poor mileage

[jamies@rkmi.com]

Gruppe,
I have completed a tune- up on my 87 5KCSQW, yet it still only manages about
14-15mpg in the city.  The car has a TAP chip & spring, but I have been driving
economically for the most part.  The idle does have a madulation of about 150
RPMs, which from another listers comments, may be an indication of a failing o2
sensor.  Also, the bottom hose on my intercooler is covered in oil.  Aside from
these two conditions, what are other common causes of poor fuel mileage in these
engines?  I have ran one can of Jectron and the previous owners records indicate
2 cans of 44K were used not long ago.
TIA

Jamie Schnell