'88 Caravan, water lost thro overflow, wild bubbles/boiling water ?!?!?!?

Mom wants her car back
Got new radiator cap. Flushed (I thought) radiator, filled & let set for 2 days. No leaks. When started, the water in the overflow bottle started running out. After water level dropped below overflow, (5-10 min) just blew bubbles, then water started really bubbling, almost erupting, water bubbled harder. Like water boiling inside bottle. Where's the air coming from ? What's going on?
'88 Caravan. - - - - - - Autogirl

Is this the first start up after you flushed and filled? When you start up a car once the engine is flushed, you cannot fill up all the high spots with new antifreeze/water mixture. You have to start the engine and allow it to run to work all the air out of the system. The easiest for you is start the engine with the radiator cap off. DO NOT add any more mixture until the engine comes up to running temp and the thermostat opens to allow circulation. The level of fluid will rise and if it threatens to overflow, put on the cap. The upper radiator hose will get hot when the fluid starts to circulate and starts drawing out of the overflow. Once the air is worked out you won't have the bubbling in the overflow.
Did you flush it because it overheated? The thermostat possibly needs to be changed. Replace the thermostat with the recommended temp. stat bacause when a vehicle overheats the stat is bad or will go bad. Once the level in the radiator drops, all you'll get in the overflow bottle is steam to blow those bubbles. If it has been changed, you may have to have the tradiator flow tested to see if it is stopped up. This won't allow propper flow and will cause the same thing again.