[bigdad]

you have an air lock in the cooling system on occasion....the bleed off bolt is the highesst point in the system...and gets air locked there or sometimes in the heater core with small passages. the revving is due to the thermosensor sensing air temp and coolant temp alternatively.....they will be at significantly different temps until the engine is at operating temp....this temp. data is sent to the ECU which then tries to set the correct idle speed and it then revs up and down.....bleed the air and fix the leak that allowed it in. For the person who gets the air lock to move by driving "crazy"" or up and down hills, he is relieving the air lock in a "unique" fashion.