Nope, doesn't erase anything, here's how you do it.
Turn the phone off....hold down the power and both volume buttons at the same time for a few seconds, a menu will appear....use the bottom volume button to scroll down to "recovery"....use the up volume button to select recovery....there will be a pause....when the android appears on the screen...push both volume buttons at the same time, another menu will appear...use the down volume button to scroll down to "clear cache partition"...use the power button to select it...it will format the cache...when its done....push the power button to restart the phone....and you're done.