High NOX - EGR?
 

Hi all,
My 92 LT1 failed inspection for the 2nd time.....high nox. I am thinking it is EGR related, as I have replaced everything else.....

Question 1 is, do I just the replace the valve itself or do I also replace the vacuum solenoid?

Question 2 is, how hard are these jobs?

Thanks! Much appreciated.

See if you can get the valve to open at idle by either using a vacuum pump or by pushing on the diaphragm. Does the engine run rough or stall? This tells you that the egr passages are open. Some engines have been known to get clogged up.
Tee in a vacuum gauge to the egr valve connection & take it for a drive (must be up to temp) & see if you get a vacuum reading under acc. & cruising conditions.
Egr isn't supposed to open at idle, when the engine is cold, or at wot.

Thanks ex

How high is it? Mine is usually a couple points below the max. I have the test station put a shop fan in front during the test or it will fail. The problem is you have to get the cats hot and keep them hot while waiting in line and the motor is getting too hot. Yes the fans come on at 228 but if your motor has high miles and parts are worn it may be border line pass or fail on the nox, the shop fan helps a lot. My tech even runs a hose on the radiator though I do not think it is needed. Since I take all 9 of my cars to him he gives me great service.

Interesting. Thanks Kimmer. My usual guy closed his shop! I'm in trouble!!!

I wouldn't think that the EGR valve would open during a test. I could be wrong on that. I've been wrong before...

Yeah,, some egr valves only open while driving & others have a valve that senses back pressure to make the valve open.

Did you replace Cats? I had a 96 Dodge Dakota 318, I drove back from Il. to
Cali and it failed because the salt had dissolved the cat, new cat pass smog.
Good luck

I replaced one of the cats based on the codes...everything was calling driver's side.....didn't help.

NOX is EGR related, you need to trouble shoot the valve operation. The valve opens during cruising to lower the flame temperature in the combustion chamber and it adds in additional timing to try and make up for the slower burning flame front. If the valve is not opening or if the passages are plugged you will need to fix that to pass the NOX test. Verify valve operation and fix if it does not open, if it is opening verify if the passages are open or plugged for the exhaust.

fAILED TWICE
 

I failed twice on high nox replaced both o2 sensors and passed. Just sayin

Another thing that can cause high NOX is running lean. Not sure of CT but in CA in the 25mph test or in the non-enhanced areas the 2500 rpm test is specifically designed for the EGR to open. EGR opens at moderate throttle (1500-3000rpm). It will be closed at idle and full throttle.