It would effecting mixture if it is stuck or sticking. It is easy to test the EGR for sticking: put a vacuum hose directly on the hose nipple of the EGR and suck a vacuum on the hose by mouth; you should see the EGR valve's stem move as your reach the right vacuum level. And the above test blocking the hose closes should close the EGR valve... IF it is working properly. If it is stuck partially or fully open, then blocking the hose won't change anything and the EGR will be leaning the mixture all the time; it should lean the mixture just under certain operating conditions.