question.... was burping some air from my radiator since i had done a thermostat change a while back. anyways i raised the front end up so that the front of the engine is higher and removed the radiator cap after a few minutes coolant spewed from the filler neck, actually it did that twice before i put the cap back on. I kept a check and all the time the heat gauge stayed in the middle and you can tell when the t-stat opened because the gauge would drop to just a little below halfway on the gauge. I have never had to add any coolant to my system nor does it overflow from the recovery bottle.(bottle is always on the full mark/line)... does this sound like the radiator needs flushing ? I did a dry compression test and all the plugs looked normal...this was the results:
#1 = 169
#2= 168
#3= 169
#4= 169

highway

 

The coolant gets hot and may be above boiling at various points in the motor.
This heated/boiling coolant is going to generate some pressure. You'll get bubbles and eventually some volume expansion in your cooling system.

It's normal for it to overflow if you don't cap it and let it get hot.
Worry when you let it sit and it builds enough pressure to push out past the cap into the coolant tank.