I just went though the process to get my 94 passed over the last two weeks. The NOx was the issue for me as well. The limit is 4.000 and I was hitting 8.700 to start. I read through as much as I could on this Forum and thanks to several of you guys including some on the 100 series section as well, I was able to get mine passed this morning.
1st step I took was to figure out if the EGR was working. Did the vacuum test on the EGR itself with the engine off and could hear it working, and it would hold the vacuum as well.
2nd I started the motor and let it idle while I engaged the EGR with a hand held Vacuum tester. The engine did not change so I figured the passage was clogged up. I bought this 94 over a couple others as this one only has 163,000 and figured it might not need as much done to it. But at 163,000, the EGR passage in the top of the Intake Manifold was completely clogged at the Throttle Body.
I took the Throttle Body off and found it was clogged, and proceeded to clean it out with various thin screw drivers, baling wire and finally some small braided cable. Once i felt like I had got a majority of the gunk out I loosened the EGR and used compressed air to blow the passage out toward the EGR and still got some more junk out. After putting it all back together I tested the EGR wit the motor at Idle and it labored hard and then died, showing the EGR was now working.
At that point I wnet and got it tested again and the score dropped down to 4.600. Still needing more work I checked the timing and found it was set around 2.5 deg at the base timing. I set the timing to 7deg after reading some posts on timing, added 6 gal of e-85 gas and when I tested again the NOx went up to 5.900 (rounding the numbers a little).
I read a little more and found I need to retard the timing to help with NOx and so set the timing back to 0deg, base timing. (it wouldn't go any further) Did one more test and now at 4.111
So read some more on the forums and found a thread on Seafoam. I ended up following the instructions on a can of Seafoam and just ran 1/2 a can through my pcv valve (sucked it up slowly with the pcv), put the other half in my gas tank and finished out that tank of gas this week.
Last night I filled back up with half a tank of Premium (91) and drove about 15min. I then took it out and drove for 30 min this morning and then tested it and passed with 3.689 on the NOx!
I post this to help anyone else who might end up on the line for NOx since the limit has been lowered and fairly tight for our vehicles. The Seafoam seem to help a few point on the NOx scal and possible the Premium Gas. Who knows, I may still need to replace the Original Cats at some point but I am good for now.
). But I'm subscribing for future reference.
