[MetroMPG]

Correct: only goop came out. The check valve stopper was intact as confirmed by cleaning the part (as you do).

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Here's a question: how would a malfunctioning PCV valve hurt fuel economy?

I suppose if it's stuck open, it'd be continuously feeding some amount of oxygen-depleted blow-by gasses to the intake. That in itself should be slightly good for fuel ecnomy (like EGR) assuming it's a modern car with a properly functioning oxygen sensor: diluted intake = reduced power potential of the engine = wider throttle opening & reducing pumping losses.

But what if it's stuck closed? It'd increase crankcase pressure and possibly cause oil leaks (so I read).