Doing that will not affect circulation, but depending on how "half opened" you cap is, as s60 stated, you will probably lose coolant w/o pressure in the system. if you car is overheating, try and find/repair/fix the problem rather than tolerate it with a "half" baked solution.....

__________________
Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.