AST/reservoir hose and thermostat on order!

Just had something weird happen. I noticed when I was topping off the coolant before driving that the level kept sinking in the filler neck, and everytime I would add a few more ounces, it seemed to be filling the overflow reservoir. Thinking the AST cap must not be sealing, I swapped my brand new OEM AST cap (which I had recently replaced; ordered direct from Ray Crowe) for my old OEM AST cap, and was able to fill the filler neck as normal without it bleeding off into the overflow. Now, I'm not convinced this was the source of my coolant overflow problem since I had this issue even with the old AST cap (that's why I originally replaced it), but it seems the new OEM AST cap perhaps wasn't functioning correctly.

 

Have you looked carefully inside the neck of the AST to see if it is cracked, deformed or otherwise messed up?

 

I have looked at it and didn't see anything that jumped out, I will inspect it more closely though when I return from my trip. Will update this thread as well... thank you to everyone who has helped diagnose and get me pointed in the right direction.

 

Any small irregularity on the sealing surface can cause the cap seal to not seat properly, so look very closely at it. I've had that sort of thing (pitting on my aluminum coolant swirl tank cap seat on my racecar) which caused similar issues.

 

Just chiming in here... I had this same problem recently. Overflowing at the overflow tank, but no visible leaks. Dry plugs and no evidence of a bad coolant seal either. My car routinely held only 12psi of pressure in the coolant system, even if I pressurized it above that mark. It turned out to be corrosion on the water pump housing inlet. I cleaned the corrosion off with sandpaper and scotchbright and retightened everything and wound up with a system that holds pressure. It didn't leak much, and it never made it to the ground. It was caught in the pockets of the underbody tray and would evaporate quickly. If you're still loosing pressure, look very carefully around there. My car is also pretty low on miles

 

This is another case of anything under vacuum not sealing perfectly in the cooling system (suction side of the WP in this case) causing air entrapment issues.

 

Wow. Very rare and uncommon failure.

 

So wanted to update this thread, I have not gotten a chance to change out the reservoir line or thermostat, but I'm starting to think the issue was just the new AST cap.

I had originally ordered a new AST cap when this issue first started, and it doesn't seem to fit the original AST like it is supposed to - it is very tight. I don't think the issue is completely solved, but I do think the overheating and vigorous boiling that started more recently was caused by the fit of the new AST cap, as it hasn't done it since putting the old one back on. Still need to replace the line and thermostat.