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Im Stumped

Discussion in 'Chevy Trucks' started by purpleranger519, Jan 23, 2008.

  1. purpleranger519

    purpleranger519 Senior Member
    from Kansas
    Messages: 536

    little help guys. What I have is a 1994 Chevy 3500 HD it's 2-wheel drive not 4. This truck has been WELL cared for and has had all new hoses, a new radiator, and a new heater core with-in the last 6 months. I just came in from putting a new 195 degree thermostat as well. It's the 7.4 liter 454 motor as well.

    My problem is this .....when I am running down the road with the heat on...everything is fine. Now when I come to a stoplight and come to a complete stop...I lose heat. My heater now blows cold air. Light turns green and you put your foot on the gas and let off the brake and you immediatly have heat. So my heat is working ONLY when the truck is moving

  2. Detroitdan

    Detroitdan PlowSite.com Addict
    Messages: 1,937

    have you burped the system completely? If you've got air in the system, maybe...
    I'm trying to think how an electrical problem could cause that condition, like a temp control switch or blower resistor, but I can't.
  3. purpleranger519

    purpleranger519 Senior Member
    from Kansas
    Messages: 536

    oh yeah ....I keep going back ...but its not it.
  4. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    I can pretty much guarantee you its a water flow problem.

    The two most likely causes:

    1) Air migrating to the heater core during low flow conditions (like at idle). A major cause of it not keeping the air out of the system, even after you "burp" it is a faulty rad cap. Might want to check it.

    2) A restriction in the heater hose connector in the intake. Not all of these trucks had them but many had a restricter make into the heater hose fitting that's threaded into the intake. These will become restricted by rust or scale, thus restricting flow through the heater core.

    The next time you have the truck up to operating temp and idling, bump it into neutral and give it some RPM's (about 1200 is plenty).. does the heat instantly come back then also? If so, then it's definitely a water flow problem.
  5. streetfrog

    streetfrog Senior Member
    Messages: 337

    When you replace the rad cap get the lever pressure release type. After installing it get the truck to operating temp then pop the red lever. Drive a couple of miles like that repeatedly stopping and going. Then close the lever. That should work. If not have the system flushed.

    KINNCO Senior Member
    Messages: 256

    I would feel which heater hose is the warmest. Pull them both off the the engine and back flush them with a garden hose attachment that will fit securely in the hoses (through the cooler one first, then reverse the flush)
    It worked for me.
  7. purpleranger519

    purpleranger519 Senior Member
    from Kansas
    Messages: 536

    Wow....looks like I have some work to do....didnt think of looking into this. I'll let you know.
  8. Indy

    Indy Senior Member
    Messages: 704

    It sound like a air intake or output problem with the heater fan or a water movement problem. My guess is the heater fan. Does the strength of the air movement change at the stop or just temp???:confused:
  9. purpleranger519

    purpleranger519 Senior Member
    from Kansas
    Messages: 536

    Just the temp...Heather fan is working fine and blowing hard at all RPM,
  10. purpleranger519

    purpleranger519 Senior Member
    from Kansas
    Messages: 536

    have tried everything you guys suggested....still have the problem
  11. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    What did you find when you gave it a little throttle in neutral?
  12. streetsurfin'

    streetsurfin' Senior Member
    Messages: 770

    Vacuum problem (leak) signalling a heater control valve, too weak to hold the valve open at idle? I don't know if Chevys use one similar to a Dodge's valve in the heater hose or not, so don't mind me if this makes no sense.
    Last edited: Jan 24, 2008
  13. abbert55

    abbert55 Senior Member
    Messages: 170

    I know how fustrating this can be. You might have a problem with the water pump. I saw the same problem a few years ago and the impellers were pretty much eaten up and there was very little circulation at idle. As soon as you reved it up a little..... heat would appear like magic. Hope this helps, Al
  14. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    And thats exacty why I asked about the RPM test again...:nod:
  15. iflyhelis

    iflyhelis Member
    Messages: 53

    Three things I can think of that is causing your problem:

    1. A vacuum leak causing the air control dampener to creep open for full cold air at idle.

    2. A clog in your heater hose.

    3. If you are running Dex-tron antifreeze & it is old, it will gel up on you.

    So, when you replaced the radiator & heater core did you back-flush the heater hoses?

    I had a GMC Jimmy that clogged on me while running at speed, no heat in a heart beat.

    I cleaned it out, & the heat came back, I changed the Dex-tron also, less than 2 years later it did it again.

    Dex-tron is known to destroy water pump seals & cause all sorts of problems, due to the "silica" that is in it. I have heard of alot of people changing back to the old standard Prestone green.

    Good luck with your problem, I am sure it will be a easy fix!
  16. Vinster01

    Vinster01 Junior Member
    Messages: 5

    A quick test for your vacuum leak; turn the fan on & blow it out the dashboard vents. Find a hill or just stomp on the gas pedal from a stop. If the air stops coming out of the dashboard & defaults to the defroster- you gotta leak. You usually notice this problem in the summertime when using the AC. Good luck
  17. purpleranger519

    purpleranger519 Senior Member
    from Kansas
    Messages: 536

  18. iflyhelis

    iflyhelis Member
    Messages: 53

    So, does Bingo! Mean that you found that you had a vacuum leak?
  19. purpleranger519

    purpleranger519 Senior Member
    from Kansas
    Messages: 536

    it does.
    now where is it
  20. iflyhelis

    iflyhelis Member
    Messages: 53

    Two choices:

    The hose end that is connected to the vacuum supply which is probably located under your air cleaner housing. You will see a short metal line which is ported to your intake manifold, the end of the rubber hose (about 3/16" in diameter) will most likely have cracks in it, of course this is attached to that metal line. A lot of heat there drys out the rubber.

    Or you will find it attached to your vacuum reservoir, which could look like a black plastic ball the size of a grapefruit. Just follow the vacuum lines back from the heater box or the vacuum source.