I have no idea what the idle is supposed to be (even for the 750). Sorry dunno about the head gasket either. Haven't had to do it yet I can ask my friend who is a BMW mechanic? he is working on the A/c on my 750. I took it to him a few months ago cause it was just blowing air come to find out it needed a new expansion valve and aux fan resistor. It cost my dad $600 and it worked for about a week so now I have it back over there. But he's a really nice guy too bad you don't live here I would highly recomend him cause he's way cheaper than the dealer.