I have gone through all of the threads in the forum and cannot find anything to explain the symptoms shown by my car. It is or was in good repair with 200k miles in the relatively decent climate of Houston TX. Overheating problems began with slight losses in coolant; the leak for which could not be located until the tank end of the radiator (passenger side) failed and then it was easy to locate. A shop replaced the radiator and then a different type of overheating began, again with loss of coolant, and eventually loss of heat from heater. Read here about expansion tank caps going bad and it was replaced - no change. Again here, read about checking water pump by removing heater hoses and while there is flow through the hose that attaches to the "inlet" closest to the driver's side, I have no idea of the amount of flow to be expected by a properly function water pump - going with the assumption that flow at idle suggests that water pump is functioning. The thermostat opens between 90-92 C. The cooling fan cycles on and off in seemingly good concert with overheating. There are no leaks that I can detect. Compression is even across the 6 cylinders - so have ruled out head gasket (no water in exhaust or oil). Pressure test of cooling system shows that system holds pressure. Heat core is no leaking in the cabin and I see no evidence of leaks in the engine bay.

Here are the symptoms as of today:
1. Steam issues from under the cap on the expansion tank when car is driven less than a mile or 6/10 km (the cap is less than 1 month old). Is this common and the only route for excess pressure to escape the system? OR does it mean that the expansion tank needs replacement. Volvo dealer has been spectacularly unhelpful with this and other questions.
2. Car overheats in less than 300-500 m with AC on.

Even though the new radiator is less than 6 months old, it (besides the expansion tank) appears to the be culprit and in need of removal and cleaning. I am at a loss regarding the extraordinarily short time it takes for overheating. I am asking for any thoughts or advice on the cooling system. Is there something I have overlooked? Thanks for taking the time to read and respond.

 

Heat the engine / pull the spark plugs / pressurize the cooling system with a pressure tester and let the engine cool down. When all is cool, place a piece of cardboard over the spark plug holes and disable the ignition. Crank the engine a few times. If you see coolant on the cardboard then you have an issue where coolant is getting into the cylinders somehow (probably head gasket or warped head).