Where is my coolant going??

[AZthumper758]

Need a little help,

68 Camaro Convertible
383, with Victor Jr Aluminum heads.
Aluminum radiator (don't know the brand), vented cap, with overflow tank.

The car runs 170-180 down the freeway or normal driving and typically heats up to the 200-210 range when stuck in traffic or long stop lights but always cools right back down. If its in the garage idling with no load it'll sit around 205 all day.

Here's the issue, I am consistently adding coolant to the radiator. The level will drop 3-4 inches which each long drive or afternoon out. After a 400 mile run from California, the car never got about 180 but the coolant was a good 1/2 gallon low when I got home.

There is no evidence of leaks from the water pump.
The heater outlets are plugged so no heater hoses or core is in play.
I had the radiator and motor pressure tested by a reputable shop when I first got the car and noticed the issue and they said it was fine except for a slightly loose lower hose clamp which they tightened. Head gaskets, everything else was good.
Switching to the vented cap did help, the coolant loss seems slightly less since I switched it.



Any ideas? The car runs great and has no real issues other than this.

Thanks