cooling problem

My 91 corsica, 3.1, refuses to cool. I've replaced the thermostat, the temperature sensor sending unit. The electric fan will come on while the engine is running, and occassionally seems to be effective, but does not run when the car is turned off. Running the ac only makes it heat up faster and can't be run without getting dangerously hot. It's not losing coolant, but is still overheating. The engine idle has aslo become rough, however only while in gear. When it's in park, the idle seems fine. Help!

Make sure there is nothing blocking the air going across the radiator. You should be able to see light through the fins. Was the thermostat put in right side up? An upside-down stat will not work very well. The pointed side is up, with the side with the ring and the pill in the middle goes down towards the engine. Remove the cap and look at the ends of the tubes inside. if there is a build up have the radiator removed and checked for flow- if there isn't enough flow through it won't cool

This may seem stupid, but, you do have the coolant mixed with water right? Should be 60 -40 in the summer, 60 % water 40 % antifreeze.

Did you change the water pump?

Not only check the water pump, but also check the cat converter. This can cause all sorts of problems.

What is a good, quick way to eliminate the water pump as the problem? What issues can the Cat Converter cause for cooling? Thanks for any replies

Id flush the entire cooling system. The radiator is probably all clogged full gunk and that wil make a severe impact on its ability to cool the antifreeze that is supposed to be flowing through it.
The water pump uses an impeller to move the water through the engine. Its made of steel so unless its broken (unlikely) the water pump is not the cause.

You should be able to feel the coolant surge through the hoses. The water pump probably isn't a problem. A clogged radiator could cause the problem. As far as the cat. The most important cooling system in the car is the exhaust. If there is too much back pressure it could increase temp. You best bet if you don't have a non-contact thermometer is to take it somewhere. They'd be able to see if there's a problem with the radiator and cat that way.

You know, my 90 Corsica exhbiterd these symptoms. The cat can cause a water pump failure or anything else related to cooling. It usually sounds like loose marbles under your car if the cat goes out. The water pump will leak. Since the hole is on the top, it may be harder to realize that it is leaking.