make sure its coming from there before you get too excited.
Dorman pn 615-180is the upper intake manifold. come with the redesigned egr(some say the original melts the upper intake). the plugs are optional, i think the coolant being in the plastic is bad idea in the first place, all it does is warm the TB. i live in western new york and have not noticed any difference. the new manifold comes with all the gaskets, and is thicker in the problem areas. the lower intake manifold gaskets are plastic too and they will be a problem at some point, and when then leak they let coolant in the cylinder sometimes hydro-locking the engine. and in some cases the engine is done. if you disturb the elbows and you put new plastic ones on it, might be trouble trying to reseal it. the plastic breaks down after a while.
1998 Bonneville SSE
2007 Monte Carlo LT-KYB, Clarion, Polk audio, Kicker
1999 Yukon SLE-KYB, Air lift, Reese dual-cam, Tekonsha
1987 Firebird-355/th400-nothing stock