Pulled vacuum cap off manifold, filled with fluid.

[Mosinguy]

There's a bunch of vacuum lines and a few capped off plug ins on the manifold of my 3.0. I happened to accidentally knock one of those caps off, and it was full of some type of fluid. Keep in mind this is hooked onto my manifold. It doesn't smell like brake fluid, gas, or oil. It is clear and is slightly oily. I have no clue what it is or why it is there. I'm trying to figure out why I'm getting 8 mpg in my ranger.

 

[RonD]

Could just be moisture with some oil vapor mixed in.
Intake pulls in outside air, Dry or Humid, and when engine cools off the external parts cool faster, any warm air in the intake will have some moisture in it, that moisture will condense in cooler places, like the vacuum cap.

PCV valve pulls in oil vapor


Fuel pressure Regulator(FPR) on the fuel rail has a vacuum line attached, if FPR is leaking, raw fuel will be sucked into the intake, lowering MPG, check that vacuum line for gas or the smell of gas.