98 K1500 Vortec 350 occasionally overheating

[James09]

I have a 98 K1500 with a Vortec 350 and it is driving me crazy. I can drive it 3 or 4 times and it will be fine and then the next time I drive it and the thing will overheat. I have changed cap and thermostat. My brother pulled the intake and the gaskets looked great but changed them all while he had it off. THe wierd part is that it is so inconsistent. One time it will do it at an idle and the next time it won't. Sometimes it will overheat when you drive it hard and sometimes it wont and driven easy it will and the nothing next time and so on. Never under the same load or the same situations. I have asked so many people and nobody knows. I just know it runs very smooth and has great power. It only seems to loose a little antifreeze when it does the ocassional overheating. No antifreeze in the oil. If you let it run the needle will climb to 260 and the heat will go cold but everything under the hood is cool and the top hose gets really hard with pressure and then about a minute later it will return to normal and never do it the rest of the day. And when the guage reads 260 it runs just as smooth and still plenty of power. I am just frustrated. Help! thanks

[jonnik]

Sounds like you may have a flakey temperature sending unit or dash gauge. Also, when it shows hot, does your fan clutch engage properly? When it shows hot, shut the engine off and try turning the fan by hand. It should be fairly tight.

[dewaynep]

Sounds like you have a restriction, possibly in the radiator. I would try to backflush the radiator and heater core.

[J-Ri]

Is it using coolant and is the coolant full? The heat going cold means that there's no coolant flowing through the heater core, either it's not flowing, or it's not there. A leaking head gasket can let air/exhaust gas into the cooling system, and the pockets of air can cause overheating.

[cposey]

I have a 97 gmc with 350 vortec that is overheating also its not leaking from the water pump but on top the enging above the water pump its not the thermostat ir the hose i cant find it no where i can here it come out but cant find it

[Chevy-SS]

Get one of those hand-held infra-red temp sensors and check the radiator in a few spots at 'normal' temp, then when it starts overheating, measure the same spots on the radiator to see if it really is overheating. Obviously, if the temps are still consistent with 'normal' readings, then it's likely a bad gauge or sending unit.

If it is indeed overheating, then I would do as others suggest. First, clean all debris from the radiator, and also AC condenser (if that's in front of radiator). Blow radiator out with strong blasts of compressed air. Then check the clutch fan to make sure it's moving a lot of air. If that seems OK, then I would do a thorough flush on the cooling system.

If none of the above work, then it's pretty much got to be the water pump.

Keep us posted.