rough idle, poor mpg, running rich.

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.

You could install a rebuild kit in the throttle body. Make sure everything is clean.

It shall be considered soon, but I'll be listening for either possibilities as well

Check your coils.....A weak one will cost you gas for sure. We bought a 91 new waaaay back and put 235K on it. Apart from EGR valve, head gaskets, and a IAC we only had to replace common stuff like water pump, alternators etc. over the years for the most part. but I have had several GM cars including it that needed a coil once in a while. Check your spark on each cylinder, and watch for a weak fire. Normally you wind up with a weak and poor fire on one of them. There are three coils which sit on top of your coil pack. On the 91 it is a bit of a pain to change if I remember as mine was mounted low where newer ones were mounted on top of motor and easily accessible. If you find weak fire change them all out....save troubles down the road. They are already old and might be the problem.

I would scan it and make sure the coolant temp sensor is sending the correct temps to ecm, especially when warmed up.