its overheating....help

[jared87350]

just put a rebuilt 350 in the truck and it is overheating.
i really dont know what it could be.
it has a new water pump,and i have gone through two diffrent thermos.
belts are tight and has great water flow.the rad. is not new , but in good condition. it has plenty of air flow with a mech. fan and two electric.
ran the engine without a thermo. and it did not overheat. it ran for hours at 160 at idle and 190 at low idle in gear.dont now what the hell is up. Spent a lot of money on this thing and this is pissing me off.
Its an HEI system and the base timing is at 12 advanced.

please help.

jared

[ak_nut]

So did you reinstall the thermostat?? Because you said it didnt overheat while the thermostat was absent.

[Mike_D]

i would first see if you have trapped air in the system, next I would look at the radiator. I would take a heat gun and shoot different spots on the radiator and see if it has an internal clog. If so you could have it re-cored/dipped

[raws67]

Is it boiling over or is it just the gauge reading hi? Ive seen before when it was the gauge going bad when it warmed up and started showing high.

[donut]

In addition to what eveybody else mentioned, The thermostat isn't upside down is it? (been there). Can you throw the thermostats in a pan of hot water and verify that they open? (I've had a couple recently that didn't)