99 STS weird overheating problem

[Blenderhead]

OK, at the shop we got a 99 cadillac STS in, that has a weird problem. It overheats after almost exactly 30 mins at 2500rpm/highway speed. You can drive the car around town all day with no problems. You can leave it idling all day with no problems. There is no smoke or misfire or anything like that. You can take the car out and flog the hell out of it, and it won't overheat. Leave it at a steady 2500rpms, and it will overheat after almost exactly 30 mins like clockwork. When it overheats, the fans are working fine. Another shop already replaced the thermostat and did a coolant flush. Also on wierd thing is when it overheats, the message center says "battery not charging", but that goes away as soon as it cools back down, so I don't know if that's just the alternator getting too hot when the engine overheats.

Anyway, has anyone ever had something like this? It's a 4.6 Northstar by the way.

 

[lhheston]

Wow, that's as weird as my AC blower quitting, then running for a few days, then quitting (while the wife is out in our Blackasnight '99 STS in the middle of a 95degree day). I would guess belt tensioner, but the water pump is on the left, and the alternator is on the right, so they run on different belts; however, the alternator is water cooled (!!) and if it heats up, as you noted, it may not charge as well. If you have any small leaks around the water pump, it could easily be sucking enough air at the constant speed to prevent good cooling, which would not be obvious other times. The only other thing I have heard about northstar overheating is myriad posts about head gaskets leaking the charge into the water, causing lots of bubbles and superheated coolant in the overflow tank, but never showing any oil. I haven't had that problem (knocking on LOTS of wood).
Best of luck with it!