‘97 Tacoma 2.4L 350K Miles Running Rich Despite It All

*Apologies for the long read*
Hey all, I’ve had my ‘97 with ~350k miles for about two years now. Been sorting out its various issues over that period of time and it has always run rich to a fault. i.e. trouble starting, rough idle as well as inconsistent idle speed, ~14 mpg city, but better on the highway at ~23 mpg, slow and noticeably bogged down acceleration, and I’m just not making great power. Not that the 2.4L 2RZ engine is known for power, but on rare occasion when my vehicle has elected to run as god intended for about ten minutes before returning to its regularly scheduled poor state of tune, there is a noticeable improvement in performance.

Repairs:
Initially, I believed that the car had convinced itself to run rich as a result of large crack in my exhaust manifold. Less exhaust gas reaching O2 sensors -> the car thinks it’s running too lean -> enriches the fuel air mixture. Replaced the exhaust manifold, no more exhaust leak, still running rich.
Since then I had replaced O2 Sensors, MAF Sensor, and Engine Coolant Temp Sensor. Still running rich.
Most recently, I replaced all four fuel injectors with brand new OEM denso injectors. Which, while they have fixed (so far) the starting issues, there was no difference in the running condition of the engine. Except, maybe, what you could consider “efficiently rich.”

With all that being said, does anyone out there have any insight/ideas on my issue? I’m at a loss, feels like it could be any number of things. Thank you all for your wisdom and consideration.

 

Could be a vacuum leak somewhere and maybe air intake system. Does it idle high rpm when started, then goes down to regular idle after a few minutes. Could be a vacuum leak. I had that problem with my 2.7 in my 2001 Tacoma.

 

What has indicated “running rich”? What does your plugs look like?

 

Do a cylinder leak down test, not a compression test, she could just be worn out.

 

Plugs look okay, but the main indicators of running rich have been black exhaust smoke, heavy black carbon buildup in the exhaust system, exhaust smell, engine harmonics, gas mileage, and Engine Catalyst Efficiency error code.

 

Idle changes at random intervals throughout any given drive. Can be as low as 800 and occasionally as high as 3000.

 

Even though that engine is known to run forever , that kind of mileage is a lot . I would really check the overall health of the engine itself