Mark 4.4 v8

Hi, I'm not sure of if the car has had previous cooling system work. Is there anything that should be done around 100,000 miles? I changed the expansion tank cap last night this solved the hissing problem straight away, the car hasn't lost any noticeable amount of coolant since. Is it possible that could of been the problem? Thanks for all your help

Hi, I'm new to the x5 scene. Just purchases a 2002 4.4 v8 sport model which I love. I have a few teething problems which will hopefully turn out to be nothing. I noticed when checking my coolant tank it was fairly low so topped up yesterday, this morning I went for a drive, a few hours after I got back I went past the car a noticed the expansion tank cap was hissing and the coolant had slightly dropped from max, is this normal? Secondly the sat nav keeps coming on and off as if I have a faulty connection or damaged cable, any idea of where to begin looking or a likely cause? Thanks Mark