A hypothetical situation, as if I need to think any more about this:

Maybe the black junk inside my intake plenum is throwing off my egr or something else, resulting in O2 making it run rich, resulting in terrible mileage (down to 11mpg now).

Could running very rich somehow result in excess moisture in the crank case?

Side note: coolant level remains steady, who knows why.

 

The black junk in your plenum is carbon partially caused by having a EGR. Best bet it to clean your plenum, throttle body, and EGR. This would eliminate all of them as the cause. Then it's up to checking the vacuum plumbing, replace the old hoses with new silicone ones. Fairly cheap and eliminates cracked hoses as a potential issue.

It about a weekend job to clean all that stuff. Have a gasket kit and take lots of pictures. I soaked my EGR in mineral spirits, (not sure if it's a good thing or not), but it came out clean. Lots of black gunk came out in the morning of the second day. I used spray carb cleaner and stiff nylon brushes to attack my plenum and throttle body. Used compressed air to dry it all out and had it back together by the end of the second day.

 

Yeah I think this is going to be my next step. Just did silicone vacuum hoses and verified their route/plumbing. I like the idea of a carb cleaner scrubdown instead of a Seafoam smoke fest, seems like it would be friendlier on the O2 sensor.

 

Ck your thermostat make sure its not stuck open ,would cause the engine to run cold cause moisture buildup