Eeeek! A rebuild at only 63K?!?!?!? Oh, a toasted crank...what a drag. Sorry, dude.
My uncle, a bona-fide car nut (he's still working on trying to put the 3.5L, w/the 4-speed tranny, into a *K*-car (can't quite figure out how to shorten the axles)...) claims the 3.5 is one of the best built engines he's ever seen, and he's built more than a few rail dragster motors. I mean, he was taken aback by how solid it looked, the first time he saw one. Claimed ours would last as long as I wanted it to.
Hmmm, a 'dirty' TB, eh? Interesting. Make sure the EGR valve isn't 'stuck', as that can introduce all kinds of nasty, oily stuff into the intake. I've not had the intake apart on this car, but is the TB before or after the injectors? If after, you may be seeing it running rich. Also, given this is a rebuild with low miles on it, are you sure it's "right"? Hard to explain, but if it's "wrong", you should be able to feel it. I wonder if the shop who did the rebuild got all the computer connections back, correctly....
Yeah, check for codes. Might not be any, but it can't hurt. If they did a full-on rebuild, it might be 'tight' for the first thousand miles, but still, 17 MPG ain't right.
Kelly B