I changed the coolant on my FD this past weekend. I took a couple of drives with refilling before the system was completely burped. I went to check the coolant level this morning (mid-50 degrees F) and when I removed the radiator cap on the filler neck it made a very distinct pop sound very much like the sound that occurs when you open a jar for the very first time. The coolant was about 1/2" from the top of filler neck. I know that the system is under positive pressure when the car is running, but after the car cools off (and sits overnight with a temp difference of at least -10 degrees F), does the drop in temperature cause the system to pull a slight vacuum?

 

The cooling system is a sealed system. When pressure gets to the rating on the cap (around 14psi) excess pressure is pushed into the overflow tank. As the car cools down and contracts, it will pull excess coolant from the overflow tank back into the system.

So, yes, what you're seeing is normal and good.

Dale

 

somewhat, a failed vacuum spring in the cap will not allow the coolant to be drawn in from the overflow tank. some of the aftermarket silicone sealed caps i have noticed have too much return spring pressure in that respect.

but if the hoses aren't collapsing and the hose to the overflow tank isn't obstructed i would say it is fine, it takes some vacuum before that valve can open so some suction is going to be present when cooling down.

pull on the dongle in the center of the AST cap to make sure it isn't stuck closed or cooked to the rubber seal.