I had trouble with idle surges and replaced BVSV’S and all hoses. Used sea foam for intake, cleaned EGR and tube, replaced EGR modulator and it got better after warmed up. Took in for smog and failed NOX 565 - 1775 @15 MPH and 522 - 1682 @ 25. Had a shop due Smog Diagnosis and came up with defective TB, no vacuum from tube E, took TB off clean and it did have a lot of gunk cleaned it and installed it drove it for about and 30mins. Hwy and surges we’re fixed and headed for smog but it failed worst @15 MPH 565 - 2191 and 522 - 1072 @ 25 MPH, Gross Polluter @ 15 but @ 25 went downs a little I’m stuck and before I spend more time $, Would really appreciate any help from 
or other members with similar issues in their past, 
Nox emissions are created when combustion temperatures become elevated.
The purpose of the EGR system is to introduce a small amount inert gas into the combustion process to lower combustion temperatures.
The contributing factors that cause high Nox emissions are,
*A malfunction EGR system
*An engine that’s running hot
*An engine that’s running lean
*A defective 3 way catalytic converter
Because your vehicle has the OBDI engine management system, makes it all that more difficult to determine what’s causing the high Nox emissions,
Tester
Good advice above. I own a Corolla of similar vintage. The EGR systems are probably similar. I’m presuming you’ve never had NOX emissions problems before now, so something has changed. Maybe it is just the cat has given up the ghost.
If I had that problem I’d probably just replace the EGR valve first. And I’d make sure the electrical controlled vacuum switch for the EGR was doing it’s thing. I’d probably just replace it along w/the EGR valve. Another thing to consider is that in the process of cleaning/ removing the throttle body the small vacuum hoses that control the EGR flow were not connected back to where they should go correctly. There’s 3 or 4 very small ports on the throttle body, and part of their function is to control the EGR flow as a function of throttle position. The hoses that go to the ports have to be connected to the right place on the other end of the hose. And the very small openings in the throttle body for those ports have to be open, not clogged. I presume as part of the throttle body cleaning those ports were cleaned out too. If not, that’s probably something worth trying. Some other things to ask your shop to check
- The throttle valve position at idle(foot completely off the pedal) must be correct. As well the warm idle rpm.
- Try replacing the PCV valve. Unlikely to make a difference, but you might get lucky and it is cheap. It could be related to the cause of the idle surging.
- Use a timing light to make sure the base timing is correct, as well as the advance curve.
- I had some trouble with idle surging due to a faulty idle air control gadget. It’s coolant controlled on my Corolla. I fixed that problem by effectively disconnecting it. Wouldn’t work in a cold climate but in San Jose it works ok.