2001 Ford Ranger, 5 speed, 165K miles

At low RPMs, there is a serious loss of power. The idle is just fine. No error codes.

It hasn't a tune-up in a long time. But things have been getting much worse over the last few weeks.

What's the most likely cause, a plugged up fuel filter?

 

could be a filter a line electrical ignition mechanical among many other things if the filter is restricting flow then it should show up on a fuel pressure test do you have a pressure gauge. my 94 had a similar issue but it had a plugged converter. and as far as the error codes go i would not bet on these type diagnostic systems are particular about when they will actually set a check engine light did you get the codes read or do you not have any idiot lights on.