I have solved this problem by removing the nut and allowing the coolant to
drain and then drying the surfaces and reapplying the nut sealant.  Don't
fill the nut as you will get hydraulic lock and not be able to get it all the
way on and then it won't want to come back off either.  Just put it liberally
around the edge of the nut and tighten back to spec with a torque wrench.
The dealer wants around $14 for a tube of this yellow stuff.  I sell it for
$9 or it comes with all of my engine gasket kits.

Thanks,
Ken Wilford
http://www.vanagain.com
John 3:16
Office (856)-765-1583
Shop (856)-327-0027
Fax   (856)-327-2242