you raise some interesting points.
Whilst i am pretty knowlegable, i don't profess to know it all...and am keen to continue this discsusion for the benefit of all.
With the mention of headgasket your initial post appears a little vague.
IF i was to think headgasket, i would be forced to ask the obvious...is it using water??
You mention steam out the pipe. they all seem to do this, as the rear resonator (which you replaced) acts as a water collector...they ALL rust out. My next question to you, or in problem evaluation....does it clear afer 5-10 minutes....if it does, then its condesation in the res...if it doesn't then the lean is towards head gasket.
Someone mentioned water infected oil. On E-series this is not common with head gasket (but not unheard of) . Water affected oil/coolant is normally a cracked head.
If you are using water, then a pressure test is required to eliminate hoses etc. another sign for a headgasket with rough running is a water washed spark splug. For this to happen the gasket has to be leaking into a cylinder (for milky oil its into an oil gallery..not comon on E-series). Leaking into a cylinder however is...normally #1. eventually if bad enough, you will get to the point where you hit the key and it will spin and go clunk, and hen it won't spin....as the cylinder is full of water..and water is considered incompresible. (have PERSONALLY experienced this)
700 sounds a tad expensive....but if they are going to send your head away for testing, then its about right. From the sound of what you have described, you haven't cooked it, so the head should pass....if it doesn't then expect extra!.
On the unconnected plug. If you cannot find a joiner for it, then it is probably there for an extra not fitted to your car. Ford often use common looms, whereby they provide wiring for extra's, so you wind up with a multitude of unconnected wiring....take a look between the brake booster and rear apron........most of these are for diagnostics......apparent unconnecteds!
without a pic of your unused plug, and the full wire colours i have no hope of telling you what its for.
If you want to know....send me a pm/email with a pic and wire colours and i'll look it up and tell you
The AutoGuide.com network consists of the largest network of enthusiast-owned enthusiast-operated automotive communities.
AutoGuide.com provides the latest car reviews, auto show coverage, new car prices, and automotive news. The AutoGuide network operates more than 100 automotive forums where our users consult peers for shopping information and advice, and share opinions as a community.