I can see that the stem seal is staked into place similar to the accel pump and TPS plunger seals in the airhorn, just no way to access it.
The valve was closed, so the stem seal would see whatever was in the intake. There's likely a way to access it, but it may require special tools, and even if you could, who makes/sells just an EGR stem seal? I'm sure it's likely made of something that can take the heat, so maybe some sort of graphite rope packing-ring-like substance similar to the junk we used repacking steam valves on the submarine. Never had one apart so I don't actually know.
Unfortunately, I don't know if any other way to re-seal the one you have without replacing it. If possible, get an ACDelco unit. I've always had the best luck with those. And I can't say that's your only issue causing the rough idle. It's a factor to be sure, but how much of one??
Is the original stamped 17084375 on the top hat?? If it is, then it's GM p/n 17110475 from the parts information.
Here's a slightly used one it appears. $42 and change plus tax. But it has no P or N on it, making it a Ported type. Meaning a simple vacuum pull test will make it open.
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Or a brandy new one for 51 and change plus tax.
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GM predominately used 3 types of EGR operations on G-bodies. Note: A vacuum check will NOT work on a Positive backpressure EGR valve. It will on the other 2 types, Negative backpressure and Ported. They all have a spring on the top of the actuating diaphragm so it fails to close.
So if it's moving when you apply a vacuum, it's either Negative backpressure (breaks vacuum when negative backpressure is felt or closes when vacuum source cuts off) or a Ported. On the ported, vacuum opens it, if vacuum cuts off it closes. Simple. Negative and Positive backpressure styles have the hole in the pintle (main valve disc) and actuating stem, but work with opposite pressure sensing.
Do a search on here for more info if necessary (this is just a call out for everyone if you're really wondering how one of these POS's work). Keep the pintle tips clean and hopefully you'll have many miles of smiles.
My old computer took a dump on me, sort of, so after I got it into "limp-home mode" I went through and ensured I put whatever files that were on the hard drive onto my external hard drive where I keep most of my crap (I call it my personal "cloud"). Then I ran across some EGR information I had...
gbodyforum.com