Hi everyone, I have Chevy Blzer 2001 2WD. I've recently encountered the following problem: the engine starts slowly overheating and AC does not blow out heat. The air that is coming is completely cold. Then suddnely, at a very random moment the engine starts to slowly cool down and AC starts blowing heat. These 2 conditions iterate as I drive. Sometimes it can be dangerous and the engine tempereture is almost at the red zone. So what can be the problem?
I'm going the change the thermostat tomorrow. Though I'm not sure it's the problem because is the thermostat is stuck in closed position, the AC is supposed to blow super hot because of the overheating engine, right?
So the problem might be in the water pump..
What do you guys think?

 

I would start with the thermostat , Are you leaking coolant? if not then water pump replacement would be wasting money.. are you low on coolant? how old is the antifreeze? have you ever flushed the rad and heater core? easy to do disconnect lines that go to fire wall add some clr wait 10 mins flush with water works wonders.

 

I got this vehicle used with no prior maintenance records. The coolant level was already low, so I added some. Then it ran perfectly fine, no overheating, nothing. Today as I was driving my car overheated and some coolant spilled out from somewhere, but not much.
The coolant looks so-so. I wouldn't say it's too old, but the system will definitely need to be flushed soon.
150k miles on the car.

 

Is there Dexcool in the system? Dexcool clumps up and clogs the radiator after years of not changing it. If you grab the heater hoses after its hot are they warm? You could have a number of problems. I would dump the fluid in the radiator. Remove the radiator cap. Remove the connection of the lower radiator hose at the radiator. Stick a hose down the fill of the radiator and see what kind of crap comes out. You need to drain the system anyway to replace the thermostat. Use a AC or a Stant thermostat the cheap ones stick. While your at it flush the heater core. You can buy a kit for like $10.00 . Dont leave the kit in as they do fail. You will need about 6' of hose on one side to drain in a large bucket and a 4' piece to go on the other to feed from. flush both ways until clear.

When the car is running and you get that situation (overheat) try and switch the AC to max AC and see if hot air comes out the top ducts. Your blend door maybe stuck or the blend door motor not working and your never getting the heat from the heater core. The default position uses the outside vents I believe. Not sure on the later models.

 

I've decided to start with the easiest step. So today in the morning I checked the level in the radiator. The level was really low. I managed to fill almost 3/4 of a gallon. After that I ran the engine with my cap off to remove any bubbles. Then I closed the cap and made a test drive. So far working good. Warm (not hot though) air from the AC + no overheating at all, no matter how I try.

@Chevy Nova That's a good idea, I will definitely need to do that soon because as I looked inside the radiator I did not notice any serious gunk like some guys show in their videos on youtube. However, the inside area near the cap did not look good.

The problem is that since I had a half/full radiator, the coolant disappeared somewhere. So there must be a leak. That may be a sign of a serious problem...