I have a stock 89 that lost the rear rotor, and found an 88 for a good buy with 70k on it. I did basicly a core swap because emissions and such. replaced all manifolds and gaskets, front cover, oil pump, all of it from my 89 to the 88 core. However, the 88 had this big ugly egr valve on top which interfered with the installation of the manifold so I removed it and installed a blankoff plate. Car started up right away and went to high idle and as soon as i put the shifter in gear the engine dropped rpm and sputtered and died. Only fault i am getting from the ecm is a bad port air valve sel. I can keep the car running by using the throttle however, there is a lot of popping and it is very rough. to me is acts like a egr stuck open but it does not have one. been scratching my head for a week now trying to figure this one out, any suggestions would be helpful

Thanks!!!

 

The s4 center iron may hinder the s5 manifolds from actually sealing properly,It has been "documented" that Guys find this when doing a S5 Upper on a S4 keg.
If you look at that aspect when searching you should find a pic of what I am talking about.
That sounds like your problem.
The manifold may seem Ok when going on,But it just doesn't sit right when tightened down and gives the manifold/engine a vacuum leak.

 

Any easy fix?