I've got a 97 4Runner with about 175,000 miles on it. Recently it has started overheating after driving for a while in the hot Texas sun. Basically, the temp gauge will run up to its usual spot and hold there for sometimes as long as 30-45 minutes. Then it will start to rise steadily all the way into the redline. Definitely hot as water flowing into the overflow. Had the thermostat replaced, but still happens.
My mechanic thought it sounded like a blown head gasket, but his tests on the cooling fluid didn't test positive for combustion gases. Also, if that's it, why would the gauge run up to its standard warm position and stay there for a while before deciding to then overheat?
Any ideas would be greatly appreciated.

 

Was it a toyota thermostat? Do a search on the thread, if not a toyota thermostat, then it may be the cause. Just a stab in the dark.

 

This really isn't much of a solution, but it is soooo hot right now in Texas (at least around San Antonio). I have never had cooling problems with my 4Runner until recently a few weeks ago when the temp gauge would start to creep up. I don't have any problems with city driving, but after about 45 min or so on the highway going around 75, the coolant gets real hot. As soon as I slow down or turn the heater on for a few minutes, it goes back to its normal position. When the sun goes down, it doesn't do it at all, so I attribute it to sheer ambient heat. Is this how your 4Runner is acting?

 

rpe it sounds like you have a head gasket problem... that is very typical of a leaking headgasket.

For the both of you I would suggest a leak down test. First see how much the stealer or your mechanic will do it for and if its more than $50 than just buy your own gauge set and then use someones air compressor, that way you will have it for future and friends vehicles. This will not only tell you if your head gasket is gone, but MANY other parameters as well (as far as compression goes).

Good luck

 

this just happened to my 95' 4Runner it turned out to be a small crack in the head gasket which allowed the coolant to flow into the sixth cylinder, be sure to get this taken care of soon if caused me to get new heads into my 4Runner. if your rig start idling rough and bouncing a bit it's to late stop the vehicl and get every thing checked before it gets worse. Hope this helped and gooc luck.