How long has this engine been sitting idle?
"Brown" coloration of the cooling water is most probably a symptom of the antifreeze's rust inhibitors being consumed. In cast iron engine blocks the normal course of action is - the constantly flowing HOT water produces a layer of 'black' or 'blue-black' ferrous rust ... and this is protective. Upon long standing and without repeated heat/hot cycles the ferrous oxide slowly converts back to "red" or ferric oxide which is destructive. Long term storing such an engine by draining it (but leaving high humidity in the cooling passages) is the worst possible scenario with respect to ferrous to ferric oxide formation.
Suggest you drain the brown water, refill with at least 50:50 mix of antifreeze/water .... then RUN THE HELL OUT OF THE ENGINE (hours and days) to be sure that you redevelop the important 'black' ferrous oxide (black rust) layer inside all the cooling passages and channels; make sure that the engine is 'soaked' with 180 def. F water temp.
If the engine HAS been recently and long term been run hard and hot .... change as above and if the discoloration reappears, .... then check for a 'blown head gasket'. Your local 'automotive machinist shop' can give you 'tips' on adding a 'blacklight' dye to the water, etc. to check for blowby carbon monoxide/dioxide in the cooling water ... from a blown head gasket.
Last edited by RichH; 07-30-2009 at 02:24 PM.