Why is my truck overheating?

So my truck overheated two weeks ago, took it to the mechanic and he thought it was the thermostat so he replaced it. This past weekend, after driving it about 300 miles in hot weather with the A/C on it did it again. I open the hood and the coolant resavior was boiling over and spilling antifreeze on the ground. I let it sit, topped the resavior off with water, and was able to finish my trip by not using the A/C the rest of the way and cranking the heat full blast, not very comfortable. (The resavior was bone dry by the time I got back but when I open my radiator cap, the coolant was full). Thought it might have been the fan but that works no problem. My truck is a '99 with about 70K so it shouldn't be anything major, but why would it be overheating with new thermostat, radiator in good condition, and a fan that works?

is your taco a 4 or 6cyl.?

the fours tended to run a little hotter.
did the mechanic flush the system when he did the t-stat?
don't just assume the radiator is good. you said 70K, and the '99 had the DexCool didn't it?
check the antifreeze with a tester. a 50/50 mix should be fine for southern states (and even northern if it's a four cyl), and also, if the fan clutch is slowly losing its clutch, it will still spin, but not pull as much air as new.
the boiling over says blockage to me, but if it was overheating while running, it could be fan clutch, or possibly the water pump bearing starting to give (this usually starts a squeeling or the weep hole leaking though), but check antifreeze first, then radiator and hoses.

post back or pm me if needed,
good luck,

chris
nrt racing

Its a 4 cyl and the mechanic did flush the system when he did the thermostat. Thanks for the info, I'll bring up the suggestions when I bring it in.

Its a 4 cyl and the mechanic did flush the system when he did the thermostat. Thanks for the info, I'll bring up the suggestions when I bring it in.



a common mistake people make is failing to replace the rad cap...on some Toyota's they will weaken over time....and they are critical to keeping your cooling system performing properly....and best part, they are cheap.

Check to see if the radiator is uniformly hot or are there cool spots - indicating blockage.

I ran into a simular problem with my 94 Mitsubishi Galant it had about 74K on the engine, the mechanic I spoke with told me next time I startup warm the car up to what the temp should be on the dash guage stop the engine remove the radiator cap if you hear a pop You need a new water pump, well it did, I replaced the pump and it ran fine. Don't ask me about this pop thing I don't know.

Well that sounds like the radiator cap. If its spilling too much into the resevoir then the radiator cap is letting too much coolant pass it and into the resevoir. The radiator cap not only applies pressure to the cooling system but it will regulate the coolant into the resevoir when it gets too hot. But if the cap is bad it will let it go into the resevoir no matter what.