I apologize... I've not read all the postings... I'm just responding to the symptom described in the headline... and I only know a tiny bit about it because it just happened to me on a '90 Ciera SL, to wit:
Same symptoms as described in the headline; and it turned out to be a little s-shaped black rubber tube right up on top of the engine where it can't be missed. I'd have to look at an engine diagrm to see what it attaches to, or what it's called.
All I know is this: When the one on my car got so disintegrated and limp that it no longer had the stiffness to stay attached at one of its ends, it kep popping off, and my symptoms were exactly as described in the headline of the thread-starter's first post. I kept sliding it back onto where it belonged, and it was so wimpy that it kept popping off. It may also have had tiny cracks or holes in it since it was so weak/soft and disintegrating... so even when it was on, it might not have been well sealed.
So I ordered another one... fourteen dollars, as I recall (I had predicted $8). Took a week to get.
I removed the old one, and replaced it with a new one, and end of problem.
If you have such a little black s-shaped little tube (the whole thing, if stretched out, would be maybe 2 to 3 inches), and it appears in place and well seated, check it for softness and maybe tiny little cracks. If it's anything less than very firm (so it will stay in place without any clamps or anything) then it's probably leaking...
...and that was 100% of the problem on my car.
I know I haven't described it like a mechanic would (and if there are any here, please chime in and comment), but I'm just telling you what did it on my '90 Ciera SL.
Hope that helps.
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HarpGuy