High Nox - Southern California

morfeo_ur · Post by **morfeo_ur** » 25 Jan 2017, 01:12

Hello, I have a 1997 GLT that has failed the smog inspection twice. The car is working fine. Starts right away, has a smooth idle and does not seem to have any performance issue. The first time, a check engine light came on minutes before inspection. It was a P0116, and the results showed Nox levels of 650. Thinking that the high Nox was due to the faulty ECT sensor, I replaced it with a Volvo unit, and changed the thermostat. Also replaced the radiator and a couple of vacuum hoses that seemed old and brittle. Took it to inspection again and the car failed with worst numbers. This time the results showed Nox levels of 750 at 15mph.

Right now, I am suspecting a failure in the fuel delivery system, because there is a high pitched noise coming from the back of the car. Is there anything else I should consider?

The car has something like 150.000 miles.

Also, does anyone know of a good independent mechanic that can help me out with this around the Orange County area? I don't have much time to work on the car and need to get this fixed as soon as possible!

Thank you very much for your help!

Post by **abscate** » 25 Jan 2017, 05:18

Keep on looking for those vacuum leaks or replace all the vacuum lines.

High Nox is lean running so either your fuel delivery is bad (fuel pump relay, pump) or air is leaking in, which is torn/split/holes hoses.

jake2011 · Post by **jake2011** » 25 Jan 2017, 22:54

I'm the guy who sold him the car. After I did a complete overhaul to engine several years ago, I never had smog issues and reassured him that it'll pass smog. Ofcourse, now it decides to not pass.

Well it looks like that narrows it down to those two areas. Since the vacuum hoses are now all in good condition it must be the fuel pump or relay. Do you want me to order the fuel pump for you? It seems like a straight forward job so labor doesn't look like it'll be that much.

Let me know so I can order it asap.

Post by **abscate** » 26 Jan 2017, 07:19

Take a look at your fuel trims to see if you have a vacuum leak

If they are floating at +10%, you could be running lean enough for a NOX fail but not enough to trigger P0171

If they drop when you rev to 2500 rpm, you have a leak

morfeo_ur · Post by **morfeo_ur** » 26 Jan 2017, 15:41

Ok I just took a look of the fuel trims. When the car is idling they stay close to 0 but go up and down between 5 and -5, which I believe is normal. However, when I reved up the car they went to +10.7 one time and to -12.00 the other. So the car seems to be having problems in both sides.
I did not understand what you mean in the last part of your post, abscate. You mean if they drop to +10, like they did, or if they drop from there to minus something.

Post by **abscate** » 26 Jan 2017, 16:58

That is normal. You need to check fuel pressure at the rail next, the Fuel Pressure regulator, and finally injector quantity.