This is a recent to me car, my first Volvo, so I am still figuring this out. I planned on fixing it first, but due to my regular commuter Camry being wrecked I had to quickly get it going.

I just clocked the mileage and it appears to be around 16 mpg. That seems high. This made me take stock of some other things, and I believe the engine is running rich. The idle feels high, and I can smell a bit of gas in the exhaust.

There is no black smoke. The "check engine" light is not lit.

Things I have done so far:
cleaned throttle body and idle air valve
replaced worn vacuum hoses
put in new plugs and wires
cleaned flametrap and replaced o-ring and hoses

While I was replacing hoses, there was no sign of leakage from fuel pressure regulator. However, when I pulled the vacuum hose off of it, there did not seem to be any change in how the engine ran.

The cleaning was extremely necessary, as the small passages in the TB were completely plugged and the flametrap was pretty clogged as well.

One thing I am wondering about is that the car came with a second airflow mass meter in the trunk. I assume the previous owner either swapped or intended to swap them. I know an incorrect AMM can cause all sorts of grief. The one on the car has an adjustment screw of some kind and the one in the trunk does not. How can I tell which is original?

How can I test the FPR? What is the likelihood this is caused by the oxygen sensor and how would I test that?