I've been fooling around with the emission system on my '92 roadster. Failed testing 3 months after it passed here in Georgia. It’s a long story as to why I had it tested so close together. It failed the NOX part at both 15 and 25 MPH test which points to the EGR. I have checked the EGR solenoid and valve and the AIR system – all good. Showing no “Check Engine” light but haven’t checked the codes. I’m expecting I have an o2 sensor beginning to fail. Until I can look at the codes I won’t know. Even if I pull the codes will it tell me anything unless the “Check Engine” light is on? I do know the AIR has not been running lately. I drove it 20 miles Sunday and looked at the EGR. It was closed and the AIR blower was quiet while sitting at idle. So, I am wondering just how the Emissions system works. What tells the EGR Valve to open/close when? What commands the AIR to operate? What exactly do the O2 sensors do? Thanks in advance to everyone for their response.

 

First step would be to check for codes, there are some that will not illuminate the MIL. The AIR, EGR and many other solenoids and relays are controlled by the ECM based on input from the sensors.
Typically battery voltage is applied by the ignition switch and the ECM grounds the circuit when operation is desired.
Great book here that does a good job of explaining the EFI system and emission controls including the basics of sensor operation, highly recommended.

http://www.amazon.com/Corvette-Injec.../dp/0837608619

 

Wow, good question and could probably write a book on what you'd like to know. Here's a quickie primer that may or may not be correct. (Standard forum disclaimer). AIR pump runs for about 4 minutes maximum or until the engine goes into closed loop operation. Bottom line, if the engine's warm the pump shouldn't be running. As for your statement about the driving 20 miles, the AIR pump should have been off. When your car is cold, open the hood and start the engine. It should be running.

You can have a code and no check engine light. Check your codes first. That's pretty important. Search function has a slew of tips on this.

The EGR is the most likely culprit if your NO is high. NO is caused by excessive combustion temperatures. I think anything over 2000 degrees is not good. In a nutshell, the EGR just mixes some fairly inert gas (Exhaust) into the combustion air in the intake manifold. This will allow less heat producing oxygen to enter the combustion area. The result is a cooler combustion event and less NOX.

The EGR doesn't know what to do automatically. It need input from several sensors. Namely, the TPS, ECT, engine speed, and the MAP sensors. For example, if the coolant temp sensor is telling the PCM that it's cold that will change the information that allows EGR to happen. Same with throttle position, etc.

Just because you can visually see the EGR working doesn't mean much. The ports into the intake or from the exhaust can be clogged so even if it is working electrically and mechanically it may not be functioning correctly. Sometime you just gotta bit the bullet, pull it off and clean the ports.

Any condition that causes a lean burn situation can increase combustion temperatures and thereby the NOX emissions. A lean air-fuel ratio results in high cylinder temperature and excessive NOx emissions…A lean air-fuel ratio may be caused by low fuel pump pressure, partially plugged injectors, a vacuum leak, or defective O2, MAP, ECT, or IAT sensors. A lean air-fuel ratio causes high NOx, high HC, high O2, and low CO and CO2.” If your other smog reading were in line I wouldn't think that this would apply. Back to the EGR.

I guess you could take the vacuum line from your EGR valve and apply it directly to a source of vacuum. If the engine falters or dies I would think the ports are not clogged and it is functioning normally.

The oxygen sensors actually "look" at he exhaust and monitor for what they believe is the correct fuel/air ratio. They do get dirty and they don't maintain their accuracy forever. They could be allowing a lean idle situation by incorrectly monitoring the exhaust. That would be the same as the lean condition above and increase NOX. If they have a gaggle of miles on them it wouldn't hurt to change them out. You may even notice a change in your mileage. Many have.

Although I haven't mentioned everything, I hope I've mentioned enough to at least get you starte on your search for the offending NOX problem. Have a great day and happy troubleshooting.