1999 Mercury Sable Gunk in Coolant

I think if you check the tranny lines, they do not go into the radiator, cooler is in front of your radiator. Think heater core.I used a pair of hose clamps, clamped the long hose in back of the engine, and this brings your heat back. The hose you will clamp tees off the water pump one end goes to heater core, the other runs all the way around the back of your engine, that's the clamper.I know this is unconventional, but mine has been like this for more than a year. No problems.

If you have a different engine it could be radiator cooled, you need to look at the rad...anty lines screwed into it? Mine looked like coffee grounds when I got the car, hadnt been flushed in five years. if you look low inside the bottom of the grille, you can see the tranny cooler, that is if it's a V6 SOHC.
In fact there are two rubber hoses used for tranny flushing inside the left lower nose piece. If you have foamy stuff, but not oil, check for a head gasket problem...a block test will detect hydo carbons in the coolant if this is the case. If yours looks like this, and the foam is pinkish, you need a new radiator, or side tank.

You should also check the tranny fluid, if coolant is getting into it, the trannys friction plates will be ruined. Then another flush is in order...You can also remove the lines, use a pressure tester, if the tank is cracked antifreeze will come out of those holes!