So long story short failed an emissions test with high NOX while everything else looks fine. Natrally the first thing i decided to check out was the EGR system. After determining vacuum was being applied to the EGR valve as it should the next thing i checked was the valve itself. Being that this car is also my means to get parts i decided to go ahead and buy an EGR valve ahead of time in case i needed it. However in comparing the new one to the old ive noticed a few interesting things.

1. New EGR valve dosnt hold vacuum as the old one does. Its a fairly slow leak but definitely leaks my guess is around the fitting where the vacuum port attaches (its like a swivel fitting designed to be able to twisted to point into any direction unlike the original which was fixed). The old one did not leak at all. I was not expecting any leaks but I'm wondering if it was designed with the idea in mind that this slow leak would not hinder its function.

2. When blowing into the exhaust port on the old EGR valve there was no flow unless you really blew into it relatively hard and then you could hear a slight whistling but flowed better to the point of free flow as you manually opened the valve which was as i expected. The new EGR valve however flows with little pressure applied when the valve's closed, definitely restricted, but flows. It flows more as the EGR valve opens but i was not expecting any flow when the valve should have been "close."


Does it sound like I managed to get lucky and bought a bran new defective EGR valve or is my old EGR valve bad and the new one is how it should act. Also both are negative type EGR valves. Thanks for your responses.

 

There are two different kinds of EGR valves; negative and positive pressure.

IIRC, the negative pressure EGR will hold vacuum as your old valve. The positive pressure EGR will only hold vacuum if there is exhaust pressure at the valve pintle.

Check the stampings on the EGR valve cover. There should be either an "N" or a "P" stamped on it denoting which type of valve it is.

http://www.asashop.org/autoinc/dec97/egr.htm

Edit: My apologies. You already know all this. It does sound like the new valve is defective. It should hold vacuum.

BTW, I'm dealing with the same emissions failure as you are and have been for the past few years. I'm failing NOx at 15MPH test speed. The EGR vacuum system checks out as does the EGR valve. I installed a new cat and this helped a little, but still no dice.

I actually pulled off the upper intake to make sure nothing was plugged. Everything checks out. I did find a few injectors (originals) that had broken tip protectors, but I doubt this is an issue. I'm installing a set of FIC flow matched replacements, a new EGR valve (just because it's easy to get to now) and I'll button it up and re-test. We'll see what happens. I'm not too confident, though.

 

Yea same here except i failed the 15 and 25 for NOX by about 500 PPM in each. I cleaned the ports in the plenum and today im going to check the ports in the runners, intake, and head. Like yours i had replaced the cat with little effect as i had this problem last time. The only thing that really makes me wonder is that this new valve will flow exhaust gas while its "closed" should they normally flow some exhaust gasses when their closed? From everything i know about EGRs tell me they shouldn't and its kinda confirmed by the fact that my old one doesn't. However then again i did fail my NOX emissions so maybe its not such a great idea to compare a new part against a possibly bad one lol. It just seems like even though i dont think it should be flowing when it should be closed that would make sense if my EGR valve isnt flowing as it should that would make the NOX be a bit high consistently like that.

 

I thought there were 3 kinds of EGR, at least according to books and other aritcles. Positive, negative, and then the one that just works off of ported vacuum. I thought the one on my TPI 5.7 for example was neither positive nor negative. I have a couple of AC Delco EGR's at home. I don't believe there is an "N" all by itself clearly stamped. There is an N stamped but it is inbetween other letters numbers if I remember correctly. I'll have to look again more closely when I get home.

I've battled high Nox eversince I opened up the exhaust with 1 3/4 SLP's, etc, etc. Always fail Nox at 15 and 25 mph. I have new cats with 1K miles on them. Everything else is new as well, sensors, etc. All the other smog numbers seemed to be fine. I found a Dyno Don recommendation to drill the EGR opening (where the vacuum line plugs in) .02 greater then what it is. The stock opening is already .02. So I drilled it as recommended to .041ish. Took it back to smog, failed the 15 mph but passed the 25 mph barely the knox did go way down almost more than half of what it was. The other numbers went up a quite a bit too, close enough to be almost failing, in my opinion.

Going to do more checks tonight. Cranked up the fuel pressure from 42 psi to 46 psi. I'm going to do another pre-test tomorrow morning to see what changes in the reading are if any.
Gonna check vacuum again I think.

 

You are correct there are 3 types in my case there negative and they seem to work best. On factory EGRs they will have a N or a P stamp (factory TPIs are N type) but many aftermarket ones to not follow this convention. One thing i would mention is this if your unsure of which type you have (as it should not be the just strait ported type) that could be your problem. If its a negative type EGR it should work fine however positive type EGR valves work on back pressure in the exhaust and by adding a high flow exhaust your EGR will basically never open if there's not enough back pressure. Just a though.

 

http://www.shbox.com/1/EGR_ID.jpg