I'm assuming you're talking about the plastic plug on the battery side of the radiator. Well, I'd start by pulling the radiator, turn it upside down and pick out the pieces. Then use a garden hose to clean out the fins, they are probably clogged with debris.
There's a good chance the radiator tubes are clogged and you need it rodded, but you could try it again before taking it to a radiator shop to get it checked.
The fact that the plug blew out, however, could very well mean you have a blown head gasket. There are tests for combustion gases in the coolant. NAPA sells a kit or a radiator shop could probably do it.
http://www.napaonline.com/Catalog/Ca...006_0361139896
Tom Rowe
Four wheel drive allows you to get stuck
in places even more inaccessible.
62 88 reg
67 NADA x2
74 Air Portable - The Antichrist (tag 6A666)
95 D1 - R380
95 D90 - R380
97 D1 - ZF