1999 Jeep Grand Cherokee Vacuum / Idle / Stalling Problem

Hello all,

I have a 1999 Grand Cherokee Laredo 4.0L which was running perfectly
until tonight. I went to start it but it wouldn't turn over. I thought
maybe I hit the gas pedal when I tried turning it over and flooded it
or something. I then tried starting it holding the gas pedal to the
floor and then quickly letting it off if it turned over, that worked,
it started right up but would just stall when it hit low rpm idle
speed. If I held my foot lightly on the pedal it ran smooth at 1000 rpm
but the second I let it off it would stall.

I then pulled out my trusty Haynes manual and looked under the
troubleshooting section. There I saw the word "vacuum" which raised a
red flag in my mind. When I bought the truck about 8 months ago the
only HVAC option that worked was defrost and my cruise control didn't
work. After searching the internet I found some postings about vacuum
lines controling the HVAC venting. After a few minutes looking under
the hood I found the culprit, the vacuum line that runs to the intake
manifold was just hanging there. I hooked it back up and sure enough, I
had cruise and other options besides defrost. At the time I thought it
was a freak thing like back pressure that blew it off or something.

This all leads me back to tonight. After a lot of head scratching I
thought "why not disconnect the vacuum line that was hanging there
unhooked 8 months ago, after all, it would only take a few seconds and
if it didn't work than that was just one more thing I could scratch off
my list of possible solutions" SURE ENOUGH! just by disconnecting the
vacuum line that hooks to the intake manifold the truck started right
up and idled fine!!!

I also notice the steel cable running from the throtle body to
somewhere under the battery had a lot of slack in it (that's a mystery
zone for me since I haven't looked under there yet and have no idea
what it runs to). Does that have some thing to do with the idle speed?
Maybe a vacuum actuated choke?

I would just leave the vacuum line unhooked but there's a reason it's
hooked up to the manifold at the factory (I think?).

Could I have a bad resevoir or valve that's not opening?

Could a vacuum problem lead to poor gas mileage? I average about 17-18
MPG.

Sorry for the long post, I just wanted to get all the facts and some of
my theories out there. Also, I will post the results when I get to the
bottom of the problem.

Thanks,
Mike