Appears to run better is more accurate, you probably have a clogged pcv
valve (or crimped or leaky plumbing, clogged intake, etc.) and the
release in crankcase pressure obtained by removing the cap helps the
Pull the pcv out of the grommet and see if it rattles, if not replace
it. Check the grommet, is it a snug fit? If not replace. Check the
pcv lines to the throttle body are they cracked or clogged? If so
replace. All of these parts will cost you a buck or two individually.
Pull the pcv line off of the throttle body, are you getting vacuum?
If not, it is clogged and needs to be cleaned.
My guess anyway...