Black coolant / PCM location

Hi - two questions:
My coolant turns black/very dirty - if i flush it out and change coolant, about 3 monthes later it will get dirty/blackish. I thought it was oil at first, but my mechanic friend said it would be like a milkshake since oil/water dont mix. Any help appreciated.

Also - where is the PCM located on a 1992 Buick Park Avenue? Is the PCM called by any other name? And if anyone has a guess to how much they are - thanks

pcm /ecm is located under dash ,behind glove compartment it is slide up in place ,held there by a nut and clamp, it has three plugs on it one green and two black,all on same side of box.box is about 8x10x2

pcm /ecm is located under dash ,behind glove compartment it is slide up in place ,held there by a nut and clamp, it has three plugs on it one green and two black,all on same side of box.box is about 8x10x2 about 85.00 in store 40.00 in bone yard

My coolant turns black/very dirty - if i flush it out and change coolant, about 3 monthes later it will get dirty/blackish. I thought it was oil at first, but my mechanic friend said it would be like a milkshake since oil/water dont mix. Any help appreciated.

I had the exact same thing happening with my 1992 Caddilac SedanDeville.
it is not oil, I kept some in a jug and it never separated like water/oil does.
I replace it with a new radiator, I thought that fixed it but then it happened again, then someone told me to try the new 5yr red/orange anti-freeze that it was safe for all aluminum engine blocks. (or the volkswagon anti-freeze)
never found results of that since i stop driving car due to other problems.