After a couple of weeks of checking the floor, hoses, radiator etc. I think I finally found the source



This bolt at the back of the intake manifold had green around it

I was going to take it out and put some thread seal on it, but when I started loosening it coolant started to flow out so I tightened it back in.
Should tightening it be enough? Or do I need to do something more?

 

You need to drain some coolant out of the system to repair as the plug is below the current water level.

 

X2.
There is a cleaning trick where
you can use a good battery terminal
cleaner to do the job for the hole if
you don't have a round steel brush.
A wire brush on the plug followed up
with some teflon tape, good to go.

 

It appears to be a 3/8" NPT plug. The factory used pipe plugs. Seal it with Permatex or RTV.

 

Teflon tape, maybe, but it looks hokey IMHO
RTV, no, wrong application for that stuff
Form-a Gasket #2 would be perfect, or teflon based pipe sealer spooge in can.

Drain the radiator down to as far as need be, make the repair, re-fill. '

One advantage of the F.A.G.#2 over say teflon tape is that you don't have to tighten as much to get a seal, and with tapered threads in an AL casting, excess torque can lead to a broken casting.

 

Pipe dope works fine.
Teflon can be wrapped
as many times as you need
to go around the plug.

Much cleaner version and
removal is a snap.
Just one man's opinion.