Can the line from under the manifold be removed and plugged at the heater hose with no ill effect or will coolant run from the exposed port in the lower manifold? I cannot see where the coolant goes to, does it just sit in the manifold with no flow?

 

I think that is the return for the heater core.

 

If you are talking about the 3/8" or so line from the tee in the heater hose, it will leak if disonnected and left unplugged.

Think about it for a second. If it were to fail to leak, then where would the coolant be flowing? It could only come & go thru the hose. Not likely. It has to flow thru somewhere or it is useless.
tom