I have a 97 civic hx 5 speed. When the heat/ac is on the warm side it runs fine, but when it is on the cool side it starts to overheat.

 

the air comes out of the vent hot still or the cars temp gauge gets hot?

 

does the car overheat when you are moving? or just at a traffic stop

 

Ensure you are full of coolant first off. If not identify the leak and correct it first.

If the engine does not overheat with the heat on, this determines the water pump is physically moving coolant, no problem with flow on the "Motor" side of the cooling system.

This leaves only one cause, a blockage. Either the thermostat is failing to open, radiator is blocked or a hose collapsed. Most likely the first two.

Replace the thermostat, radiator cap, refill, and recheck.

 

bleed the system before you do anything else make there are absolutely no bubbles of air

 

2X

 

I haven't replaced the thermostat, but i did have to change a hose the tore. I also replaced the radiator cap and flushed the radiator. Its still doing the same thing, I'll replace the thermostat and see if that's the problem. Thanks everyone for the help!

 

After you replace the thermostat, be sure to bleed the cooling system.

 

it sound like the thermostat isnt opening when its supposed to. replace it and go from there.