I have an '88 Formula (2.8) that recently failed a smog test here in California. The only problem was high NO emissions.
NO emissions were high across the board, but I only failed at 15mph. I squeaked by at 25mph.
When I see high nitrous emissions I generally think the EGR system is to blame, or the timing is too far advanced.
I have checked the timing and it looks OK.
I have checked the EGR and it seems to function correctly. Please check me:
When I apply vacuum to the EGR valve, the engine stumbles. So I assume the EGR valve is good.
I connected a vacuum gauge to the input of the EGR valve and drive the car around.
I saw no vacuum at idle. I saw up to 4 in HG when going up a steep hill.
That seems a bit low, but I cannot find any reference to tell me if it is low.
Does anyone know?

the O² sensor may be to blame.

https://www.fiero.nl/forum/Forum2/HTML/132445.html

Pretty sure the EGR does not operate at low speed (ex. 15 mph and below).

[This message has been edited by Kevin87FieroGT (edited 02-12-2019).]

Nox is a byproduct of combustion.. it forms in excess when combustion temperature is really high and that can be caused by many different things. There’s is a lot to it, but if I suspect the EGR is not working, I do the “stumble” test and then go for a test drive with the vacuum gauge “T” into the vacuum line going to the EGR and see if it’s actually being told to activate and when. But honestly, most of my emission problems were due to a bad catalyst.

-Crasian

quote Originally posted by Crasian: Nox is a byproduct of combustion.. it forms in excess when combustion temperature is really high and that can be caused by many different things. There’s is a lot to it, but if I suspect the EGR is not working, I do the “stumble” test and then go for a test drive with the vacuum gauge “T” into the vacuum line going to the EGR and see if it’s actually being told to activate and when. But honestly, most of my emission problems were due to a bad catalyst. -Crasian

Exactly my thinking!
That is exactly what I did...after replacing the catalyst.
I guess I should have mentioned that.
So I tried applying 4 in HG to the EGR valve (with my vacuum tester) to the EGR valve while the engine was idling. It stumbled noticeably, but did not die.
Can I rule out EGR now?

Not quite...
V6 EGR is controlled by solenoid that's controlled by ECM.

Do what said above. T the line to EGR and watch vac gauge while you drive. Don't need to watch exact #, just when shows vac at various time, most over 35 mph or more but ECM can demand EGR On at any time. Really need ECM scanner have have help to watch both to match. IOW Scan say EGR On then Gauge should have vac too.

Emissions failures like High NOx can be several things... In no order...
Iffy EGR.
Iffy Cat.
Iffy Ignition Timing or other problems w/ HEI.
Iffy O₂ sensor.
Exhaust leaks can let ail In the system cause iffy O₂ data and/or fake out the Cat.
Burning oil or coolant can make Cat and O₂ problem and poison both that will kill them often w/o Codes setting before O₂ completely die. Poison/Dead cat can be High NOx, fail blocking exhaust, Overheat that can cause a fire, etc.

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