new 350 rebuild running hot!!

I rebuilt a sbc 350 in my new to me pick up last fall. Rough weather hit a few days after the install and I had to wait for spring to finish all the little odds and ends.......I have found that the coolant is leaking at the hose spigot above the thermostat and the temp gauge is freakin out around 255-260F if it is accurate. My theory is that maybe the thermostat is no opening and the coolant is building pressure and finding its way out....Other than this it seems to be ok, knock sensors and all. Anyone have any input?

I hope your temperature gauge is NOT accurate. At those temperatures if it is accurate, you've done some damage or will have if you ran it to long. your guess on the thermostat is probably right. No water getting into your engine to cool it. A quick test is to feel your water hose from your radiator and if it's the thermostat, you'll feel the hose turing warmer as the thermostat opens up to whatever its set at.

hose feels HOOOOOT! thermostat is opening and coolant is circulating?
Ignition was really advanced.....reset to factory specs with the extra wire disconnected as per the manual. it is a new stant thermostat and I can replace it fairly easy. Engine has only run for 3 10 minute spurts, total of thirty minutes with fans on it entire time. Not ready to road test until I know everything is decent, could the lack of moderate airflow be causing issues?
Engine oil is heavy for air temps and normal operations, could be causing problems. I will have to make some adjustments to establish a decent baseline for a good diagnosis.

I hope your coolant temp isnt 250 degrees, thats way too hot.

i know that max is around 220F.....I am in the process of making some "given" adjustments. I have tested the thermostat in water and it opens at 180F which is 15F cooler than factory. I will replace the radiator hose that is leaking to rule out a $15 fault. I am changing the oil and putting in 10W30 as the ambient air temps are around 60F here. I have also verified the ignition is firing on time and correctly. I have a lot of new parts in this motor and I am trying to keep them cool while I verify that they are all working correctly. My main concern is that everything here has happened while idling and limited, controlled engine rev cycles. This is my first complete chevy rebuild and I have been very meticulous. I really just want this to end up a really dependable truck.

What are you using for fans? Do you have the fan installed on the motor? Do you have a fan shroud installed?

I had a pair of 36" industrial fans in front of the bumper about 3' away so I could walk in between to inspect things. The engine fan, mechanical version, is operating as well. There were no shrouds installed, would that cause this big of an issue? I will definitely make sure they are in place for the next trial to rule out extraneous issues.

Excessive timing advance will lead to overheating .

With the radiator cap off, warm the engine and verify that the coolant is circulating inside the radiator. I keep a bi-mettalic thermometer with a 12" stem for verifying such things. You can probably find one at your local grocery store or cooking store. Just make sure the range is well over what you want to measure because the dial types are typically not accurate below 20% or above 80% of the scale.

http://www.ertco.com/bimetallic.html

Engine fans don't work very well without a shroud.

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Originally Posted by threearmsinjune

I rebuilt a sbc 350 in my new to me pick up last fall. Rough weather hit a few days after the install and I had to wait for spring to finish all the little odds and ends.......I have found that the coolant is leaking at the hose spigot above the thermostat and the temp gauge is freakin out around 255-260F if it is accurate. My theory is that maybe the thermostat is no opening and the coolant is building pressure and finding its way out....Other than this it seems to be ok, knock sensors and all. Anyone have any input?

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If it has been at 255 -260 very many times your rings will have lost their tension.
In other words you loss your seal you lose hp and you will start to use oil.

did you put on a serpentine belt? if so, you need a new fan. Serpentine runs the fan backwards.

And the water pump.

I, for one, have installed a thermostat upside-down. Make sure the pellet and spring go toward the motor, not the radiator.

My guess is its the water pump. Depending on the setup, you may have installed an incorrect water pump. Try replacing the water pump with a new one. If you are running a serpentine setup use a reverse rotation. Heck it might not have even been your fault. I have gotten incorrect water pumps brand new from fairly big name stores(and catalogs). There were some years of trucks where they came with both serpentine and v belt setups. The part numbers might be close enough to make the mistake.