BMW Repair: Coolant problem, 2 quarts, head bolts

Question
2000 528I
Don't know were the coolant is going, I have been adding 2 quarts of 50/50 a day! Presure tested at the dealer, new tank cap, no visible leaks, no white smoke of the exaust pipe.
Dealer can't tell me what is wrong.
Any suggestions would be appreciated
Thank you


Answer
Two quarts a day and they cant find it?Unbelievable.
Anyway, the headgasket is a common failure.If you wait till it really overheats then the head bolts will pull out of the block and the hole engine is ruined.  BMW had a policy of extending the warrenty on these engines if you were a "good customer".  
Check the spark plugs for white or green color.