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2008 Silverado 5.3 Heater issue - strange

Discussion in 'Chevy Silverado Forum (GMC Sierra)' started by mattwave, Jan 12, 2013.

  1. mattwave

    mattwave New Member

    I have 2008 5.3 Silverado, love it, never a problem, but my heat suddenly started acting strange, if I have truck stopped the air blows cold, once I am moving it heats up. I can leave it running for an hour or if its running all day and I am stopped, same thing. Been doing for months but now its cold and I need heat. The temp gauge is where it should be so I thought not a thermostat issue, its full of antifreeze so that cant be issue. Blend door maybe?? Anyone had this issue?
  2. j cat

    j cat Active Member 1000 Posts

    you may have an air pocket in it . very common..

    with the vehicle cold remove coolant cap start engine and allow to run until at the normal op temp then rev engine a few times to 2.ooorpm.. if you see a drop in coolant add to the full hot mark. exactly. then see if the heater issues is corrected. if this works and then repeats and you have to add coolant again then you have a leak

    good luck

    temp gage should be 190f area.
  3. RayVoy

    RayVoy Active Member 1000 Posts

    Unless the coolant was changed, there shouldn't be an air pocket (maybe I'm wrong, and these things develop air pockets, I haven't had mine long enough to get any HVAC experience). You say it is full of coolant, was it ever low and coolant was added, this could have produced an air pocket. However, I've gotta tell-ya, the description sounds like low coolant, is the overflow tank filled (to the correct level) as well?

    You say the heat stops when parked. I'd get the hood up and feel the heater hoses, do they become cold when the truck is stopped? Drive it, then park it and raise the hood (do this as fast as you can) are the hoses hotter?
    If the above points to hoses being cold at times, you have a coolant circulating problem.

    If the hoses are always hot, you could have a temp door actuator problem, but they don't normally "fix" themselves as you drive.
  4. mattwave

    mattwave New Member

    Coolant never changed, secondary never topped off, hoses hot, I definitely hear a thump in the heater box when I turn heat on, when it fixes itself the heat seams to be lacking in temp though.
  5. RayVoy

    RayVoy Active Member 1000 Posts

    The "thump" usually indicates air. Try parking on a hill with the nose in the air. Let the engine run for awhile, maybe the air move to the high point (radiator)
  6. rr1963

    rr1963 New Member

    Not sure about this model year but the door actuators and/or the heater control valve could be vacuum controlled and a vacuum hose could be leaking. Just a thought.
  7. mattwave

    mattwave New Member

    Ok I will try the hill resolution, where would I be looking for leak in vacuum hose, whats the best way to determine if there is a leak?

    - - - Updated - - -

    Thinking its a vacuum leak, I can hear a hiss driverside engine compartment. Any thoughts on best way to diagnose and fix?
  8. j cat

    j cat Active Member 1000 Posts

    did you do the coolant air purge proceedure I posted in #2.

    air can get into the system . leaks do develop if you do not properly maintain the coolant .
  9. mattwave

    mattwave New Member

    Yes I did check.

    - - - Updated - - -

    So my question is now, if I were to execute a smoke test to check for vacuum leak, where would I want to blow the smoke into?
  10. RayVoy

    RayVoy Active Member 1000 Posts

    On the '08s, all motors are electric.

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