Can anyone explain how an EGR valve operates, and how it behaves when it fails?

I failed emissions testing because my EGR valve was disconnected by the previous owner. When connected to the solenoid, it runs poorly in the low rpms. I reconnected it to go through the testing, and passed easily. (Failed at 1200 NOx, second try was 61 NOx). I don't really understand this, because I though it opened when a vacuum was applied to the valve. It I leave it disconnected all the time, it will be closed all the time, correct? But why should opening the EGR valve cause a stumble?

Basically what I'm trying to figure out is why the stumble is occurring when the EGR valve is opened, which is supposed to happen, and the ECM is expecting to happen! The car behaves fine when the ECM expects the EGR valve to open, but it doesn't! Weird.

 

https://www.thirdgen.org/newdesign/t...diagnose.shtml

 

What has happened, is that something in its control circuit is making it stay open 100% of the time. That's not the factory setting.

But hey, if it goes through inspection with it connected; and runs fine with it unhooked, it can't be all bad, no???

However the car might cruise a little better and possibly even get better gas mileage on the highway if it worked right.

 

Alex,

Some of what you say may be true, depending on the fuel system you have. There are three types of common vacuum pilot EGR valves, and only one of those will open directly when vacuum is applied. the other two require exhaust system pressure differential to open.

And other than the older carbureted engines with temperatur vacuum switches and timed vacuum ports on the carburetor, most later systems use a solenoid valve to modulate vacuum to the valve itself.

So, what type system do you have? Year? Model?

 

Oh duh, you'd think that after the several years I've been posting on this site I would learn to post that stuff.

89 Firebird TPI L98. ZZ4 cam, otherwise stock.

I know that the solenoid works, I've tested it and got vacuum. But I guess I should look further and see if the vacuum goes on/off. My memory is a bit fuzzy about this, the last time I worked on it was last year at this time.

 

That engine should have been equipped with a negative backpressure EGR valve. You can follow this flowchart for diagnosis: