Thanks to all who have responded. At Ken's suggestion, today I removed the expansion tank cap to have a look and attempt a pressure test. Before I got to a test, out of curiousity, I sniffed it for exhaust smell. Three guesses what I smelled and the first two don't count. I do wonder though, when the engine is running, if cylinder pressure is pushing exhaust into the coolant through some failure, wouldn't one expect that cooling system pressure would then force coolant into the cylinder when the engine is stopped and cooling? If this was happening wouldn't the engine miss or stumble some on the next startup? Ours doesn't, it starts and idles cleanly. Oh well. Looks like my fate is kinda sealed. Now, any ideas how I tell which head is leaking into the coolant? As I said in my first post on this thread, the engine was a purchased rebuild (Fast German Auto) about 20k ago. Shouldn't be a need to pull both heads if only one has a problem. I know that the list has beat the head gasket issue 'til it's screaming for mercy, but I still don't understand why the wbx has so many head related problems. Would a failure like this be corrosion? I've been using the orange stuff...