Trying to pass emissions in my SHO

[mechjames]

I'm having a hard time trying to pass our aircare test with my 94 SHO.

Originally after I bought the car, I took it through the test, and failed on Hydrocarbons only.

The readings are:
HC g/km= 0.7553 MAX 0.5000 FAIL
CO g/km= 2.1388 MAX 9.3200 PASS
NOx g/km= 1.0371 MAX 1.2400 PASS

So I proceeded to do a full tuneup, seafoam (worst seafoam i've ever done) plugs, wires, air filter, fuel filter, Pennzoil Ultra oil change, no PCV valve on this car (factory).
Someone also told me to put a litre of methyl hydrate in the tank to mask the HC's. I think it boosted my NOx's instead.

I then tried again, and it got worse.

HC g/km= 0.8558 MAX 0.5000 FAIL
CO g/km= 3.2470 MAX 9.3200 PASS
NOx g/km= 1.2483 MAX 1.2400 FAIL

Now I just realized that the previous owner was using 87 octane 10% ethanol, and that's the gas that I have been testing with. I've now got a full tank of 92 octane ethanol free. Could using the wrong gas cause different readings on a car designed for premium fuel?

The only other thing I haven't tried yet is changing the O2 sensors. With the age on them, they probably could be done and bump my readings down a bit.

If I do that, should I reset the computer again? (I did it after the tune up so that it can relearn the mixtures, then I took it for a 60km drive.) Does the EEC-IV have a drivecycle to get the leanest mixture? That could be also why I didn't pass, maybe I didn't drive it long enough.

Other thing I could think of is the Catalytic converters. There's 3 of them on this car. Not looking forward to the cost of replacing those.

 

[swetrid]

Did you change the oil before the seafoam treatment or after? Seafoam treatment will add hc's to the oil and contribute to higher emissions. Your real problem is probably the cat converters, i replaced alot of them back in the day when we had dyno emissions tests. Probably wouldn't hurt to change the o2 sensors if they are originals.

 

[swetrid]

High nox could be from EGR or cat converter problems also.

 

[mechjames]

Did you change the oil before the seafoam treatment or after? Seafoam treatment will add hc's to the oil and contribute to higher emissions. Your real problem is probably the cat converters, i replaced alot of them back in the day when we had dyno emissions tests. Probably wouldn't hurt to change the o2 sensors if they are originals.

High nox could be from EGR or cat converter problems also.

Odd thing happened, they thing finally threw a CEL this morning on the way to work. Will check when I get home.

Lets say if it finds the O2's aren't switching, the computer goes into limp home and runs on a baseline mixture, which would fail me too?

Now what about the EEC-IV drive cycle. After I change the O2's, how long should I drive the car before it recalibrates itself?

EDIT: I changed the oil after I ran the seafoam.

 

[mechjames]

High nox could be from EGR or cat converter problems also.

The high nox only happened on the third test. Maybe the seafoam took out what was left of my cat converters.

 

[mechjames]

Alright, got home from work, and at operating temperature I initiated an EEC test.

Key on engine off revealed my continous memory code was a 172. The regular KOEO was a 111-pass.

During the run test, it also came up as a 111. I then proceeded to do the cylinder balance test, and came up with a 99=all cylinders equal.

Code 172 is

HEGO sensor circuit indicates system lean (right side)

So looks like it should be my oxygen sensor. If it's wrongfully indicating that it is lean, that means the mixture is richening up, failing my test.

 

[mechjames]

After changing the O2's and driving for a day:

So latest test results:

HC g/km = 0.8971 MAX 0.500 FAIL
CO g/km = 3.4085 MAX 9.3200 PASS
NOx g/km = 1.6923 MAX 1.2400 FAIL

At this point I decided to remove the SPOUT jumper, as instructed here. Results got better but not perfect to pass.

HC g/km = 0.6266 MAX 0.5000 FAIL
CO g/km = 3.3641 MAX 9.3200 PASS
NOx g/km = 0.9225 MAX 1.2400 PASS

But not enough. A shop I know is going to test the cat converters, but i'm sure now that is the issue here. New cats should bring my HC down enough to pass emissions.

Also, aftermarket cats usually only last a few years, and they look like they have been changed already.