Hey guys, Just some back-story to help explain. I just bought a 4WD 98 blazer LT 4.3L with 160k miles from my uncle.

The problem is- he replaced the heater core 3 years ago and the radiator 2 years ago but it just lost heat... AGAIN.

So i'm trying to figure out where to look first. Could a 3 year old heater core really clog up that quickly? Or should i be looking into things like thermostat or if there is a flow control valve to the heater core? possibly stuck shut?

My uncle thinks it is the heat/ac control panel in the cab but he is not a car guy so that's just his blind guess. Plus all the controls work fine so i don't think it is the panel.

Any help or guidance is really appreciated!

 

There is no flow control valve in the heating system. Modern thermostats are "fail safe", meaning they stick in the open position if the break. A problem with the blend door could cause your no heat problem. Brown gravy looking sludgy old dex cool could be the problem as well. If your coolant is in good shape, then investigate the blend door and actuator.

 

Great, thanks for the reply. I'll check the coolant condition and if both in/out lines to the heater core are getting warm when i get home tonight. If they are warm, i will look into the blend door and actuator.

I'll try searching for it now but is there a simple way to check the blend door/actuator to see if they're operating properly? like an easy access from engine bay or under dash?

 

its possible. i work in a shop and every year right when it comes to cold weather we have a guy who comes in a older grand prix due to his heater core clogging every year. previous owner put a crap ton of stop leak in his engine for whatever reason. so we flush it out and just wait til next year.

 

Search around about Dex-Cool (the original GM pink/orange coolant) and you'll find plenty of reading material. Basically it's fine if -- and ONLY if -- the coolant replacement service intervals (5 years or 150,000 miles) are stuck to and there are's no air in the cooling system. However, and a lot of people just never replace it and it leads to problems.

Unless you're confident it's had a proper flush recently it's probably a good idea to flush the cooling system anyway, and if the heater core is clogged there's a chance it might help clear it too.


Depending on who you ask this might be a good opportunity to change over to conventional green antifreeze; personally I stick to Dex-Cool and just keep up with the service intervals.

 

On my old 99 blazer LT I had lost heat, I unhooked the heater core lines and took some deep breaths and blew into each side. It wound up being clogged and blowing into it unclogged it. When you blow into it, you will know it is clogged if you get resistance. just blow until it comes free and presto, you will have heat. But you also want to do a good flush as mentioned above. Do not use an air compressor, you could blow the heater core, just lung power.

 

so i got some time to look under the hood and both the IN and OUT hoses to the heater core are getting equally hot when i let the blazer heat up. The coolant looks like **** so i'm still going to do a radiator flush and try to flush the heat core out but the fact that both hoses heated up equally and fairly quickly makes me think it might be the heat mixer flap/actuator.

Is there an easy way to access this mixer flap under the hood or dash?

 

Hey guys, another update. I got some time and did a full heater core flush with garden hose and some CLR and WOW the crud that came out was bad! After flushing the heater core i test drove the blazer and wouldn't you know it.... i have heat!

It seems like i have either full heat or full cold though, if i put the temp selector on anything except the hottest setting it gets instantly cold, Would that still be the mixer flap or should i look into the control panel? its odd that its 100% or nothing but hey! im just happy i have the option of heat now!

 

I have the exact same problem, I never did figure it out. I also had no heat a year or two back and flushed mine the same way, using a water hose and CLR. I have had good heat ever since.