temp gauge on my 97 blazer normally runs 1/4 of the way up and heat works fine. all of a sudden heater/defroster begins blowing cold air and needle goes up halfway then almost 3/4 ways then back down to halfway. It goes up and down like this all the time I'm on the freeway doing 60 trying to make it home before it gets in the red zone (for about 20 mins or so). It finally got real close to the red so I pulled over. Popped hood and I heard some hissing and saw steam coming from the top of engine by where the two heater hoses run from water pump to firewall and the other one runs from manifold to firewall (Obviously a leak somewhere). I think these two connect to heater core??? It did not exhibit the typical heater core malfunction symptoms like steamy windshield or leaking onto passenger side floor area. The one peculiar thing I remember is hearing an intermittent sound from behind the dash like a tapping inside a box every now and again. Was this something malfunctioning in the ducts/vents??? Any ideas about where to start looking is greatly appreciated. Yes it's winter and cold up here in Minnesota. It was around 15 degrees above when this occurred last weekend. Right now it's 15 below where I'm at and it was 28 above earlier today. That's a 43 degree change in temp in less than 12 hours. Gotta love it!! Thanks again!!

 

The clicking could be the blend door actuator which typically fails by cracking the gear on the output shaft that connects to the blend door itself. The blend door is what controls the airflow over the heater core in the HVAC system.

I would recommend a complete cooling system service. Flush out the block, radiator, and heater core. Replace the thermostat & radiator cap. Clean the overflow bottle and make sure that the hose running between the radiator and the overflow bottle is clear.

When filling the system back up, make sure to remove all of the air from the system. It helps to have the front passenger corner higher than the rest of the truck to facilitate air removal. Run the truck up to operating temperature then let it cool down, topping off the radiator with each cycle; cycling it at least 3 times or until the level doesn't drop down all that much when it cools.

To clean the overflow bottle, I have found that a shop rag, some diluted simple green, and a few medium sized nuts makes quick work of the crud that can build up inside.

 

flushed heater core and replaced t-stat. when pouring water into radiator, it started leaking out pretty bad from under middle of engine roughly from above the oil pan, tranny pan area, not from radiator/water pump area. Any ideas what this might be. As always, thanks for any input.

 

check your heater core hoses against the firewall

 

Check your intake gaskets as well. That was the problen with my 01 Blazer TrailBlazer

 

I don't know if water would "leak out pretty bad" while pouring it in if the intake manifold gasket is bad, but you might consider one of the freeze plugs have failed (or rather done their job due to the coolant freezing and expanding).

 

You say you where pouring water in, Hope thats not what had in it before. Could have caused the whole problem, should be 50/50

 

Did a search and found this thread. I am having the same issue, not exactly but similar. My heater core became clogged years back and the heater quit working. I flushed out the heater core and it worked fine for years. I replaced the radiator about a year and a half ago because it was cracked and leaking around the hose connection up top. I tell you all of this just to give a little history.

This past winter which was pretty brutal I topped off the antifreeze just getting prepped for the cold and my heater quit working again. I flushed out heater core again and it was not clogged. I went through the winter with barely any heat. Engine would never get as warm as normal either. I assumed the stat was clogged but never replaced it because I had to leave town for work for a while.

Been back and driving it. Runs great but engine still doesn't get warmed up and no heat to speak of still. I drove it the other day heading to the store. Had to get into it a little to get past someone on the road and when I did I started smelling antifreeze. I got it home and saw some leakage towards the back of the motor from underneath but couldn't see the leak. I drove it again to warm it up and only went a few miles. Engine started getting really hot, when I popped the hood there was antifreeze on the intake around the stat and heater hoses but stat area was dry. I assumed it was the heater hoses and I just took them off but couldn't find a hole or split in them. Radiator looks to be empty so when it blew, it apparently blew out it must have blown most of it on the road when I first smelled it.

Any thoughts? I was thinking of changing the stat and the hoses I had to cut to get loose so I am going to replace them. Dunno what else to look for. I'm just trying to make her last a little while longer. I dropped a bunch of money in her last fall with a new fuel pump, catalytic and muffler, other sensors, new tires and brakes. I'm pretty tapped until I get back to work.It's a 2000 4X4 V6 LS.

 

Dozer your stat could have been stuck open in the past causing continuous flow and never getting warm. Does sound like it may have shut and stayed that way. I would change it. First though I would would fill it with water,start it up, dont drive just leave hood open to see if you can find the leak. Why change all the hoses

 

Just noticed you cut the hoses