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Why does my heater occasionally blow cold?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by bigk, Dec 15, 2001.

  1. bigk

    bigk 1/2 ton status

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    Sometimes on my 88 Burb the temp will go up and then down quickly, and the heater will go from blowing hot air to cold. I just put a new thermostat in a couple weeks ago. Could it be sticking, or is it something in the heater? Someone told me about a door or something that could be sticking. I've never been into the heater before. Thanks in advance.

    Karl
    Temporarily Unemployed Underwear Model
    87 3/4 ton
     
  2. LKJR

    LKJR 1/2 ton status

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    Coolant is low that is why it's fluxuating too

    Like em big and topless
    Excuse me...Would you mind removing your import from under my chevy?
     
  3. Butch

    Butch 1/2 ton status

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    If it does it just under power or just under decel you may have a wrong vacuum line hooked to the valve that controls the heater core.

    I thought I was wrong once,
    but I was mistaken
     
  4. high&liftedup

    high&liftedup Registered Member

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    I had a similar problem & found that I had a small coolant leak on the intake manifold/water neck... With coolant low..there's little 'hot' water to warm you up...

    HE will always get you out of the mire clay!!!
     
  5. chevyracing

    chevyracing 1/2 ton status

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    you have air in the cooling system. Need to bleed the system and get the air out.

    John

    Like to go sloppin' 'round in da mud in a rapid fashion....=)

    <a target="_blank" href=http://coloradok5.com/gallery/albun31?&page=1>BLAZER PICS</a>
     
  6. txbartman

    txbartman 1/2 ton status

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    I haven't had the problem with the temp guage fluctuating, but hot air fluctuating is fairly normal. Especially if it does it at consistent times (i.e. at idle, sitting at a light, etc). Your engine and heater core share the same fluid. It's primary purpose (the coolant) is to keep your engine at the proper temperature. If it gets too hot, it will circulate coolant through the heater core and radiator to get freshly cooled fluid and send out the hot fluid for cooling. If your engine is too cold, it will stop circulation and keep the coolant in the block and not allow it to circulate through the cooling devices. Your thermostat is what controls this.

    So, if your thermostat is bad and isn't circulating or holding the fluid properly, you will see this problem. OR, under normal circumstances, if your engine starts to get cold (real cold outside, low RPM so engine isn't generating much heat, or blistery cold winds over cooling coolant, or low coolant level) your thermostat will keep the coolant in the engine and out of the heater core. What is in the core then turns cool and your heater will blow cold air. When the coolant in the engine heats up and the thermostat allows the circulation again, the heater core will heat back up, the hjeater will blow hot air again, and your temp guage will drop as cold coolant is brought into the engine. The whole cycle starts over.

    This is why if your engine ever starts to run hot in the summer, turn on the heater full blast. Your heater core then becomes another radiator helping to cool the fluid.

    Brian
    Check out <a target="_blank" href=http://tx85gmcguy.alloffroad.com>My Jimmy</a> with all her projects!
     
  7. Brian 89KBlazer

    Brian 89KBlazer 1/2 ton status

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    It's as simple as what chevyracing said! Just spent a while messing with this stuff on the wife's Lumina. After a thermostat &amp; a pile of asking around; I bled the system &amp; presto!! What a difference. Faster warm up &amp; nice &amp; steady heat.

    Give it a try; just have lots of coolant on hand



    Brian
    89KBlazer

    Build it Right or Don't build it at all!!
     
  8. Calclips

    Calclips 1/2 ton status

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    how do you bleed the coolant system? Or better yet... how do you do a complete coolant system flush?
     
  9. chevyracing

    chevyracing 1/2 ton status

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    BigK,
    Try this. Take off the radiator cap, fire the truck up and let it run to operating temp. If it gives you a volcanoe of coolant in the air at some point you had air in the system. With the engine running, top it off with coolant mixture and enjoy your consistant heater

    John

    Like to go sloppin' 'round in da mud in a rapid fashion....=)

    <a target="_blank" href=http://coloradok5.com/gallery/albun31?&page=1>BLAZER PICS</a>
     
  10. chevyracing

    chevyracing 1/2 ton status

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    Oh yeah, turn you heater on full blast while doing all this. The key thing is to let it run for a while with the cap off so all the water circulates through the entire system and pushes the air out.

    John

    Like to go sloppin' 'round in da mud in a rapid fashion....=)

    <a target="_blank" href=http://coloradok5.com/gallery/albun31?&page=1>BLAZER PICS</a>
     
  11. bigk

    bigk 1/2 ton status

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    Thanks, if it's not fixed, I"ll have to give that a shot. I checked it and it was 1 1/2 gallons low on coolant. Guess I didn't get it filled back up completely when I changed the thermostat. I only drained a gallon out when I did it, but it must have needed more.

    Karl
    Temporarily Unemployed Underwear Model
    87 3/4 ton
     
  12. chevyracing

    chevyracing 1/2 ton status

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    If it was 1.5 gallons low you definately had air in the system, heck, you pretty much had an entire atmosphere in there.

    John

    Like to go sloppin' 'round in da mud in a rapid fashion....=)

    <a target="_blank" href=http://coloradok5.com/gallery/albun31?&page=1>BLAZER PICS</a>
     
  13. wheelnut46

    wheelnut46 1/2 ton status

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    On this same subject - I'd like to ask a question;
    My temp guage only reads a quarter and my heat is warm, but not warm enough. I changed the thermostat (190 degree) and let the engine run while I topped it off. I know that I got at least one big bubble out. Tonight I stuck my wifes meat thermometer in the radiator but after the engine got hot the water came surging out, then stopped, then surge, then stop, then surge, then stop - I could see the water rising, then spilling out, then settling down again, repeating this over and over. Weird huh? The truck runs good. Any ideas?

    Bill
    '77 GMC Jimmy
    '46 chevy pickup
    '67 Chevelle Malibu
     
  14. wheelnut46

    wheelnut46 1/2 ton status

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    Please offer any ideas you may have. I appreciate the thoughts.

    Bill
    '77 GMC Jimmy
    '46 chevy pickup
    '67 Chevelle Malibu
     
  15. Brian 89KBlazer

    Brian 89KBlazer 1/2 ton status

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    Wheelnut

    That's exactly what happens when you have air in the system. Not to sound like a parrot here [​IMG] but again; like chevyracing said; if you remove the cap &amp; turn your heater in the vehicle on bust; then run your engine for a while; the air will bubble up spitting out some of the coolant with the air. Have a big drip pan under the rad area &amp; let the engine run til no more air comes out.

    Another note (don't ask why I know!!) but have plenty of coolant on hand to keep the radiator near full while it runs. If there's alot of air; you'll lose a fair bit of coolant!

    Best of luck

    Brian
    89KBlazer

    Build it Right or Don't build it at all!!
     

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