Air inside engine
If your engin light also is on, you need new oxigen sensors!
The lack of air intake designed to flow into the motor.
Check the timing belt to see its off.. If the timing is correct them most likely you have a failed camshaft sensor..
check the coolant
A 1991 Subaru Impreza might overheat because of a leaking cooling system or because of a damaged thermostat. You might also have a damaged or leaking radiator or a cooling fan that is not working.
your water pump is leaking and traveling down to the cover. Water pump needs replacement.
The extra load imposed by the air conditioning compressor is overheating the engine. This could be caused by numerous things, but the most common would be either a broken thermostat, low coolant, or a badly tuned engine. Take it to a mechanic.
You don't.
show me the fuse panel for the 91 ls 400 an witch one is for the alternator
at least 91 octane
Premium, 91 or higher
press the button of Overdrive lock in front of the Shift knob panel . then shift out of park. That is a temp solution. one of the sensor in the transmission need to be replaced. aproximiately cost you $100-200 part and labor. I experienced same thing on my 91 ls400. Let me know if it works. Good luck.