2003 Pontiac Grand Am a/c will only blow cold when engine cooling line is crimped closed ?

There should be a actuator valve on the line that determines if the heater core gets hot water or cold water. When the heater is selected, the actuator opens and lets hot water into the heater core. It sounds like that actuator is corroded and not opening.

You could have a bad (broken) vacuum line going to the dashboard area from the engine compartment. These cars use vacuum to open and close the flappers under the instrument panel that control the heat and cooling air routing. The heater controls on the dash usually just open and close valves that route vacuum to the vacuum motors that move the flappers. I had a bad vacuum line under the hood on my '95 mustang last summer. The A/C wouldn't cool the passenger compartment even though the compressor was doing fine and the fluid lines from it were cold. The vacuum line break was on the firewall in the engine compartment. It was real obvious once I figured out what to look for. That could also explain the loss of power. If there is a vacuum leak anywhere in the car, lots of other things that use vacuum don't work right either, including the engine computer etc. Also, the engine could be running lean if a vacuum line is broken, The engine is sucking extra air in through the broken vacuum connection and reduces fuel/air mix robbing you of power. The faster you run the engine, the worse it will be.

If the large ac line(low side) is cold going into the firewall,then the temp door is not working,check it out.

Most likely a bad water pump and/or low coolant level

check you climate control vacuum hose. it may have come loose or crimped. its located beneath and under battery mount box. also check to see if your coolant resvouir is full as well as your resivour cap is tight.

is the temp gauge working and showing the engine is warmed up? also check the heater hoses in the engine compartment ensure both are at the same or close to the same temp. if they are not could have plugged heater core. try flushing. possible blend door could be stuck.

Pre 1996 cars have r-12 refrigerant. If you used the r-134 on your 1993 grand am, get your car to an authorized ac repairshop immediately.

check the temp control switch in the dash

You would need to buy a factory service manual for that blend door diagram. Try eBay for a used manual set.

The heater core might be clogged. It's located behind the dashboard usually. Just follow the supply and return lines through your fire wall in the engine compartment. Run the engine for awhile then carefully hold the return line, if it's cold you have a clog and the core needs replacement.