Recently got this 91 V6 Corsica and replaced a few things:
Original plugs and wires
All engine/trans mounts
Exhaust, plenum, egr, oil pan, and other such gaskets
Water pump
Vacuum lines
Gas cap, pump, and filter
Air filter
Trans filter, fluid, gasket, and tcc solenoid
Also while the plenum was off I cleaned the throttle body, intake, plenum, egr parts, and checked the fuel injectors. Managed to keep the brakleen off anything that didn't look like metal and let it dry before assembly to keep seal damage to a minimum.
This was done to resolve a cylinder 4 miss, poor vacuum, 15-18mpg, and the rich condition. It still has a shaky idle even when cold, but its much lighter than before, as is the gas smell in the exhaust, and it no-longer stumbles on sudden acceleration. Did and still does run fine on the road, just gets terrible mileage. I'm an aggressive driver, so I'm only expecting 25+ instead of the 34 my previous Corsica managed on road trips. Seems like the next step is to try replacing the digital egr valve, but that's a $90 purchase that might be entirely unnecessary.
Maybe the roughness is the ancient muffler, or clogs from running rich for so long? Additional backgroung: before I bought this car it had been sitting for 4 solid years. But in garage-kept condition and less than 100k miles, $1000 seemed like a good deal to me.