Rep Power: 0

So my car only starts to run hot when I turn the heater on. Could this be a ln indicator of the heater core being bad? I've run it all summer with no issues so I don't believe it's the head gasket. As soon as I turn the heater on it starts running hot.

 
Rep Power: 126

How’s your coolant level looking? Is the overflow reservoir gaining coolant/overflowing?

 
Rep Power: 225

Maybe there is air trapped in the heater core. Check fluid levels, add/remove as necessary while bleeding the coolant system.

 
Rep Power: 0

The overflow is not gaining fluid. The coolant was a little low. I added some. Now the engine temp goes way down as soon as I jump on the highway, The needle goes all the way down to just above the Cold marker. Is it possible someone put the thermostat in backwards? Will they even fit in backwards?

 
Rep Power: 368

easy to open it and check.
Also, several reports that an aftermarket thermo does not work well in Hondas

 
Rep Power: 0

Ok after some research there is no way the thermostat would fit backwards and I'm going to look at an OEM thermostat now. The one I'm using is called a Failsafe. Anyone else have any experience with these thermos?

 
Rep Power: 126

Failsafe is the style of thermostat, not the manufacturer. It's designed to lock the thermostat open for maximum coolant flow when overheating occurs. It sounds like it's stuck open.

 
Rep Power: 0

Yeah I'm going to replace that with a OEM thermostat and see if that helps. Also going to raise the front of the car while I bleed the system which I didn't do before.