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Engine running too hot only under load
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!