Best bet - Motor mounts.

Left side is the typical culprit that rips and the fluid drains out and then you have a pretty steady vibration after that.
Right side will go sooner after the left side goes.

 

I was debating on doing those, the most expensive option of my fixes - doesn't solve my fuel economy though

I initially thought I was losing an ignition coil but I don't notice any lack of power or bogging when I get on it.

The car could also be sucking in unmetered air through the 5 plenum seals, don't know if they've ever been replaced and I very recently separated the manifold from the plenum to do the VVT solenoid.

 

With no codes I'd suspect the motor mounts as well. If the right (avenger's left) side mount has never been replaced it's almost assuredly bad. Actually, at 140k all of them are suspect. It's pretty easy to pull the brace off the top, jack the engine up a bit & do a thorough inspection.

 

I doubt either mount has been replaced at this mileage though I have no service records to confirm this. I've taken a good look and I don't see excessive rocking while idling or sagging typical of mount failure.

Would mounts really be this intermittent as well?

Sometimes it's quite bad and other times it's not as harsh. That and the question of my disappearing MPGs..

 

No, mounts would not be intermittent & they don't explain poor fuel mileage.

If you truly have those issues something else is going on. It's just that the Volvo management system is quite sensitive & will trigger a CEL pretty quickly with any sort of misfire or poor mixture control. Perhaps you'll get one shortly to help us out.

 

Back to revisit this, my gas mileage has gotten better and everything seems to have smoothed out in that regard.

I ended up replacing all 3 motor mounts, and for a week it was better.

Now I'm getting the same symptoms of a shot front right (the one near the coolant bottle) mount - pulled the top brace off, re-torqued both motor side bolts, then reinstalled the brace and torqued that...still lots of vibration when I input throttle, especially getting up to highway speeds.

My mount looked a little deformed as well (top pic is before it went into the car, brand new from FCPEuro)

 

Weird, that certainly doesn't look right. Any signs of fluid loss?

 

Not that I can see after removing the top bracket.

FCPEuro is going to warranty replace the mount, looks like the direction I will be taking...