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.
- 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
- 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.
- 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
- 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