Thanks, it was a surprisingly easy fix:
-Order $65 OEM replacement off Ebay ($24 part was described by Ebay seller as correct for my car, but was different part number and problem soon reappeared)
-Unclip old vacuum modulator diaphragm(thanks ATGParker!) and disconnect three hoses.
-Connect the three hoses to replacement, clip in.
-Disconnect car battery for a minute (see my old posts for the reason I cannot just take out the 15 amp EFI fuse).
-Start car, notice that engine idles nicely and the check engine light is gone.
Here is a picture of the new and old valve side-by-side: