I drive a 1997 SS, with a T56 and had roughly 138k (85750 miles)
Currently my car is on jack stands.... Once my temperature heats up a little over 100 celsius (210 Fahrenheit) coolant begins to spill very rapidly over my coolant resevior stem. Though the car does not overheat. Let me includes that a previous owner for some reason or another chopped this stem down so it only extends rougly 3" over the bottom of the battery and a cap will not fit onto it.
For the past few weeks I've been losing mass amount of coolant, though It hasn't appeared to be leaking while I'm driving or parked. After I top up the coolant the car drives normaly and doesn't overheat until a couple weeks later when the coolant level drasically drops.
There is a new water pump, thermostat, rad cap, upper, and lower rad hoses.
I did a home flush today of the rad using a kleenflo 10 minute rad flush I bought from Lordco, and then flushed with water through the top of the waterpump and the rad until clear water ran through the drain plug. (Took about 20 gallons of water before it ran clean! very lengthly process)
I asked an mechanic expericanced with reverse flow systems and he asked if the water pump had been changed recently, and said that there are water pumps that will bolt onto the car that will pump in the opposite direction. However I cross refrenced with different parts stores and they all told me I had the correct water pump. He also told me to check the thermostat and rad cap.
I tested the thermostat even though I just replaced it within the past month, and it opened abrubtly in boiling water. I also ran the system without the thermostat and coolant still erupted out the resevor stem.
When the coolant level in the rad is low and the car is running I can see that there is coolant draining through the hole which is connected to the hose running from the throttle body.
Hot air blows through my air vents, there is no smell of coolant in the car, and the passenger side carpet is not damp.
The oil and exhaust fumes look normal.
I bled over both bleeder screws during the filling process until solid coolant poured out.
I have not yet pressure tested the system, but a friend will be coming over with a pressure tester.

Hopefully I didn't forget any details, to anyone who is still reading I'm looking for any other possible causes for this that anyone can think of any possible solutions to fix my problem.

 

Well try posting in the LT1 section and you might get better results. Are you sure your fans are tuning on? Lt1 PCMs are notorious for having the fan ON/OFF PCM setting just dissapear. Happened to my car, fans just stopped turning on one day, had to keep MAF unhooked to keep the pcm in limp mode to turn the fans on until i got the pcm reflashed.

Have you tried a new engine coolant temp sensor? There are two on the engine, one by the water pump and one in of the heads if i remember right. The one in the head is your gauge temp sensor, it could be bad. The only thing that could be causing you to lose coolant through the overflow is a blown head gasket otherwise. The force of the combustion process will pressurize the cooling system and force coolant out. I know this because it happened on my Duramax. I also had intermitant heat. You can do a compression test to see.