No heat while idling, Only when cruising/accelerating?

Discussion in 'General Chevy & GM Tech Questions' started by Kady, Jan 23, 2013.

  1. Kady

    Kady Epic Member 5+ Years GMTC Chick 100 Posts

    So, I noticed a little problem with "timmy" today. I went out to start it 10 minutes before I was leaving for work, turned the heater knobs all the way to hi, expecting to get in a warm vehicle 10 minutes later... Well, I got in and it was as cold as it was outside (9 Degrees) Air was blowing ice cold, and I assumed it was just because I was used to how fast my commuter car (nissan 240sx) heats up. So I lowered the knobs, and started driving. Almost instantly, I felt the heat kick in. so I blasted it again. Great! So I pull up to the coffee shop, and while I'm idling in the drive thru, I notice my heat turn slowly to cold... Start driving again, and it heated up again! What could be causing this? I don't know much about heater cores, but could this be the issue? Any ideas, or suggestions about how to diagnose this would be greatly appreciated... I'd rather not have to put my truck in neutral and rev it at red lights to keep my heat on.
  2. stchman

    stchman Active Member 2 Years 1000 Posts

    Not knowing what make/model/year "timmy" is, I guess I'll assume "timmy" is your 2000 Tahoe.

    It's probably not your heater core, but more than likely the actuator that controls the antifreeze going into the heater core. It might be the rotary switch on your dash as well.

    The NBS trucks heater cores require the dash to be completely dis-assembled to replace them.
  3. Kady

    Kady Epic Member 5+ Years GMTC Chick 100 Posts

    Yes, You would be correct to assume that. But how could it be the rotary switch on my dash if it works while i'm moving, but not when I'm standing still?

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    And the actuator on the heater core? What would cause that to work only when I'm driving?
  4. stchman

    stchman Active Member 2 Years 1000 Posts

    I don't know that much about that year Tahoe, the switch on your dash is FAR less likely, a lot of the actuators on older cars are vacuum controlled, someone on the forum can correct me, and when you rev the engine up your vacuum pressure can increase.
  5. Kady

    Kady Epic Member 5+ Years GMTC Chick 100 Posts

    Hmmm, i haven't checked it in a while, but maybe low coolant? bad thermostat?
  6. stchman

    stchman Active Member 2 Years 1000 Posts

    A bad thermostat would affect coolant temperature only, you did say that when you are moving the air is hot. Is the air as hot as it usual?

    Low coolant is a possibility, when at idle the heater core might not be filling up with hot coolant.

    My girlfriend's Cobalt appears to have a bad air flow changing mechanism (I guess that's what you call it), and will only do floor and defrost. Her car uses electronic actuators to rotate a plate to change each flue's position. She's living with floor and defrost, hopefully the mechanism isn't too expensive to replace.
  7. Kady

    Kady Epic Member 5+ Years GMTC Chick 100 Posts

    Just checked, my coolant is low. So that's what I'm thinking it is. When I'm moving, it does blow as hot as it used to... I remember being in the truck the other day, trying to defrost the front wind shield so I could see out of it, and it wouldn't clear. Now I know why.

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    My dually was stuck on defrost too, but that was a vaccuum problem..

    And the reason I said thermostat because maybe it was faulty and stuck open? But that wouldn't make sense because I'm getting head while moving.

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    So, due to someone talking out of their a$$ on fb, i went outside now, at 12 degrees, to figure out what was wrong. topped it off, and heat cranked like normal. And when I said the coolant was low, that was an understatement. Now tomorrow project is to figure out why coolant was that low. I haven't check it in a while, but it could have been something from the accident.

    Attached Files:

  8. Pikey

    Pikey Moderator Staff Member 5+ Years ROTM Winner 5000 Posts

    Sounds like you had a pretty crappy day. Hopefully a pressure test tomorrow reveals something easy like a bad factory installed hose clamp.
  9. stchman

    stchman Active Member 2 Years 1000 Posts

    If you are that low on coolant it's either leaking out (preferrable) or leaking into the engine (bad).
  10. Charlie Tango

    Charlie Tango New Member

    I have exactly the same symptoms as the OP on my new 2014 Silverado but my coolant is full.

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