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1997 Z28 M6. Fresh stock build on the engine. Drove it about 800 miles without issue. Then one day in traffic, it overheated. With the car running, we'd open the bleeder on the heater hose and nothing would come out. Assumed that meant the water pump wasn't pumping or there'd be pressure and it would push some out the hole.

Replaced the water pump and bled the system the same as when the engine was first installed. Opened the bleeders and filled it up, closing them as coolant came out. Started it up and let it run for about 2 minutes and checked the level. Repeated that a few times until the radiator was still full when the cap was opened.

Tried to let it go through one heat cycle to make sure everything was good, and it boils out half the coolant. Temp gauge only showed 225, which is just before the fans would normally kick on. Is there anything other than air in the system that can cause this to happen?
 

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Are you sure you hooked up all hoses to the right spots? I believe shoebox site has a diagram of where everything goes. www.shbox.com
#18 & 19 in diagram/drawing/exploded view section

I ask because I remember reading other threads with coolant boiling out. And some of those were caused by hoses not being in the correct location.

Did you hook up the steam pipes on the back of the heads? these are very important on the reverse cooled lt1 engine.
 

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Ethylene glycol and water mixed at 50/50 will boil at 226*f. If your radiator cap is bad (not allowing pressure to build to 15 psi) it will boil. A good radiator cap will allow the system pressure to increase to about 15 psi and increase the boiling point to 255*f. So you could check or replace your cap.

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