kimmer

Hey guys looking for a little input here. Well failed smog due to high NOX. HC was below average and CO was a bit above average nox just passed at 25mph but failed at 15mph by 136. First thing that comes to mind is a bad EGR valve and maybe 02 sensors. Would like your input, thanks ahead of time.

zr1fred

I would start with an inoperable egr system. That would be the most likely given the other readings.

kimmer

I tend to agree with u on that but could it be the egr solenoid or valve?

kimmer

???

zr1fred

Both. If you're not getting a code, look for a mechanical failure.

kimmer

Mechanical failure as in what? Thought those failures would affect HC's

zr1fred

Sorry, I meant mechanical failure in the EGR system (faulty valve, broken lines, or in the case of my LT4, blocked off) Google LT1 EGR system,

https://tse1.mm.bing.net/th?id=OIP.c...5&pid=3.1&rm=2

mike100

in this order-
EGR functionality check and or repairs
new O2 sensors
take the car to the smog man cold and just throw it on the rollers.


if the cats have a ton of mileage on them, a cold coolant temp will help the nox quite a bit. Also if you have a reprogrammed ecu or chip with lots of ignition timing...go back to stock or have them program it to the stock curve below 3500 rpm,

ex-x-fire

You didn't list the year, can you apply vacuum to the egr while its idling? It should stall. You could have a bad egr solenoid that doesn't throw a code.

John A. Marker

Kimmer, the NOX is a direct correlation to the EGR opening or in this case not opening or staying open long enough. When the EGR opens it draws the exhaust gasses into the cylinders and actually cools down the combustion. The NOX is created at HIGH temperatures. The O2 sensors will have no impact on NOX. And a cat will not scrub the NOX away from what I understand.

I would replace both thru Rockauto part.....less expensive.

John

kimmer

thats what I will do

Paul Workman

w/ others, for the most part.

Scan it and see what is going on. Verify that it is in CLOSED LOOP operation,for starters.

NOX= primarily high combustion chamber temps, bolstered by low HC readings (methinks). That can be affected by:

LEAN condition
Spark advance too high
Faulty O2 (reading high, yet the integrator value is saying LEAN)
EGR valve NOT FUNCTIONING/blocked
Coolant temp being high

What does the scan say?

kimmer

Since it failed by only 100ppm wouldnt keeping the engine temp down [lets say to 197 ] be enough for it to pass? I did chg the egr solenoid and thinking i should get an oem egr the one thats in it now is one of thoses generic autozone pos. also since i have no mufflers i will need to make an exhaust restriction.

96LT1AUTO

First I will say find the problem and fix it correctly. But if you want to possibly pass the smog test cheaply and painlessly listen further.
My 96 is also a victim of high NOx and I heard of a product called "GUARANTEED TO PASS" that may mitigate this problem... and it does work!!! You can buy it at parts store.
Follow directions...I like to drive car a little hard right before I get to smog shop to blow out any crud in motor.
I was skeptical also. But ultimately my o2 sensor failed but it worked great til then..good luck

mike100

just diagnose the egr system the old fashioned way with a vacuum pump/gauge. it should get vacuum present just when you crack the throttle when taking off or when you step on it from a low load condition (coast). you should see the vacuum on the gauge-tape it to the windshield. the pos autzone egr is probably fine.

02 sensor(s) most definitely affect NOx. Even the slightest deviation into lean territory under load will elevate it. There are lots of rich/lean graphs online that show 5-gas curves for gasoline engines. It makes sense when you see the charts. If you don't want to delve into it that far, just buy a new pair of o2's as preventative maintenance and call it a day.