I have noticed the coolant disappearing in my 94 eldorado 4.6 litre. And have read in amongst the forum about the problem commonly being the headgaskets. Does anyone have any experience or insight about the possibility of it being the air plenum? I know that the gm 3.8L engine has a big problem with the air plenum walls corroding from the years of pollution, and results in an identical problem, being the coolant disappearing. Has this happened with the 4.6L to anyone's knowledge?