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I share your worry about it being the head gasket again. Happened to me once where a head gasket went a second time in under 15K. One of my trusted indies did the work and like you, knew what he was doing and did everything by the book. We could only guess at why it failed again so soon.
Try a simple test. Remove the rad cap (okay, expansion tank cap), start the engine and fully warm it up at idle. When the thermostat opens and the level starts rising, if it rises rapidly and looks it may start bubbling then suspect the head gasket again. A B230F should be able to idle almost forever with the rad cap off.
When there's over pressure from whatever cause, I always expect the rad cap to be able to release all the excess pressure as it's supposed to do. In the real world there are often other weak links in the chain that get in on the act. Some are small and subtle like a weeping water pump top rubber gasket (a probem I currently have that I just discovered yesterday) or the water pump itself. Often the next weakest link is the heater valve (esp. the original Volvo valves). Then there's the major nuisance stuff, like the heater core and heater hoses, before getting up into the more serious stuff like the expansion tank, rad and rad hoses.
If things aren't too bad, maybe it's a failing thermostat allowing the engine to get too hot:
o Has the dash temp needle been noticeably going above the mid-point closer to the red zone before it drops down?
o Has the rad fan been noticeably running on during use or just after shutdown?
Short of having it properly tested, putting in a new rad cap is probably a good idea. With my B230FT, the turbo can make for a hotter and harsher environment under the hood, which is likely harder on cooling system components, which is why I immediately switched it to the lower pressure black cap (75 kPa, 11 psi) which were stock on many B230F/FT engines in 240/740s. If the pressure starts rising for some reason then I'm hoping the black cap will do a better job of releasing excess pressure without overstressing other components. The black cap lower system pressure makes no noticeable difference in operation, although likely makes a tiny difference in engine efficiency (to keep EPA happy). In our B230F, on the other hand, I'm still running the higher pressure stock green cap (same 150 kPa, 22 psi as the grey cap, but supposedly a better design that lasts longer). If I ever replace the cap, I'll probably switch to the black cap as well.
Also thinking along the lines of running too hot, any idea which thermostat temp is in the car? If the dash temp needle is always running on the high side of being straight up and down then it may well be a higher thermostat. The thermostat specs for B230F/FTs have long been an item of debate among Volvo types and even varies in Volvo literature. A lower 89 deg (87-90) thermostat might be a bit friendlier to your cooling system than a higher 92 deg (91-93) one.
Has the cooling system had a good flush in living memory? I can't imagine that being a problem with one of your cars. If the rad cores are badly scaled up you might be running too hot because of that.
And lastly, don't forget to pray to the Volvo gods every time you put the key in the ignition. You may not be religious, but I'm convinced our bricks need all the help they can get to keep the devil out of them.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now
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