grey black sludge in my radiator???

When my Dodge 2.2L K-car had a head gasket leak (common problem), I would get a frothy brown "mousse" at the top of radiator. It would also over-heat after ~20 miles hwy. Drove it that way for a few months, changing the coolant regularly (straight water), then junked it due to many problems. There is a chemical kit to check for combustion gases bubbling out of the radiator cap. I had one, and recall using it, but not seeing the color change (blue to yellow), so don't know how conclusive those tests are. I have a separate trans cooler in my three 60's Mopar's, bypassing the radiator. Some don't advise that, but the oil stays cool and I don't have to worry about water ruining my transmissions.