I have an 88 Mustang ,I'm losing power going up hill . What could cause this?

Catalytic converter plugged up.

If you dont have a exhaust problem, your transmission may be slipping under the load of a hill, when was the last time you had you transmission fluid and filter replaced, if it has been over 4,000 miles, this may be your problem. Remember, when going up a hill, gravity forces all of your fluids to the back, so if you are low on trans fluid, you could burn up your transmission going up hill.

Why are you driving it don't Mustangs become really expensive after time. Take care of that baby and stop going up hill.

plugged cat, timing getting ready to fall apart, The vaccuum diaphram not operating properly anymore

If you are in an area that doesn't check exhaust, you can cut the cat out, run a straight pipe and see if it fixes the problem. If it does, then put a new cat in.

Need to service, bad spark plug wires, dirty air filter a 88 you may need a overhaul. Throttle body may need to clean.

Plugged fuel filter, failed oxygen sensors, bad map sensor or air flow meter depending wether it is speed density of mass air flow.

well if your blowing smoke while your going up the hill then you could have a bummed out pistion ring or your head gasket could be shot

you could be really low on gas, fuel filter dirty, air filter dirty, plugs are wore out, cataletic converter pluged up, spark plug wires old or touching exhaust manifold, coil is breaking down under load, and finally the wind...off my camaro as i blow by you.

Release the parking brake.

The wind.