Ever since I got my car (93' DeVille) back from the shop a few weeks ago, it has had an very high idle. It was in the shop for a smog inspection, and to get some oil leaks fixed. My mechanic said he replaced some engine seals, and tightened some bolts, to stop the oil leaks; he also fixed the coolant leak I had, which was from the water pump seal, so he replaced the water pump as well; he removed and welded the PS pump/belt tensioner bracket back together (it had broken near the bolt); he fixed a leaky injector; and finally he replaced the catalytic converter. He said he tried a lot of other different things while trying to get it to pass smog, until finally he replaced the catalytic converter and it passed, so I'm not sure what else he did before that.
Anyway, after getting it back, the idle is extremely high, it can coast for a few miles at 40mph easily. I have already tried readjusting the ISC motor (which seems to be working fine [i.e. it retracts and extends when I tell it to in the diagnostic menu]). This is causing extremely low mileage in town (6-7mpg), and I also have to ride the brakes at speeds below 25mph. It still seems to get around 25mpg on the freeway, however I can occasionally feel it accelerate by itself when coasting down hills.