Dropping Coolant While Driving

[philipjayadler]

Well I was driving yesterday when the coolant light began to flash, temp hit 200, so I went home and filled the coolant, which was really low. When looking through the fill, there wasn't any coolant all the way down. I put in about 3 quarts [ capacity 2.5 gal.]
which was a lot of loss. But There wasn't any leaking. [?] The level stayed steady at the 1/2 min-max mark.

Driving today, about 10 miles back and forth, not only did the light come on but the temperature shot up. I pulled into the driveway just as it hit 250.

No check engine light, no other diagnostic light, just the flashing coolant light.
[meter/check codes?]

When I got out of the car, I could hear a loud whine from the trunk.
Opened it to find that the coolant cap was venting hot coolant steam, and of course the loud and unnerving hissing-whine, from the cap.

After a few moments it stopped. Waited till it all cooled down and slowly and carefully removed the cap. Same issue as above. No visible coolant in fill. I again put about 2 quarts, level hit between min-max.

No leaks under car, coolant level hold steady.

The mix is 1 gal. Porsche coolant [bought though Pelican], and 1 gal. distilled water.
Shaken, not stirred to mix. Let sit. Yes, nice and pink when poured.

The coolant tank is brand new, perhaps has a thousand miles on it. Cap came with the tank. Porsche, not OEM. Put on by a reputable shop which specializes in German cars. I screwed that cap down properly, real tight.

No overflow. No leaks. I'm loosing coolant when driving, only.

Any ideas?

Obviously I can't drive it. Next steps is to load it on a flatbed, and drop it off at the shop.

{Poor Boxie}

Thanks, as always!