My buddy has an 04 Suburban that's a beautiful truck, he's had it for about 2 years now. Was having overheating issues really bad that would shut his truck off constantly, AC wouldn't work, and then it wouldnt start, so he had it sit for 8 months until I came back to town to fix it.

I spent all day replacing his water pump and gaskets (they were shot), his fuel pump (car wouldn't start because of it), thermostat housing, gasket, and thermostat, AC belt and his Serpentine belt (both were cracking pretty bad), and cleaned the mass airflow sensor.

Truck starts and runs great now! Works better than ever, throws no codes, and purrs like a kitten. However, when we were sitting in about 90° in idle in a parking lot with the AC on, it only took a few minutes before the temp started slowly rising up to about 3/4 of the way up before we turned the truck off, to immediately drop back down to normal temp.

Any ideas as to what that could be? I'm at a loss..

 

Hmmm - running hot/overheating on highway is typically indicative of bad radiator, engine issues. But "90° in idle in a parking lot with the AC on" is really pointing to an air flow issue. If the 2004 has a belt-driven fan, I'd replace the fan clutch. If the 2004 has electric fans, I'd make sure that both were operational when the AC is switched on. Of course, this all assumes that the fan shrouding is all in place and that all the air was bled out of the cooling system.

 

If the fans check out then it's radiator time. Especially if the water pump impeller had parts of it missing.