Here are the particulars. 1990 Coupe, stock L98 with about 95K miles.

About two weeks ago, the Vette started idling very rough, particularly when warmed up. It seemed to "surge" at ilde and up to about 35 MPH, then it seemed to smooth out some. I took it to a local mechanic and he found that the vacuum line to the EGR was (at some point) incorrectly installed and the EGR valve was not functioning ... at all. He also found that the IAC valve was not operating correctly and that the #8 plug had a crack in the ceramic. All items were then replaced with genuine AC Delco replacement parts, the throttle body got a good cleaning, the fuel filter was replaced, and everything was put back together.

When I picked up the car the other day, the mechanic told me that the idle was still a bit rough, but that should smooth out once the computer recalibrates to the new parts and improved vacuum. He told me that everything else checked out fine ... injectors, fuel pressure, and all of the other diagnostics. I drove it this morning up the interstate for a while and then rode back into town. Honestly, I don't think it's running (idling) any better than when I dropped it off at the mechanic and it still seems to surge, particularly when it's warmed up.

Is there any truth to the claim that the computer will recalibrate to the new parts/improved vacuum? If so, how long will that take? Do I need to run a tank or two of gas before I notice a difference?[/font]

I may be incorrect because I am not that familiar with the 90. In most cases the IAC resets every time you shut the engine off. So when the engine is started the IAC meters the air required to meet target idle at what ever temperature the coolant is at.

If it has a rough idle but stays close to the target idle look for spark plugs, ignition wires, fuel injectors. If it will not stay close to target idle look for air leaks.

The EGR should not function at idle and if it did not function at all you should have got a check engine light.

When ever I hear about rough idle all of a sudden I generally start out believing that it is a vacuum leak. Check all of you hoses even the PCV hose.

take some brake cleaner and start spraying it around the engine compartment... on hoses, intake manifold, things like that. If you hear a rev up, you have found the leak.