14.0 Detroit 2005 engine

Please help! Can you water test a N3 injector cup without injectors in place?

 

Coolant in oil .018% in 15,000 mile change, #6 cylinder trickling water back side of sleeve, pulled injector and inspected. Not leaking from top of cup. I had to order injector tool and it will be in today, I'm going to retest new cup after installation to see if I still have leak. What should I use on injector threads? Any ideas, I've read detriot option, but a local engine builder told me something different. If not injector cup, head gasket possible because of intrusion or cracked head. Don't want to pull head if I don't have to. Thanks for response. Also should I blow out water system before I pull cup to avoid water in chamber and were would be the best place to do this, that would clear water from head.
Thanks again.