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I live here in AZ and drive through the salt river canyon a few times a year towing my trailer witch is only about 3000 lbs. I case you don't know the salt river canyon is a very long and very steep mountainous curved road. When it is over like 90 degrees outside my transmission easily gets up to 230 degrees then I make sure to let off to cool down. When I say let off I mean like 15-20 mph. Also on the really steep straight grades my engine will easily get up to 230 or so. And that is not really accelerating that much just trying to make it up the hill. Has anyone else had this problem or know of any solutions I have a 2009 GMC Sierra 2wd crew cab with the 5.3 and a 6 speed an 3.42 rear end and 285/70/17. I am contemplating a blackbear tune, transmission and/or engine oil cooler, also maybe 4.10 rear end. Any advise please help.

I have also noticed the same thing driving my mother inlaws 09 GMC yukon with 5.3, 6 speed and 3.42 rear end but stock tires and it does the same thing. It actually got as high as 260 at one point.
 

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Your plans sound good to fix it. I saw my trans temps as high as 212 one day driving around the city on a hot day, and I installed a Tru Cool 40K cooler and I don't think I've seen it over 145 since. The oil cooler would keep that cool, and the gears would help you accelerate easier. In a tune, I believe the tuner could have the fans come on sooner as well.
 
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