exceeds the 'ambient' air pressure. Ambient in this case is the pressure inside the coolant overflow tank. About the only way this can happen is if the pressure cap isn't, the tank itself is leaking, or if there is another, higher pressure source of vapor (like combustion gases through a leaking head gasket!) that introduces gases into the coolant.

First case, check/test your coolant cap or if it's 5 years old, get a new one..current volvo color is green as I recall. New design vacuum and pressure valves, separate ones.

Second case, replace the expansion tank. Not a bad idea if it's ten or more years old, BTW. If it cracks/loses your coolant while running, instant/rapid overheat. Same with plastic radiators. Or not...wait until it cracks, it may not.

Last case; run an exhaust emission test on expansion tank vapors...HC in the coolant vapors is prima facie evidence of head gasket failure. May be able to confirm with cooling system pressure test and/or compression test.

/// There are several possibilities. I always suspect a pin hole in a heater hose. A leak at any coolant sensor will also cause a slow coolant loss and the system will boil over. After a small amount of coolant escapes the hole in a heater hose is above the water line and only steam escapes.

Fill the coolant system and run the car to operating temp. Park over a dry spot and turn the engine off. Look for wet spots.

The water pump has a weep hole but it is covered by the pulleys. Coolant will only drip from the back of those pulleys at pressure. A leak here indicates a failed seal in the water pump. The bearing will soon fail also.

A restricted coolant flow is also a possibility but you would notice an increase in the temp gauge. If the problem is a leak a longer drive will eventually cause it to boil while running.

Greg -- I'll take a stab at it:

Your pressure cap may be on the fritz, or maybe it's not sealing properly with your new overflow bottle. I'm suspecting that it's boiling because the coolant is at atmospheric pressure rather than elevated pressure.

I once drove a diesel VW Dasher across the country with an open coolant system. It was OK until I crossed the high country of Colorado. At high altitude, the coolant boiled at a lower temp, and climbing those mountains, the diesel put out a lot of heat. Needless to say, I boiled away a lot of coolant and it seemed I reloaded several times a day. Once I got to Kansas, the system worked OK.

Rob