Does anyone have any advice on how to remove the heat riser flap/valve (unsure the official term) in the exhaust manifold. Mine is seized & I can't find anything on how to remove it. I don't see any retaining rings or anything so i'm not sure what's holding it in or how to get it out (simply press it out?). Any advice is appreciated.
GM has a spray that is used to lubricate the heat riser flap:
https://www.amazon.com/Genuine-GM-88862628-Penetrant-Inhibitor/dp/B00BK7MLKE?th=1
You could also try Kroil penetrating oil. I don't believe you can remove the flap without cutting the shaft.
A small cut-off wheel (dremel), can cut the metal flap and leave the shaft in place. That is what I did.
YouTube video on 6 cylinder heat riser operation:
https://www.youtube.com/watch?v=9tepFx1uMJo