racerguy; I hope I am too...

I had a similiar thing happen on my Diesel Sub, cept it was the tranny cooler. It was a hollow cylinder inside the radiator that leaked. So my overflow filled up with pink foam. I was lucky it never went the other way!

On my F-350 6.9L the engine oil heat exchanger is a mysterious cylinder on the drivers side, mounted just about at the level the oil pan hits the block. This is where the engine oil gets real close to the radiator fluid separated by thin sheet metal.

Of course you could have some horrible block or head defect where the two could meet but the huge number of very detailed tests executed at the factory are supposed to prevent this.