I bought this '88 GT on ebay, about a year ago.  It "idled" too
fast then...about 3000-4000.  But I wasn't worried because I towed it
home.  I assumed it was the EGR tube, so I ordered the replacement on
Ebay...that requires threading the upper intake manifold and uses the
gas pipe.  When I removed the old tube, sure enough it was cracked very
near the mounting point under the manifold.  I had previously played
with the EGR diaphragm and it moved when I pushed it.  And pushing it
did nothing to the idle.

As I said, installing this new kit requires tapping the old EGR tube
hole.  It may sound easy but keeping the tap exactly vertical to the
manifold is NOT easy.  However, I kept it as nearly vertical as I could
and put the thing back together.

However, when I started the car, it still "idled" between 3k-4k.  I
tried various things to see what was the problem might be including
putting lots of gasket sealer everywhere I felt it might be needed.
Nothing worked.  In fact, if I put my hand over the throttle body
intake the engine does not slow down.  I had cleaned the IAC pintle
previously, so it isn't that.

Can anyone give me a starting point?  I flat can't think of where
there could be such a large leak that it would make the idle be that
fast, AFTER replacing the EGR tube.  

TIA.
John