Question about 2005 GMC Sierra

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HELP! - GMC cooling system problem

1989 GMC C1500 5.0L V8 - after replacing the thermostat twice and the temp sensor, still have this problem and not knowledgable enough to resolve - please help! after running for a bit temp will rise (temp guage indicates) to ~195 deg (thermostat rating) and then after (I assume) the thermostat opens the temp will drop to ~160 and stabilize there regardless how long or hard the engine is run... I am at a loss... I do have a higher cooling capacity radiator than the one that came stock but wouldn't think that was an issue as I thought the thermostat should still regulate the block temp. regardless... I haven't tried replacing the radiator cap but will tonight... is there any possibility that the cap is allowing the system to overpressurize and not allowing the thermostat to close once opened?... just grasping at straws here... please help! I am having the same exact problem on my '05 gmc sierra. It has always ran at about 190 - 195, i noticed today that when i started the truck (already warmed up) the temp went to 190. Then after about a minute or so, it drops to the bottom temp of 160 and stays there. Does this sound like the thermostat is bad or is the gauge / sending unit going bad?

Posted by tfdrooke24 on

  • 3 more comments 
  • tfdrooke24 Dec 14, 2008

    it seems like everything is working ok with the ac/heater, its just that this started out of no where. Its like the gauge just bottoms out and shuts off. But the funny thing about it is the gauge works ok for about a minute before it bottoms out. Its got me confused!!!!

  • Anonymous Feb 07, 2009

    Same type of issue. Temp rises normally, then randomly raises to 235 then drops back to normal. At the same time as the temp rises I've noticed that the heater quits outputting. Thinking it may be a thermostat and will try to change.

  • Anonymous May 11, 2010

    don't really understand what your problem is. if it is stablizing under thermostat rating is it blowing cold air? if not I would guess you are right is assuming gage is reading bad..sending unit

  • Anonymous May 11, 2010

    have you checked you antifreeze for consenisty if strong or weak it might cause prob.

  • Anonymous May 11, 2010

    How is the heater is it hot? I have found that the factory gauges aren't very accurate. 



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Check instrument voltage stabilizer, also if alternator over charging will do it.

Posted on Jan 19, 2009


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