I've rebuilt several dozen of these heads and have seen this twice now, on this style engine. Coolant in the oil makes a black milk shake. The head is removed because the tech suspects (but doesn't test for) a bad head gasket. The problem wasn't with the HG, but rusty freeze plugs. These heads have 11 of these under the valve cover, toward the ex side of the VC. I was able to poke a ball point pen through one of these, and another was close to rusting through. The big plug in the picture came out of the front of the head, and looks brand new. My theory is that the coolant level drops a little, and air is introduced to the system, accelerating the process. These are not stainless like what's used in Isuzu/GM or Ford PS engines. Freeze plugs

Brooks from Texas

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