Bad MPG/lacking power...help?

Hello,
I've got an '04 2.4l, and my MPG is averaging around 15-16 combined.
I was getting about 19-20 MPG combined in my '03 2.7l when I had it.
I stole a bunch of the newish parts of my '03 2.7l when it got totaled and I swapped them out, and also a few new parts on my 2.4l
The list includes:
New MAF sensor
New PCV valve
New air filter
New spark plugs
Oil and filter changed
Replaced a few vacuum lines
A ton of new relays for pretty much everything (because I already had them, why not)
And I've been running nothing but premium gas in it.
And none of this has barely knocked a dent in the MPG, but after the new spark plugs, it had just a tad more "get up and go" to it.

I still have several parts from the '03 2.7l I could swap over, just trying to find the main culprit behind this, and I'm not really sure what to do besides guess and throw parts at it.
Planning to upgrade a few things anyway, but still, I'm lost.
BTW, there is no CEL, my OBD2 scanner isn't picking up any unusual readings, either.

Anybody got any input or recommendations?
Thanks in advance

 

So the MAF is off a 2.7l and you put it on a 2.4l correct? Are they the same part number? How does the sparkplug color look? Wasting your money on premium gas in my opinion. How did it run before you started swapping parts?

 

Yeah, same part #. 99% of parts from a 2.7 will also work on a 2.4
I haven't checked the spark plugs again since I chaged them, and it runs better than when I first got it.

 

Stock tire and suspension?

 

What about the fuel filter?

 

It's getting lifted at the moment at a friend of the family's shop, letting him take his time on it. But before, yes, stock everything.

That's next on the list, when I have the time and patience to do that PITA job. Lol

 

Only if you had a 3.4 Takes a whole minute to change the filter

 

O2 sensors been done in a while?

 

Run a can of Seafoam through the fuel tank to help clean the injectors, or through the throttle body (maybe have your mechanic friend do that if he's got the truck). Stuff works really well.

 

Right? Idk why they thought it was a good idea to put the filter under the intake manifold, I take the battery out and sit in the tray to change mine. Lol

No, I do have an extra upstream o2 sensor that I took off the 2.7, I'll double check the part #s before I put it in, I just hate throwing parts at it, though.

I've done that (properly) to my Camry before, didn't notice too much of a change. What helped my Camry the most was cleaning the MAF and throttle body, and it's getting bad MPG now too (was getting 25-26, now getting 19-20). But on the Camry I'm suspecting the VVT solenoid is bad, and I can get a replacement for like $35, I've changed out almost everything air and exhaust related on the Camry, still no change, it's kinda pissing me off, hahaha