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Discussion Starter #1
Hello all,

I'm struggling with an engine cooling issue. I have an '08 Tundra Double Cab SR5, 2WD, 5.7L, with about 85,000 miles on it. Bought it used, drove it 1200 miles home without issue. Everything seemed great with the truck, until I hooked up my 7,000lbs travel trailer. Once i got it up on the interstate, the engine temperature gauge began to move toward Hot. It never completely gets there, stopping a little more than 3/4 of the way. Note the transmission temperature gauge never moves, and stays at normal.

Further experimentation revealed that not only does this only happen when towing at interstate speeds (e.g. greater than 55 MPH) but only in 5th gear. When I put the transmission in S Mode and in 4 range the temperature drops back to normal. Air temperature was around 80 the first time, and in the low 60's subsequent times.

The dealer found a leaky water pump, and pointed to that as the culprit, and replaced it. However even with the new water pump in place, I had the same issue the next time I hooked up the trailer. The dealer now sounds a bit stumped, and is thinking the thermostat is the next likely culprit... maybe.

The puzzling thing seems to be that this only happens under towing conditions. That also makes it challenging for the dealer to verify its fixed. I've been searching the forums and haven't found anything that sounds similar. What else could it be?

Thanks in advance!
 

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Sounds like the thermostat possible partially not opening up all the way but enough to keep engine cool when not under load. I would have that changed out and it should of been looked at when changing out the pump and at least checked for leakage. I think it's the next logical step.
 

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Yep. Thermostat. AISIN THT005

About $29 on rock auto and $39 on Amazon. One piece housing and all. Little aggravating to change out. It would have been much easier if they'd done it when they changed the water pump.

Either that or you've got a restriction in the block or radiator. More rpms more water moving through the system. That is why you can down shift and temp will go down. That is also a good sign that you don't have an engine issue of some kind.
 

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check fan clutch also.
if cooling in 4th with higher revs but not in 5th or 6th then it could be too much slippage,although at 55mph ram air should be sufficient.
check coolant flow in radiator as well or for any obstruction in hoses.
 

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Discussion Starter #5
Thanks all. Since the thermostat seems to be the next logical (and easier) thing to try I'll start there next. I'll post the end result.
 

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Discussion Starter #6
Just to tie this thread off, last fall I changed the thermostat hoping that would do the trick, but had parked the trailer for the winter without a chance to test the change. Finally got it back out again a couple weeks ago, and the final result is the engine temperature is now right where its supposed to be regardless of speed, gear, uphill, or down. Sometimes the simplest solutions are the best. :)
 
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