su/kdesu not working

[Dj YB]

I am sorry to have to post this problem to the list, but I got no answer in 
the forum, and couldn't find one elsewere.

I can login as root.
when trying to su to root using "su" or "su -" i get "su: incorrect password"
following some threads I added my user to the "wheel" group and tried to edit 
/etc/sudoers using visudo with no luck on both.
also tried to edit "/etc/pam.d/su" again with no luck.
I really need a solution since many operations can't be done without su 
normally.
thanks alot
YB.