168K miles, 5 speed, 4 cylinder

Serious lack of power especially in 1st and 2nd gears when starting to give throttle.

Idle is OK.

No idea when the last tune-up was done, nor have I looked at the spark plugs. I got a fuel filter just in case.

Any idea which direction to look at? Fuel delivery - fuel filter, or ignition such as plugs and wires? Or air filter? Or what. It was steadily losing power over the last few weeks.

 

If you have a CEL lit, most autoparts stores will scan the computer at no cost, then post All trouble code Numbers, as they can provide good trouble shooting clues.

If no CEL lit, with the description given, I think your on the right track to suspect a fuel delivery problem.
So maybe begin with the basics, like inspect the air filter air box & intake air tube all the way to its intake, behind the passenger side inner fender liner, as critters like to build nests, or store things in there. I just read of a Tech in the UK trouble shooting a lack of power complaint & all the usual suspects checking out, so he backed up & began checking the basics & found 6lbs of peanuts in the vehicles intake air tube & air box, stored by a squirrel raiding a near by bird feeding station!!!!! Have also heard of ground squirrels & pack rats doing likewise. Even bees, mud dobbers, wasps & other insects, building nests in there & choking off air to the engine!!!!

Dirty fuel filter & MAF sensor & acting out throttle position sensor probably belong on the suspect list too. Maybe a wimpy fuel pump, so check fuel pressure & delivery rate over time. It should pump 1/2 pint/15 seconds.
Some thoughts for consideration, let us know what you find.

 

No CEL unfortunately. I even scanned it anyway just to be sure - nothing.

I will replace the fuel filter and go from there.

 

OK, so no code clues, good idea to scan anyway, as sometimes there are pending codes stored that haven't turned the CEL on yet, that can offer up hints. If you don't know when the fuel filter was last changed, or if its due, then that's a good place to start, as is inspecting the air filter & intake air tube, which are easy to do.
Will be interesting to know what you find, so keep us posted on your trouble shoot.