So I failed Emissions

I bet the seafoam and the timing retardation will help you pass. When I used to smog my race car I would get the EGR working again (big player in NOx reduction) and retard the timing to it's lowest setting.

I've seen "cheater" plugs help with NOx too. You can run a colder plug to drop combustion temps, a shorter plug to slightly lower compression, or one of those "fouler" adapters that pull the plug out of the combustion chamber.

Getting the catalyst nice and hot is dancing on a fine line. The cat is more efficient hot, but then you may get too high of combustion temps which will escalate your NOx.

Did the Si have an egr system? If so it must be gone what with the long tube headers and all.
I probed the O2 sensor. It was weird. Everything I can find says it should be right around .45v. It started at .36, and then was unstable as hell. It dropped as low as 0.1. It seemed like the act of probing was affecting readings, causing them to fall. I unplugged it and probed it and it read a constant .014V and the car didn't car at all that it was unplugged. No CEL. I'm not sure I learned anything at all.

My 79 f150 didn't pass here in ky when we had emissions testing. Vehicles got tested every other year depending on the year of your birth. I sold the truck to my wife, who was born a year earlier, for $1 and when it was her turn for emissions testing she sold it back to me for $1. Problem solved.

mazdeuce wrote: Did the Si have an egr system? If so it must be gone what with the long tube headers and all. I probed the O2 sensor. It was weird. Everything I can find says it should be right around .45v. It started at .36, and then was unstable as hell. It dropped as low as 0.1. It seemed like the act of probing was affecting readings, causing them to fall. I unplugged it and probed it and it read a constant .014V and the car didn't car at all that it was unplugged. No CEL. I'm not sure I learned anything at all.

There's not much of a check engine light system in that age of cars.

.14V = lean, and since you pointed out that your car is tuned lean at lower engine speeds/loads- that result was expected by me. Can you change the calibration? Putting in a new cat will help- but just getting it to run stoich will take care of most everything.

I got nothing for ya … just glad that here in NC it's only '96 and newer that have to pass emissions

wbjones wrote: I got nothing for ya … just glad that here in NC it's only '96 and newer that have to pass emissions

Same here, no sniffers upstate NY, just no CEL on OBD2, or simply having a cat on 75-OBD1

Serious thank you to everyone that helped me think through this. I'm now happy.

So what was the fix?

I pulled the piggyback that was messing with the MAP signal, pulled 2 degrees of timing, and put in a new O2 sensor because I couldn't get a stable reading. My bet is the piggyback and timing were the culprit.

In reply to mazdeuce:

now there's no worry anymore, right?

Nope. I just had to pass this one year. After this the car is 25 year exempt, so I'm good.

Car is over 21 years old. Can you register it as a classic vehicle and get exempt?

nvm see the issue is resolved

We're at 25 years for that too. As of next year four of my seven cars will be eligible.