I have about 140K on my 850. For the past year it has been using more and more coolant about a pint a week. No coolant leaks could be found. Recently the coolant reservoir seemed to overpressurize with coolant spraying out of the hose connection at the bottom of the reservoir with no apparent problem with the hose. I assumed the head gasket had been leaking for some time and just got worse so I replaced the head gasket but the problem continues. According to the temperture gauge the engine is not overheating but will overpressurize even when just idling. I've noticed that the fan does not come on when this is happening but according to the temp gauge it reads right in the middle of the gauge and may not be calling for the fan to start. I'm wondering if it is very likely the engine is overheating at idle causing the coolant reservoir to overpressurize and a temp gauge reading in the mid range(faulty). Thanks for any thoughts.