Question about 1999 GMC Jimmy

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Heater goes from defost to vent when going up a hill.

When going up a long hill no matter what setting I have the heater on it goes to vent or blowing straight at me. Once I get to the top of the hill its ok. Some times the brake peddal is soft. Suspect they are both related to a vacume problem. Its on a 1999 GMC Jimmy 4x4.

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  • f_rozario Jun 22, 2009

    Thanks for the good information I will try that for sure.



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There are vaccum check valves in the system. you need to find where the check valves are and disconnect each one and blow into one end and then in the other, the air should go thru only in one direction if it is good. these check valves are usually always the shape a disc say about the size of a nickle or a quater, then the disc shape will have tip extending from each side of the thing about and inch where the vacuum hose connect to it. one side of this valve will be white the other will be black. a better discription i guess would be like two very small funnels connected together, one white and one black.
I have had the same issue on other cars and this was the fix each time and the valves are very cheap. I have one in the shed now I believe.

Posted on Jun 17, 2009

  • Michael Fenton Jun 24, 2009

    please come back and rank this answer it has helped.... thanks



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This sounds to me like the thermstat is is a low temp one, I would check and see what temp it is and replace it with a 195 degree thermostat. either it is faulty or as I say it is a low temp one. One other thing to check is that the gate valve on the heater vent / hoses is openning properly, and not stuck. Good luck and remember to rate all solutions that you get on FixYa, thanx.

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