Failing NOX on emission test

[gmtechnician]

Does anyone know if on a 91 Cadillac Deville 4.9L has torque converter lock up in 3rd gear at 40km/hr. I know of one that keeps failing NOX emissions.
Tried everything except disabling torque converter lock up.

[DFBonnett]

Originally Posted by gmtechnician

Does anyone know if on a 91 Cadillac Deville 4.9L has torque converter lock up in 3rd gear at 40km/hr. I know of one that keeps failing NOX emissions.
Tried everything except disabling torque converter lock up.

I'm not trying to second guess you but are the EGR tubes visible looking down into the TB clear? Is the pintle on the EGR valve clean? Those are the usual culprits on the 4.9 when there is an NOX problem.

[maxwedge]

Tcc has nothing to do with nox failures, as DFBonnett said this is almost always an egr issue.

[gmtechnician]

I know the 98 venture vans you have to E-test in 2nd gear because lock up
occurs too low a speed and they all fail by NOX. I had the EGR off and cleaned the tubes and the EGR cleaned. When operating the EGR it pulls the engine speed down and runs pretty rough so I assumed it is OK. I ran it again in second gear and the NOX dropped from 1600 to 1100. I tried backing the timing off and it made no difference. Is it possible the catalytic converter can cause this? I didn't think so but??? The car has 180,000 miles on it. Compression is 156 across the board. I am lost with this car. HELP!!!

[maxwedge]

Yes the cat could be bad, cyl temps are the biggest contributor to nox. Egr corrects that tendency, this is a simplification but that is the basic theory.

[ilgoldstein]

Sounds like EGR to me. My '89 had the same problem. Not too big a deal. The guy at the smog check place told me to make sure the car was good and warmed up before a smog check so that the cat was working better. Anyone ever hear that? Not just a few minutes, but a nice run.

[maxwedge]

Yes, cat temp is critical.