So winter is here and I have no heat, but after flushing my heater core an odd thing began to happen. Now I have good heat for the first few seconds after I turn the heat on (after the engine is up to operating temp) then it gradually drops in temperature until it's cool, but if I turn the heat off for a few mins and back on I then have heat for a few seconds before it cools down again. Now I am sure the blend actuator needs replacement as no matter where I have the dial I have air flow through all vents, but could the blend actuator also be the cause of this odd behavior. Before it's asked I do have a new thermostat and it's maintaining operating temp perfectly, as someone originally suggested when I said no heat, no the heater core isn't leaking and it has good flow when flushing/backflushing, the HVAC fuses are good. I am not very familiar with the heating system so I am utterly stumped.

 

There are a number of doors in the HVAC system inside these trucks. To help you more specifically, could you let us know if you have manual temperature control or automatic?

On both systems, airflow over the heating coil is controlled by the blend door actuator, an electronic actuator that moves the door. Airflow to the various functional outlets (defrost, vent, floor, etc) is controlled by vacuum for both systems, however the automatic system has an additional electronic module that controls the vacuum to the various actuators under the dash.

It sounds like you have a combination of problems; 1- blend door & 2- possible loss of vacuum pressure to the HVAC system.

 

It is the manual controls

 

Finally had some time to check some things over and get back to this thread, turns out the blend actuator wasn't even on the blend door b/c the mounting areas had been broken off rather than the screws removed to take the actuator off so that was definitely a problem. After some tedious work I have the actuator back on and "calibrated" correctly and turning it to cold definitely makes the air colder, but my heat is just barely warm then back to cool. There isn't a vacuum leak I saw the little diaphragm moving the ventilation around perfectly. I am now under the belief I have a clog somewhere between the heater core and water pump, the heater core inlet hose is hot as it should be but the outlet is still fairly cold and after removing the outlet hose for a short period and putting it back on (with the vehicle off ofcourse) I noticed the engine went a little bit over 210 degrees and after about 30 seconds went back to normal. Keep in mind I have flushed the heater core a few times, though it seemed to have better flow in the back flush rather than the straight flush. I been running some of the Prestone flushing fluid through my system for about a week and am about to flush the entire system to get rid of the Dex-cool garbage that's in it as I personally saw the dex-cool build up in the heater hoses the first time I flushed it. At this point I am just curious what you all think about my clog theory and what exactly could be clogged or if it would be safe to reverse the heater hoses at this point as a possible temporary fix.

 

There is either air trapped in the heater core, or the core is restricted. Both heater hoses need to be hot. You should only be able to hold your bare hand on the hoses for a very few seconds when at operating temperature. If the Dexcool was not maintained properly, (flushed every 3 years or 36K miles, whichever comes first) that could easily be the problem, (neglect, not the fault of the Dexcool). Your cooling system was designed for Dexcool. Yes, cooling system. There are plenty of threads, both here on the forum, and on the internet, discouraging the use of ethylene glycol in a system designed for Dexcool. It will cause ongoing problems, especially with the water pump and aluminum parts in the engine.

 

I've only had the blazer for around a year and a half so I can't say whether or not the cooling system was properly maintained, as for the heater core, how can I bleed it properly and if it is restricted what would you recommend to flush it with. The last flush I did was with CLR, I isolated the heater core for the flush since CLR would likely do more damage than anything in the radiator and block. I am to the point where I am looking at getting a new radiator, core, and hoses, just to be done with this.

 

When a heater core gets plugged up as bad as yours is, it might be best just to bite the bullet and replace it, same with the radiator. The block has to be cleaned out too or the new core and radiator will plug up again.

To "burp" or "bleed" the system, remove the radiator cap, bring to operating temperature, raise the front right corner of the vehicle, keep the radiator full, hold RPM ~2500, keep the radiator full, jazz the throttle a few times, keep the radiator full. When it stops taking coolant, keep the radiator full, put the cap back on and allow the engine to idle. That oughta do it Did I mention to keep the radiator full?

 

I really appreciate the input man, depending on how the flushing goes Monday I may leave the current core in, but if it isn't perfect after the insane amount of flushing I am gonna do then I will replace it aswell, I am hellbent on not taking the dash out lol. I thought about putting a bleeder valve on the heater outlet hose like some of the older models have, but I haven't decided yet.

 

So after four days of work I have a new radiator, reservoir, and heater core. No heat, and the engine is overheating, right now I am chalking it up to air still in the system, but if I am wrong does anyone else have any ideas.

Update - I was correct in assuming it was air in the system, bled it good and now have heat and no over heating. Thanks to everyone who pitched in.