Conflicting symptoms.....
Well...because of increased fuel burn, I'd suspect the TBI....there are 2 injectors working together...and I'd investigate if one of the 2 could be blocked opened...throwing a constant fuel flow where not needed. I think you can disconnect the injectors one at a time...so try that to see what happens...
This does not explain why covering up the air intake would make it run better...there is no air flow meter on this engine...
Verify also if the ground side of the wiring to the injectors is shorted to ground...this side of the injector is grounded by the computer when needed....(the +12V is always HOT).
www.avigex.ca/xport/fuelcontrol31.jpg
www.avigex.ca/xport/TBI.jpg
www.avigex.ca/xport/injectors31_1.jpg
www.avigex.ca/xport/injectors31_2.jpg
The other possible cause is the manifold pressure sensor....i'd do a test with this sensor disconnected in case it gives a false "wide open throttle" reading (reading low vacuum = atmospheric pressure = wide open throttle). ONce disconnected the computer should fall into "default mode" ..probably better than a false reading.
Well.. go with these first.... report findings ....and other symptoms..does it have the smell of rich mixture, unburned fuel ???
BY the way...a "huge vacuum loss" with increased fuel burn should give an uncontrollable high idle..which you do not report....so I have doubts about a stray air inatake....