Hmm, I'd take it to a different mechanic if he knows you have an exhaust leak, that's your main culprit right there for it running rich. If the leak has been there long, then the O2 is probably toast too.
Get a scan tool, scanmaster is the easiest, watch the O2 counts, if they are not changing constantly, or watch the O2 cross-counts, they should cycle from 0-255 in less than 90 seconds, if not, then its new O2 time.
But you have to fix the manifold leak first, assuming it is before the O2 sensor.
Other things to check would be fuel pressure, should be around 42psi with the vac line off of the regulator, and around 35 with the vac line hooked up...if the pressure does not drop when connecting the vac line..then thats another culprit for running rich. If the chip is a stock chip, then it should not be idling rich at least. Stock chip is good enough to sell the car, wouldn't go full boost with it though as the timing is wayy too steep for todays gas, it was programmed for gas of yesteryear
If you can take some pics of the injectors and post up, we can tell you if the are the factory ones, or go look at some close of pics of some of the low mile cars that have been on ebay / CL.
