I recently put a new alternator in my Montero which required disconnecting some coolant hoses resulting in coolant loss. I replaced the coolant back to full, bled the system thoroughly only to have it start overheating. So, I replaced the radiator cap and thermostat, bled the system thoroughly again. Fixed. No more overheating.

Now, a few weeks later, it will not put out any heat even though it is at running temperature. Radiator is only 3 years old, had system fully flushed about a year ago.

How should I go about diagnosing this?

 

Most likely you have some air trapped in the heater core. You can try to bleed it again. You can try to park on a slight incline (engine uphill) while bleeding the system. Hopefully that will help. The fact that the system was overheating after first coolant refill, indicates that there was an likely an issue with bleeding.

 

Just a question, does it get hot or overheat when your just driving or when you have the AC on? That is a common issue I have come to learn and I have no answers for that except that I put an electric fan on it in addition to the stock on a switch just for this purpose. Works fine and it cools it down...

 

Thanks for the reply. So this weekend I decided to flush my heater cores (they have one in the front and one for the rear air). I would not recommend doing this yourself, unless you like laying in antifreeze!

This was the issue. I back flushed both cores and got tons of crap out of them. I think when I was replacing the alternator and thermostat I squeezed the hoses and broke off the buildup that was inside of them. I read somewhere that when you're bleeding the system it helps to squeeze the hoses to get the air out. I wouldn't recommend doing this. I felt a crackling sound in the hose and I think all of that debris clogged my heater core.

Heat is extremely hot now and Momma is happy.

 

I actually had this problem a few years ago. Ended up putting a new radiator in it and it hasn't happened since.

 

Awesome. Glad it worked. Did not know about possible buildup on the inside of the hoses.