I had a CEL and associated P0175 (bank 1 rich) a few months back, didn't drop into RP. Checked the LTFTs and sure enough both LTFTs are running around -11/-13 at idle. Changed the air filter (no difference) and cleaned the MAF (improved a bit but went back). Haven't had a return of the P0175 but I monitor LTFTs daily with a scangauge. Questions I have for you are:
1/ Have a look at the (bit blurry) photos below. Worth doing anything about it? Haven't had a re-occurrence so I only know they're not ideal as I'm monitoring it. A fair part of me says just drive it until it gives another CEL.
2/ Conversely, is it worth swapping out the MAF for a new one? Are the RockAuto ones OK to use?
3/ Any insight on diagnosing? Fuel pressure looks OK to me and most of what I've read suggests MAF as the primary culprit. Car only has 60,000 miles on it so I don't think it's a core engine issue as there are no secondary indications such as smoke or oil loss. Similar for leaking injectors as both banks are evenly low.
I bought a Delphi MAF from Rockauto, it seemed to work for a while, but soon noticed some running issues. It would hold gears way too long, seemed to be running rich, intermittent uneven/shaky idle. Finally triggered a CEL and running rich code. Put back the OE Denso MAF and all issues went away.
I had only changed the MAF because the dealer diagnosed it as giving "faulty" readings and causing my light throttle pinging. I'd suggest you stay with OEM Denso if you replace yours.
The MAF should read 5.0 g/sec at hot idle in gear. There's a spec for at 2500 rpm as well.
Also, what are the short term fuel trims? Your total (ST + LT) should be close to zero. If the ST is also -10, and LT is -10, you have a total trim of -20%, and that's near the tipping point for a CEL. If both banks are similar I would suspect an incorrect (over metering) MAF or a fuel pressure issue.
Your MAF should read approx 5.1 g/s at idle in gear, full operating temp. Otherwise your total fuel trims look fine. It may have been running rich and now it's back to normal. Eventually the LTFT will learn the ST trim values and should return closer to zero.
The chart ties up as I got 15 g/s at 2500 RPM in Park so it's over-reading by 1.3 g/s, which explains the negative LTFTs, but not enough to be concerned about. I'll pull out the MAF and give it another clean just for stamps.
