dribble1 wrote: 1. Radiator place changed the radiator cap today from a 15lb (it was really tight) to a 13lb - is this a big deal?
Nope, that's what you're supposed to have. The 15 pounder would have been too tight as you have found 
dribble1 wrote: 2. - Radiator has had big clean out (she was 40% blcoked) + new thermostat. However, I drove her home today (before the new radiator cap) and when I turned her off I could hear her "bubbling" - temp gauge still just below half (normal for her) . Should I be worried and would the cap have fixed this issue?
The stronger spring in the 15lb cap would have allowed greater pressure in the cooling system before venting steam, which raises the boiling point of the coolant. With a 13lb cap the boiling point is lower, so the coolant is more likely to boil than with a 15lb cap. That said, if the radiator, pump and thermostat are in good condition then you're not going to come close to boiling the coolant.
I tend to find that when I turn the engine off, I can hear bubbling in the radiator (and if the coolant level is too high, it gets spat out of the overflow). This is normal -- when the engine stops, the cylinders are still hot but the pump is no longer circulating the coolant. This allows the coolant to flash to steam around the hot spots, which expands and pushes more coolant into the radiator. This is the bubbling you can hear.
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