Villiska

So, it's getting pretty cold outside now and I have an issue. If I drive my car without turning on the heat or turning the HVAC **** to anything but off, my car is fine. However, if I turn it to heat or mix, my car overheats and I'm forced to pull off on the side of the road and turn off the car until it resets. 5 minutes later and I'm fine and back to driving.

I popped the hood this time it happened when I was troubleshooting it and the coolant level was completely risen to the top of the back fill tank and it was foggy inside.

The thermostat was my first idea so I replaced that and it's still giving me this issue. Any idea what it could be? I don't see any wetness from coolant on my floor nor do I see any condensation on my windshield.

What could be happening?

Edit:

I also want to mention that even when the heat is on, it blows cold 80% of the time. Sometimes it'll blow hot but then it goes right back to cold.

PNYXPRESS

It sounds like possibly your water pump is bad. When you don't have the heater on, the pump is able to move the coolant thru the engine and radiator, but as soon as you turn on the heater it can't move the coolant fast enough to prevent overheating. Or you have a blockage in the heater core and/or lines

You can check this by running the car with the radiator cap off and when it reaches temp you should be able to see the coolant flowing. It should continue flowing when the heater is turned on.

Chromeshadow

Is there a chance someone reversed the hoses to the heater core?

PNYXPRESS

It shouldn't matter they both flow thru the water pump. And I don't thing the heater core is directional.

TfcCDR

There are two kinds of heat regulation in car HVAC. The traditional way is to use a valve to set water flow to the desired level, and the other is to have the core always hot, and regulate airflow through the core. I've going to assume that the Mustangs still use water-flow regulation, in which case a blockage won't affect the rest of the engine.

I think you probably have it with the water pump, or possibly the thermostat (partially open, not able to pass enough water for HVAC and engine cooling at the same time). I'd swap out the thermostat first, then flush the cooling system and see if that fixes it.

Villiska

I'd actually prefer it to be the water pump than something related to the heater core. The water pump, from what I can tell, looks like a $60 job and about an hour of work. The heater core would require me to take out the hard plastic dash board and tamper with all of that stuff. That seems frustrating and difficult since I'm not so savvy with this.

I've already replaced the thermostat, by the way. I think I'm using a 193 or a 195F now and before it was a 175 or something. I don't remember, but I opted for the slightly more expensive one when I purchased it.

Is this a huge issue though? It may sound silly, but I'm selling this car around August so if I'm able to fight this winter with extra clothing and save the money, would this cause issues elsewhere during the Summer?

I've had a coolant drain since I bought the car, but this overheating issue started this Winter. Could this all be linked together and if I buy a new water pump it could all be fixed? I just don't want to waste money, replace it and then still find myself with the same issue.

I almost want to say that I refuse to touch the heater core aspect of things and would rather just wait until I sell the car. That seems like such a pain in the butt from what I've seen in videos!

PNYXPRESS

It could also be a head gasket issue if you have been losing coolant

Villiska

The coolant leak only occurs when I use the AC or the heat. If I don't touch those, I can go months without adding coolant whatsoever.

Derf00

Sounds like a leak in the heater core. When you turn on the heat, your cooling system loses pressure and fluid because of the leak. that pressure drop would cause the fluid to boil at a lower temp. Yes, heater cores are a PITA because you have to remove the dash to get to them.

Villiska

Do you think that pain in the butt is worth dealing with if I plan to sell the car within 6 months? I can survive 2-3 more months of winter since I dress warm for work anyway, and the Summer is a breeze for me.

I just don't know if there's any other issues that may come into play within that time frame outside of using my HVAC *****. As long as I don't touch those, I should be fine right?