P0171: what to look at next, after MAF, O2 senors, fuel filter, and vacuum?

Question

I have been chasing a P0171 on a project vehicle for quite some time now.  This is a 1999 4Runner with the 5VZ V6 engine, 70K miles, naturally aspirated.  For this particular model-year there was no EGR.

So far everything I've looked at has been fine, and I'm wondering what the next logical thing to look at next is.

MAF sensor

out-of circuit ohmage @ 68F was within tolerance
in-circuit voltage generation fluctuated correctly, given steady streams of shop air
cleaned it anyway, and reinstalled

Fuel filter (factory original?)

Fuel dripping out of input side looked just as clean as fuel from output side
Replaced with new filter anyway

O2 sensors

Didn't know how to test rigorously, so just replaced
Now, after clearing fuel trim data the CEL comes back on even faster -- which makes sense to me, since I'd expect pristine O2 sensors to be more sensitive than dirty ones, and notice more uncombusted oxygen.

Vacuum

Squirted water at all vacuum fittings, no observable change in RPM. 
Vacuum gauge connected to port just downstream from throttle body showed rock-steady 20 inHg at idle (700rpm).  When cracking open the butterfly valve, vacuum would briefly drop before RPM increase, then head back up to 21inHg as RPMs settled to maybe 1500.  While at ~1500RPM, the needle would barely fluctuate (maybe +/- 0.2 inHg, with a period of about 1 Hz). At time of test, elevation was 700 ft, barometer was 29.88 inHg

Other

Spark plugs have less than 10,000 miles on them.  Haven't looked for fouling recently; perhaps I should.
OBD2 data suggests higher vacuum reading than the mechanical gauge: 24.5 vs 20.
I haven't checked PCV valves for proper operation; but at the least, they don't seem to be contributing to unmetered air (at least not at idle, or at 1500rpm @ low load.)

What should I look for next?  I think my vacuum gauge results rule out valve problems.  Since my P0171 code seems to only show up after highway driving, I suppose I could have an intermittent leak in one of my intake & manifold gaskets, that is dependent on engine load & RPM.  But I'd rather avoid pointless tear-downs before that becomes necessary.

I'm thinking I should circle back to the fuel system, maybe check fuel pressure.  And as far as PCV valves go, can a stuck-open one contribute to unmetered air?

Is there anything I'm overlooking?

UPDATE: freeze-frame data

engine load (calc) 31.0%
coolant temp 195.8F
STFT1 4.7
LTFT1 39.8
RPM 2915.2
speed 42.9mph
intake temp 125.6F

GdD · Accepted Answer

The main cause of most P0171 codes on the 4runner is the MAF sensor, which also has the air temperature sensor on it, a faulty air temperature sensor on your MAF can cause a running rich problem. It's possible that the MAF may be the wrong one for the engine as well, or an inferior part, if it's a project car who knows what was done to it before you got it. It's worth checking on at least.
There's a 4runner site which has a forum thread on exactly this problem, and someone posted the diagnostic procedures:

1999 Toyota 4Runner 3.4L, Eng Cfg V6, Eng Des 5VZFE, USA/Canada
Hotline Archive With OEM Direct diagrams, components, TSBs, and R&R
procedures
Number: 482996 Vehicle Application: 1998 4Runner 3.4 1999 Tacoma 3.4
1999 4Runner 3.4 2000 Tacoma 3.4 2000 4Runner 3.4 2000 Tundra 3.4
Customer Concern: Has a check engine light on and a code P0171.
Tests/Procedures:

Monitor the fuel trim readings at idle and at 55 MPH driving down the road. If the numbers are low at idle but climb on a part throttle
cruise, the airflow meter could be reading low or it may also have low
fuel pressure. The mass air flow sensor grams per second should be
3.3-4.7 at idle and 12.9-18.3 at 2500 RPMs no load, if the readings are low the sensor could be bad.

Monitor the calculated load reading on a wide-open throttle acceleration. The reading should go to 85% or more. If not, then the
airflow meter could be faulty.

Monitor the oxygen sensor inputs, both front and rear oxygen sensor signals. The oxygen sensor should not stay high or low voltage for
extended periods of time. If the oxygen sensor shows low voltage and
the fuel trim readings climb, check the performance of the oxygen
sensor.

Check the vehicle for a vacuum leak by spraying carburetor spray around the intake manifold while watching the fuel trim readings. If
the readings change, there is a vacuum leak. Tech Tips: If fuel trim
readings are high at idle and go down on a free rev, there is a vacuum
leak. If the fuel trim readings are high at idle and at cruise,
suspect fuel injectors.

Diagnostic Codes P0171
Confirmed Fix
225 - Mass Air Flow (MAF) Sensor  9 - Engine Air Filter, Mass Air Flow
(MAF) Sensor  4 - Oxygen (O2) Sensor(s)  2 - Fuel Pump  2 - Heated
Oxygen Sensor (HO2S) Bank 1 Sensor 2 (B1S2)  2 - Heated Oxygen Sensor
(HO2S) Bank 1 Sensor 1 (B1S1)  1 - Front Oxygen (O2) Sensor  1 -
Vacuum Leak  1 - Fuel Injector(s), Mass Air Flow (MAF) Sensor  1 -
Cruise Control Vacuum Line  1 - Intake Manifold, Mass Air Flow (MAF)
Sensor  1 - Mass Air Flow (MAF) Sensor, Throttle Body Cleaning
Procedure  1 - Mass Air Flow (MAF) Sensor, Mass Air Flow (MAF) Sensor
O-Ring  1 - Exhaust Pipe, Mass Air Flow (MAF) Sensor  1 - Air Intake
Tube 1 - Mass Air Flow (MAF) Sensor, Throttle Body  1 - Air Fuel Ratio
(AFR) Sensor  1 - Engine Vacuum  1 - Oxygen (O2) Sensor, Bank 1 Sensor
2

Using this as a guide compared to what you're already done I'd:

Check the fuel pressure, if it's low look at the lines and the pump
Pay special attention to the MAF, it features so much it's worth an extra look. If you can swap it out temporarily then do it
You won't have plug fouling on a lean cylinder, if anything your plug should be too clean. Pull all your plugs and put them in cylinder order to make sure you know which one goes to which cylinder, then compare them. If one or two of them look markedly cleaner than the rest then diagnose those cylinders by checking their fuel injectors
The diagnostic procedures say to check for vacuum leaks by spraying carb cleaner around the intake manifold and watching the fuel trims, it's worth following that advice

Carguy · Answer

You haven't considered dirty injectors.  Put Marvel Mystery Oil into the gas tank at a treat rate of 2 oz. per gallon, then floor the car 45-75 MPH on the highway five times in a row.

Zaid · Answer

LTFT1 39.8

This sticks out like a burr. The lambdas are sensing 40% more air than what the MAF/MAP is measuring, either due to unmetered air or insufficient fuel.

Given that the MAF and lambda sensors have been inspected or replaced I don't think this is an issue related to false readings.

If there's a gaping air leak somewhere, it can be confirmed by blocking off the intake and observing that the engine continues to run.

Remember that vacuum leaks are not limited to just vacuum hoses, any place where unmetered air can make its way to the cylinders can have the same effect.

Update after brief chat

In light of the negative LTFT at idle, the unmetered air scenario becomes less likely, since unmetered air should give a positive fuel trim regardless of load.

The question to ask then is if there is anything that could cause improper fuel flow that is load-dependent. It turns out there is one such component: a vacuum-actuated fuel pressure regulator.

The consequence of improper fuel pressure regulation is out-of-spec fuel pressure. This can be verified by measuring fuel rail pressure, preferably at both idle and under load.

Bigredox · Answer

Go back to beginning. Re-think what you have checked and why. Only 5 probable conditions/reasons for lean burn DTC, #1 is WEAK SPARK. Examine/test plug wires, ignition coils and housings. One anecdotal account to consider: Coming home from the lake I suddenly got the flashing-omg-stop now signal from the instrument panel. Lean on right bank. Long story short the #1 plug had a cracked insulator that only was an issue after about 100 miles on the new plugs freshly installed the week before when I had the wires and plugs changed because a faulty O2 sensor caused spark plug fouling on that bank from running rich.

Answered by Bigredox on May 1, 2021

Eric · Answer

Replace the MAF sensor. In looking at your freeze frame data, LTFT shouldn't be that high.
Look at Airflow g/s and it should inrange of 3.0 - 4.5.
Since your freeze frame is at 42mph rev motor up to sbout 2500 and airflow g/s should be in range of 12.5 - 18. If either are not in range replace the MAF per Toyota.

P0171: what to look at next, after MAF, O2 senors, fuel filter, and vacuum?

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