2005 GMC Sierra 5.3 heating up on hills at slow speed

[Until6855]

My 2005 GMC Sierra 1500 5.3 heats up when going up steep grade at slower speeds, usually 15-30mph. As soon as I stop the temp goes back down to just under the 210 degree mark on the gauge. I haven't pushed it enough to see if it actually goes fully to the high end of the temp gauge, it started doing this while we were out on vacation so I didn't think that was the time to really test it.  I noticed this while going offroad in 4 high and 4 low. The engine has 150k miles on it. Here are the parts I've replaced:

Full coolant flush with Valvoline Green Universal.

OEM Thermostat and Housing

OEM Water Pump

Serpentine Belt, Idle and Tensioner

It doesn't over heat at idle or on the highway. I got the truck from a family member and the radiator has been replaced at some point under 100k miles because of a front end deer collision. I've checked and the fans kick on just fine and seem to be pushing a decent amount of air. I'm guessing there was a good chance they were replaced when the radiator was replaced. Could this be normal?

[CamGTP]

Whens is the last time you changed the expansion tank cap? A bad cap can cause coolant temps swings too, it's worth a shot to replace it.

 

But low speed, high grade, high stress situations will cause a rise in temps. I'd only be concerned if you started seeing temps over 220-225 degrees. The gauge cluster isn't super accurate either, a live data scan tool is better to get the true coolant temp.

[Until6855]

Those are both good calls. I’ve been wanting an excuse to get a ScanGauge2 anyways. And I don’t know when the cap has been replaced, it’s likely the original. It’s a cheap cap anyways so might as well try it. Thanks!

 

Edit: Just went back and checked and I changed the Radiator cap out when I changed the coolant and water pump. 

Edited April 8, 2024 by Until6855