High NOx emission

Hello,

Buick Century 92 3.3 L 160k.
The car is in good shape.
No SES light.

First Emission Test failed NOx 1100 (GTA limit 450).
I replaced cat.
Second one again failed NOx 990.

The problem is this car doesn't have an EGR valve.

Could somebody tell me how can I lower NOx? Does anybody have this problem and fixed it?

Thank you.

Most vehicles built in the '90's were built with EGR valves, whether visible or not. Look on your emissions sticker for the moniker EGR. It will appear with stuff like o2, catalyst, etc. This will assure you that the engine either has EGR or not. Though possible, it is not typical for this era engine to not have EGR. If it does not, things like excessively lean mixtures, or over-advanced timing can create excessive NOx. It is also possible for an incorrectly installed timing chain to cause this as well. Essentially, excessive combustion temperatures cause high NOx emissions. Let me know ... Ray

Unfortunately there is no EGR valve on my car.
I found an article over the Inet: “dirty MAF sensor = High NOx”. The study was done on a car exactly like mine Buick Century 3.3L. Link: http://www.gatewaycleanair.com/mechanic/air4_4/dirtymaf.htm
Tomorrow I am going try a used MAF from scrapyard. A new one is 500 CAN.
If it doesn’t work I have timing chain checked and maybe a trip to a dealer to scan the ECM.

Unfortunately there is no EGR valve on my car.
I found an article over the Inet: “dirty MAF sensor = High NOx”. The study was done on a car exactly like mine Buick Century 3.3L. Link: http://www.gatewaycleanair.com/mechanic/air4_4/dirtymaf.htm
Tomorrow I am going try a used MAF from scrapyard. A new one is 500 CAN.
If it doesn’t work I have timing chain checked and maybe a trip to a dealer to scan the ECM.

a lean running engine will cause high NOx. (dirty MAF sensor will contribute)

A Buick 3300 has no EGR valve

I solved the problem.

But I tell all the steps maybe somebody is in the same situation or will be.

First I replace MAF sensor (took from scrap yard and cleaned). Nox dropped to 800 ppm. Not enough (limit 450ppm).

Next step: another MAF sensor (also from scrap yard) and Sunoco 94 gas. Nox dropped to 730 ppm. Not enough.

Last try: I swap the ECM. NOx went down to 449 ppm. Only 1 point below line. Unbelievable. But who cares. I PASSED.
The ECM was from a Buick Century 3.3L Station Wagon. Engine was running good, but tranny went crazy, shifting funny and making noise.

Anyway this is the story. Bottom line I need a new ECM, another trip to scrap yard.

Bye.