i did some much needed service to my truck today, and i happen to check the overflow tank, seemed a bit low from where it was the last time i checked, so i thought i would pull the rad cap to check level there too, and i found brown crud, it was almost like gritty mud building up around the filler neck, and it was on the bottom of the cap, coolant looks to be dark orange to brown looking too. why is this and what is this?
i guess i am going to need a flush, but i am not sure if it will get rid of all that.

 

its usually rust, flush and fill asap, take a look at the fins inside look for buildup..the coolant shouldn't be brown at all. if u open the drain on the bottom and crud pours out id pull the rad and have it boiled out. Check the coolant for electolysis neg prob on batt neg, positive in rad coolant, dont touch the filler neck with pos probe. if the voltage is greater then .10v then its happening.

 

my truck sat for most of the winter, i did use it here and there but it sat more then normal, i am guessing i probably should have driven it more.