Hey guys here i come again. My 94z is not passing emissions. I have checked O2 sensors and they seems to be fine. There is a movement on the voltage reading. I decided to go with a new cat. Before the new cat, my car was failing at 15 and 25mph on HC,CO and Nox only at 25mph. After the New cat HC, Co where super clean, but my Nox is now failing on both 15 and 25mph. Any ideas guys and girls? thankx

 

u can get acceptable false info from the 02 sensorsl so they could be the problem. have it scanned for any trouble codes that are stored. bad cat will foul the plugs and pre cat 02 sensors.

 

i have a Hyperthech programming chip install, i just read another post were they say to turn it off. What would that do?

 

I have one question. I did a quick check to see if the egr was getting vacuum. with the car running. I unplug the hose at the solenoid coming from the intake. I can feel the vacuum. But when i disconnect the the hose from the egr valve and feel for vacuum there is none. one more thing. anyone know when should the erg solenoid be active meaning open?

 

NOx is caused by excessive cylinder pressure. Make sure the EGR is working properly, car is in good tune, and the spark advance is per factory spec. Your tuner may be part of the problem (excesive spark advance) so reset the tune back to factory before inspection. Also make sure to fill the tank with premium fuel; the slower burning premium fuel will help in reducing NOx.

 

NOx is also caused by excessive cylinder temperatures. One cause can be the EGR valve as Pete stated.
Spark plugs in too hot of a heat range, or carbon deposits on the pistons or heads can be another cause.
If all else fails, you could try this: http://www.autobarn.net/ch05063.html
I've never needed it, but I've known it to do the trick for people.

 

when you say too hot of heat range what do you mean by the gap of the plug?? oh and i just remember that before i got tested i messed around with the programmer and changed something and before i changed it the car had only failed in 25mph. so i will be putting the programmer again and putting everything back to stock

 

I have checked the EGR solenoid and the EGR itself. Test results. When i apply vacuum to the EGR valve it holds the vacuum. that tells me the diaphram is not leaking by. I follow the instruction from shbox.com to test the egr system. "If you cannot get in there to push on the diaphragm, you can use a hand vacuum pump (like a Mityvac) connected to the EGR valve to open it. The valve should also hold vacuum, which would prove that the diaphragm is not leaking." I did that step with the engine not running and it held vacuum, but when i did it with the engine running IT would not hold vacuum at all. I know the solenoid is opening because i put a gauge at the end of the egr vacuum line and drove very little to make sure the pcm was giving the a signal to the solenoid to open. Any other ideas?

 

With the EGR you have to make sure that the gas passages between the exhaust and intake are clear, otherwise no matter what the EGR valve does no gas will flow and NOx will be high. You may want to consider removing the EGR and getting all the crud out of the passages and valve.

 

Ok here i come again. I cleaned the EGR valve and the inlet holes. I reset the programmer to stock. I took the car to smog, it failed again high Nox. the good thing is that the guy has not charge me no money, only once the four times i have take it.

He told me that it could be my egr valve bad, but then he connected a hose directly from the intake outlet to the egr valve. He said doing that made the car be on border line of passing. So he suggested have an tech look at the egr system. what do you guys think? He also asked me if i had modify the exhaust and the asnwer was not i know. when i bought the car i had to smog it and it passed 2 or 3 years ago.