I have had a similar prob with my 4Runner. Overheating on long uphill climbs (not necessarily steep) and/or after engine has been running at least an hour at highway speeds. Has happened twice while on the highway. Both times truck was not particularly overloaded or towing, but was full with luggage and 3 kids. Truck is 2001 with 102K miles.

Three separate mechanics (two Toyota dealers, one independent) have not found the problem. Did pressure test, which showed no leaks.

Last incident, it began to overheat while traveling to NY; luckily found nearby Toyota dealer; opened the hood and found overflow tank overflowing and boiling over but no actual leaks. In that instance, after pressure test, dealer speculated old radiator cap had some brittle rubber parts that fell down into radiator and blocked something, though he was only speculating; changed cap anyway and flushed system. Dealer also ruled out blown head gasket. We thought problem was resolved, got back on the road, and had gone about 30 miles when temp indicator started moving up near the red again. I slowed down, turned around and limped back home (about 75 miles).

The truck runs all day around town without a trace of overheating. Last week I purposely took it alone on a 90-minute drive up the highway to try to replicate overheating, but it didn't happen. However, I had no pax/luggage and the day wasn't very hot.

Radiator was replaced in 2016; coolant hose and thermostat less than a year ago; water pump reportedly replaced in '03. I am stumped, but i know very little about engines. The fact that three mechanics, including Toyota, can't figure it out, is more concerning. Sorry for the long post. Any thoughts?