My '91 305 TPI failed the California emissions test for high NOX. It passed the other parts of the test.
Since then, I checked the EGR valve and it holds vacuum and the diaphragm works fine. I cleaned the passages from the EGR which were plugged up with carbon. Replaced the Catalytic converter (it was very old and I figured it had its run) and replaced the oxygen sensor. I tested the EGR vacuum solenoid valve, and ran into a problem. The two wires coming from the ECM (one green and one pink) in the connector never vary from 12V measuraments. It is as though the ECM is not opening or grounding one of the wires, but it is always in the same state (grounded, which causes the solenoid to block vacuum) whether the car is idling, running or just sitting with the ignition on. I have no error codes in the computer.
Should I just disconnect the connector from the solenoid valve and allow vacuum straight through from the manifold to the EGR and re-test it this way? At least the EGR valve will operate some of the time. If anybody can make any sense of this posting and thinks he has an answer please let me know. Thanks

Rick

 

Manual or auto? Auto would have to be in drive to get the ground signal. Manual would probably have to be driven(speed sensor activated). What are you using to check signal voltage, signal ground? You might try hooking up a vacuum gauge to the hose and drive it to see if you get vacuum. If you're getting vacuum, passage is clear, and valve operates, you're as far as you can go with the EGR system. What is your coolant temp? Low coolant temp can cause high NOX. Especially right now with the cold weather.

 

It is an automatic. I first put a vacuum gauge in series with the EGR. Taped the gauge to the windshield and drove around for a while at different speeds with the car warmed up.
I saw no vacuum. Figured something wrong with the solenoid, so I took it off and bench tested the solenoid it worked perfect. So I connected a digital voltmeter to the solenoid connector and taped it to the windshield. Drove around and saw a steady 12 volts across the terminals no matter what the speed with the car at operating temperature. Strange case but it seems to point out to the ECM.

Rick

 

I had to back the timing off to stock to get my Nox down. See if yours is advanced then back it off to stock.

 

That's really strange you replaced the cat and cleaned the passages and it still wont pass. Do you have a scan tool you can watch to see if the computer is commanding the EGR solenoid to work?

You have to be within three degrees of the stock 6, so the minimum you could lower it is to 3 degrees.

 

I finally realized that the problem was somewhere in the ECM.
Either a bad ECM at the EGR solenoid control point or a badly
programmed EPROM.
So I looked at the vacuum coming into the solenoid from the
Throttle Body vacuum outport. I saw that it would not produce any
vacuum until it reached over 1000 RPM and after that vacuum would
increase slowly with RPM until it reached 20 Hg and then it wou
hold steady. This told me that I could connect the egr directly to
this point, bypassing the solenoid and still not have the EGR open
during idle and have the EGR fairly functional. Sure 'nuff took the
test again and passed NOX with flying colors.