]

Originally Posted by Quicksilver

In the 80s Motronic Carreras they have a brown jumper wire that made the system retard the ignition by 4° to reduce the combustion pressure specifically to reduce NO for California's emissions. (So yeah, we disconnect the wire when it was time for a smog check.)

Do you mean connect the wire for smog? I was under the assumption that it should be plugged in to retard timing. On that note, I'd love to do a comparison between smog results with the brown wire connected/disconnected to see how much headroom it gives those of us with motronic.

[Solamar]

Originally Posted by FloydPink

Do you mean connect the wire for smog? I was under the assumption that it should be plugged in to retard timing. On that note, I'd love to do a comparison between smog results with the brown wire connected/disconnected to see how much headroom it gives those of us with motronic.

That's my understanding as well. Hope we don't have it backwards!

[HarryD]

Originally Posted by Quicksilver

NO is a product of higher pressures during combustion. You only knock it down by reducing power or by using a three way catalytic converter. (The original 2 way catalysts didn't deal with NO and NO2.)

In the 80s Motronic Carreras they have a brown jumper wire that made the system retard the ignition by 4° to reduce the combustion pressure specifically to reduce NO for California's emissions. (So yeah, we disconnect the wire when it was time for a smog check.)

NO is a product of high temperatures and a lean running condition. By slightly richening the mixture, you reduce the peak flame temperature and produce more CO and HC.

This chart is helpful To see how they interrelate.

[Cals911]

My 86 carrera a couple weeks ago, I live in a “non enhanced” zip code (desert)
No no

[dannobee]

EGR problems can cause high NOx, too. IIRC, 930's had EGR on both CA and FED cars.

It's still not terribly high. I'd be more concerned if it barely squeaked by.