I had no idea, these years especially, were that sensitive to vac leaks. That's all you hear/ read about is vac leaks if you have surging, hunting idle. Everyone was so right!Biggest culprit is the ports inside the heads themselves get clogged up. The path runs front to back through the head with perpendicular pathways to the exhaust ports.
And yeah...it's amazing how sensitive these cars are to vacuum leaks, especially 86-88 speed density cars as their fuel/timing curves are based primarily on manifold pressure. A small leak will change that
Excellent diagram! After studying this, the factory EVTM pic you sent, using the wiring diagram and item description pic from my haynes manual, I come to realize the tab and tad wiring and vac lines were reversed. Your stating TAB is towards the outside of the car and TAD is closest to the engine. My manual showed that too..I dunno if you've seen this diagram, but this will visualize the air pump system. Flow is to the heads during warmup to feed the first cats (not shown here) and then it's directed to the main pair of cats when the vehicle is warm. I believe at WOT, the bypass dumps all the O2 to atmosphere.