2016 Legacy 2.5 - Running rich

[2016 Subie]

Original owner of a 2016 Legacy with more than 200k on it. Hoping to get another 200k!
All was going great until the engine sputtered a few times.
Once from 3k down to about 1k... sputtered (coughed), the next time was about the same.
MPG went down about 4-6 mpg. Mostly 55-65 mph driving avg was 34 and dropped to 29. Even worse with recent cold.
Most everything was original except spark plugs.
No CEL.
Engine still running smooth just using a lot more gas.
Replaced fuel pump, coil packs, PCV, Both O2 sensors, MAF, throttle body.
(most replaced with OEM parts from Rockauto)
Last thing replaced were the injectors.
P0172 now popping up at idle or when warming up (not all the time) - engine still running smooth when accelerating.
Checked plugs - All have are blackened but not heavy or wet deposits.
I'm not into just replacing parts but figured if keeping for the long term, I would need to replace them anyways.
I performed idle relearn after part replacement but some issues.
After picking up an scanner I noticed the strongly negative short and long term trims.
Long term is mostly at -25 to -15 at speed. Short term will get above 0 at times.

Any suggestions would be greatly appreciated.
I attached the latest freeze frames for insight.

 

[bmart]

Did you diagnose it or are you just throwing parts at it possibly introducing other issues?

 

[2016 Subie]

Did you diagnose it or are you just throwing parts at it possibly introducing other issues?

It was diagnosed as P0172.

 

[bmart]

That's too much fuel in A/F. Did symptoms change with the parts you put in, for the better or worse? Was one of those parts IACV (just curious after 200k miles).

 

[2016 Subie]

That's too much fuel in A/F. Did symptoms change with the parts you put in, for the better or worse? Was one of those parts IACV (just curious after 200k miles).

It has improved with each part. The replacement TB was complete with the IACV.

 

[keith3267]

Your freeze frame data is a little confusing. You should have fuel trim data for B1-S1, B1-S2, B2-S1 and B2-S2. You are only showing bank 1 data, I would need to see the bank 2 data for comparison.

If there is a big difference between the B1-S1 and B2-S1, I would look for an intake manifold gasket leak. Start by spraying some type of oil i.e. WD-40 or a carb cleaner/throttle body cleaner around all the intake ports to the head. If the idle is affected by spraying around one of the ports, or both ports on one side, that is the problem. It does not matter whether the idle speeds up, slows down or becomes smoother or rougher, the fact that it is affected says the gasket is leaking.

The P0172 is not saying that the engine is running rich. It is saying that the PCM is having to reduce the amount of fuel injected in order to bring the engine into the proper air/fuel ratio. The problem is that the PCM adjusts the fuel flow to all the cylinders the same when only one cylinder is off. It averages the fuel flow. That can cause one cylinder or one bank to run rich while the others run lean, or visa versa, the average is good and will satisfy the needs of the cat, but your gas mileage will suck.

 

[Dick Foster]

Could be due to several things.
First grab a vaccum gauge and see that you dont have a manifold leak somewhere. You can also listen around for a leak using a piece of vaccum hose as a stethoscope.

Next get some electronic grade contact cleaner and clean the MAF senor along with changing out the air filter. This is what tells the ECU how much air the engine is using.

Lastly checkout or have checked the main O2 sensor in the exhaust. The main O2 senor is what tells the ecu how rich or lean the fuel trim is.

 

[2016 Subie]

Your freeze frame data is a little confusing. You should have fuel trim data for B1-S1, B1-S2, B2-S1 and B2-S2. You are only showing bank 1 data, I would need to see the bank 2 data for comparison.

If there is a big difference between the B1-S1 and B2-S1, I would look for an intake manifold gasket leak. Start by spraying some type of oil i.e. WD-40 or a carb cleaner/throttle body cleaner around all the intake ports to the head. If the idle is affected by spraying around one of the ports, or both ports on one side, that is the problem. It does not matter whether the idle speeds up, slows down or becomes smoother or rougher, the fact that it is affected says the gasket is leaking.

The P0172 is not saying that the engine is running rich. It is saying that the PCM is having to reduce the amount of fuel injected in order to bring the engine into the proper air/fuel ratio. The problem is that the PCM adjusts the fuel flow to all the cylinders the same when only one cylinder is off. It averages the fuel flow. That can cause one cylinder or one bank to run rich while the others run lean, or visa versa, the average is good and will satisfy the needs of the cat, but your gas mileage will suck.

There is only 1 Bank with the single exhaust for the 4 cylinder. Perhaps I'm not seeing something but only see the 2 sensors. 1 before and after the cat.
I recently checked the plugs and hoped 3 would look normal and only 1 blackened. That wasn't the case as all looked the same. I'll see if spraying WD-40 near the ports helps.

 

[2016 Subie]

Could be due to several things.
First grab a vaccum gauge and see that you dont have a manifold leak somewhere. You can also listen around for a leak using a piece of vaccum hose as a stethoscope.

Next get some electronic grade contact cleaner and clean the MAF senor along with changing out the air filter. This is what tells the ECU how much air the engine is using.

Lastly checkout or have checked the main O2 sensor in the exhaust. The main O2 senor is what tells the ecu how rich or lean the fuel trim is.

I'll do my best looking for a leak. There isn't an obvious sound but i know it doesn't need to be large. This was going on before I replaced the MAF sensor months back. The replacement did improve the mileage a little but nothing where it should be. Main 02 sensor was replaced and helped a little as well. The CEL doesn't pop as often but it still is especially with a warm engine idling.

 

[keith3267]

The 2014 Legacy (150k) that I bought new is my first Subaru and I haven't had any trouble in that area, but all Subaru engines are two bank engines. They are not inline engines that only have one bank. All the other two bank engines that I have worked on (V and H) have a primary O2 or A/F sensor for each bank located as close to the exhaust ports as possible.

The readout you posted shows two STFT that would indicate two primary O2 sensors, but I don't know why the second STFT is labeled B1S2. S2 is for long term fuel trim (LTFT) and you would only have a B1S2 if you have dual exhausts, not twin tailpipes from a single exhaust.

The primary sensors are located before the primary cats in the manifolds. The primary cats, AKA pre-cats or warm-up cats may be built into the manifolds or are attached to the manifold before the exhaust pipe.

But then, maybe Subaru uses a single exhaust manifold for both heads, as I said, I haven't been under there for any reason. Maybe someone here will enlighten me.