Good diagnosis! Next time, use a little common sense- - - - -if you're not losing coolant out of the overflow, the coolant isn't boiling- - - -PERIOD, end of discussion! A pressure cap raises the boiling point 3 degrees F. for every pound of pressure on the system, so a 15# cap raises the boiling point 45 degrees, to 247 degrees F. at sea level. If you're losing coolant, something, somewhere in the system is either getting WAY too hot, or there's a compression/combustion gas leak that's overcoming the pressure spring in the cap. A system that will go 10 miles or more, regardless of speed or outside temperature, without puking coolant is not boiling, or getting a major pressure leak, regardless of what the temp gauge is reading.
The most common cause for high coolant temperature that's not gauge-related is a dirty radiator, or one that doesn't have enough cooling ability to handle the size of the engine. Fan shrouds and other airflow enhancers just make a marginal radiator work at the upper limits of its capacity without a boil-over. The solution is more cooling capacity,with a clean, correct-size radiator.
Jerry
