ive got an 318i bmw, 2 probs with it when it,s idling it goes up to 1500 revs then back again the second prob is when you take the header tank cap off the water coming spitting out, ive been told there's to much pressure in the system, any help would be great

 

For idling I would clean the idle speed control valve first.
As to pressure, if it does it only when HOT then that is ok so long as shows no signs of h/gasket failure, it is under pressure to raise the boiling point, if you remove the cap quickly it will spit water.
At the end of the day if it is not using water and not overheating stop worrying.
tabetha