see alot of it , in all cast parts

 

i personally never believed it was casting sand, rather i believe its stop leak...but many dont believe that to be the case....if the manufacture is cutting corners and not properly washing the blocks & clearing the water jackets, anything is possible.

no matter, i installed a by-pass filter on my superduty and the filter was plugged w/ sandy crap in less than 1000 miles of driving, my second filter plugged up in about 2500 (more) miles and ive never plugged another filter since...and many of us who got coolant filters on early have not had problems that most have had....and my coolant is always in good shape (i perform periodic litmus paper test to make sure).

 

I put stop leak in my sons cooling system on his TJ. A few months later no heat would come out of the vents. Sure enough, the stuff completely clogged his heater core. I flushed it out with a garden hose and all is well now.