The service manual is specific for this task, many have tried to do things "their way" and it resulted in similar symptoms to yours. Jacking up the car is not going to do anything to get the air out of the system. I would verify this first....I just don't like to assume anything.

Good luck

 

I know on the old 69 SS I had this and that is what I thought it was at first. I guess Maybe it is worth the cash to purge the system huh.

 

Really? What are the gotchas for refilling the C5's cooling system? I swapped out my water pump awhile back, and I didn't do anything special. Normally I just keep topping it off with the engine off until it stops bubbling while squeezing and shaking the hoses. Drive around with the heater on until the thermostat opens up, park, (over)fill the reservoir, checked it again in the morning and was gtg. Haven't had a cooling problem yet, and I've been driving in 100+°F Florida days.

I'm not saying you're mistaken btw. Just curious to know if I did something wrong or just got lucky.

 

Maybe I am mistaken.....I just don't know a lot about these C5's...perhaps someone with mechanical knowledge and experience will speak up.

 

Here is what Lucky was refering to. SIMPLE, and it works. BUT, he don't know anything about C5's??????
---------------------------------------------------------
Fill the cooling system through the surge tank.
Fill the cooling system with a 50/50 mixture of DEX-COOL® coolant and deionized water.
Start the engine.
Allow the engine to idle for 1 minute.
Install surge tank cap.
Cycle the engine RPMs from idle to 3000 in 30 second intervals until the coolant temperature reaches 99°C (210°F).
Shut off the engine.
Remove the surge tank cap.
Start the engine.
Allow the engine to Idle for 1 minute. Fill the surge tank to 12.7 mm (0.5 in) above the COLD FULL mark on the surge tank.
Install the surge tank cap.
Cycle the engine RPMs from idle to 3000 in 30 second intervals until the coolant reaches 99°C (210°F).
Shut off the engine.
Remove the surge tank cap.
Top off the coolant as necessary, 12.7 mm (0.5 in) above FULL COLD mark on the surge tank.
Rinse away any excess coolant from the engine and the engine compartment.
Inspect the concentration of the engine coolant.
Install the surge tank cap.

 

His avatar would suggest otherwise assuming that's his car.

Gotcha, no doubt that'll do the trick, but I'd think as long as there's nothing weird like opening purge valves or plugs then the air is going to work out over the course of normal driving anyway. Just keep the reservoir tank topped off and all is well.

 

MYSTERY SOLVED!
The hard way.
Well drained the rad again and followed the steps to filling it so you dont get air in the system. Took it for a test drive and it all seemed good. But the morning was a cool 76 degrees. Went down to a friend of mine to pressure test the system and he didnt have time he said take it out on I 55 and see it the temp hold. When I got on the interstate thats when it happened. I punched it to get off the ramp and out the back I see a large cloud of whit smoke. I just new I blew a head gasket. I hit the first U turn I could find and limped it back to a station off the highway. Opened the hood and there was anti freeze all over the place and my belt was shredding. But I couldn’t see anything wrong. I bought a gal of Ant Freeze put it in and limped it back with a loose rad over flow cap to my friends station. Opened the hood and he looked down to see a hose lying loose. Turns out it was the hose from the intake coolant line to the TB. Seems that if the clamp was there it wasn’t tightened so it never pressurized and it would be sucking air. Since I haven’t drove it much since the install it never had a chance to pop off I had noticed a little ant freeze on the garage floor from time to time but thought it was from the water pump going bad. Put a new belt and hose clamp on cleaned it for 3 hours and now it never goes above 223 degrees. What a relief was thinking maybe I did put the head gaskets on wrong after all. Thanks for all the input

 

Always helps to have the whole story....I'm glad I'm not your doctor..

 

Glad you have it fixed!