Ok, I have to admit, this is on my wife's car, which is a '97 F**d Escort lol, but I couldn't think of a better place to ask for help. Last summer i replaced the thermostat. I know the radiator has a small leak, but I'm not so sure that's the problem. When the a/c is run, the car will overheat, to the point of boiling, if let run long enough. Since the weather has cooled down, and the heater has been in use, the temp gauge barely moves above cold, and doesn't even reach normal operating temp. I tried the a/c again and sure enough, it started getting hot again, turn the heater back on, the gauge goes back to cold. Could the cause be a thermostat installed backwards, or something else? Thanks in advance for any help you guys can give.

 

To tell you the truth I took my AC out since its always a problem more so than the T-tops
Anyway what I think the problem is, the preassure! (i am thinking that) Since your radiator is letting some preassure out you will be running hot. Now, when the radiator sucks water in from overflow it will suck in air as well. In other words fix the damn leak or get a new rad. Also, when using AC you are only using the radiator and really putting preassure on it!! When you are using the heater the coolant is sent through the heater core and the heater core acts as a small radiator too! Therefore you are running cool. In cool weather with the heater on the car will stay cool. Not reaching the operating themperature could mean few things. Its cold and the thermo is a lower temp one!! It might be stuck open (also a cause of overheating with some cars), or its just to dang cold. However, I would fix the leak on the radiator and backflush the system (burp it), and change the thermo!
Let me know what happens!! Mr. Ford escort!!

 

I'll try backflushing first and let ya know how it does, Thanks for the help

 

I have the exact same problem and for the past 5 years i cant figure out what the problem is. IT will run 180 with the AC off on a hot summerday but then shoot up to 260 when i turn the AC on. I have flushed my cooling system have a 50/0 mix with water wetter. all hoses are fine no leaks air damn is there i have no clue were else to look i have a 160 stat i dono where elese to look.