P0172 code, need to know normal OBD2 values

[CT10LGT]

Hello, have a 2010 Legacy GT, just started throwing a P0172 code. Plugged in my OBD2 reader and checked the LTFT, STFT, MAF and O2 values. Sure enough LTFT was pegged at minimum, STFT jumping around 0.0 +/- 5.0 but lambda was at 1.00. MAF appears to be high to me at 4-5g/s, thought it should be closer to 2.5 at idle. The odd one to me is the upstream O2 is mostly at 4.0V but occasionally drops to 3.9, shouldn't this only go between 0 and 1V? So I replaced it with a new O2 and it reads the same, pegged at 4.0 most of the time. If I pull the connector off and measure across the pins (non-heater side) I don't get 4V, mostly it's at 0V but does occasionally jump around under 1V. The downstream O2 sensor looks normal as it's at .78V. Any ideas what is going on? Can someone tell me what values I should be reading for this car when it's warmed up and idling? Would like to have values at idle for MAF (g/s), O2S1(V), O2S2 (V), intake (PSI) and any others that you deem important that I'm missing. Thanks

[DrD123]

What does the MAF reading do as the RPMs increase while driving?  Sounds like it's under-reporting - perhaps it's dirty or there is a leak in the intake somewhere?

[CT10LGT]

Well, learned today that the upstream O2 sensor is wideband so you really care about the current, not voltage. 4.0V is normal, current at +/- .500 mA when blipping the throttle is what I'm seeing, which is OK. DrD, the MAF checked out clean but I sprayed it again just to be sure. It appears to increase properly when throttle is opened. After driving (warm) it's now reading closer to 3.2-3.5g/s when idling vs "sitting in garage" warm, I think this is OK. I attempted a pressure test by blocking the intake after the airfilter and putting a couple of PSI in the bypass hose (oil filler cap off), losing air somewhere so I disconnected the intercooler, capped it there an reran the test. No leaks from there thru intake so I must have a leak either in the intercooler, uppipe, downpipe or fittings. Will attempt to pressure test intercooler by itself as the very quiet muffled leak sounds like it might be coming from there. Leak sounds pretty small, can't believe this is what's causing the STFT to go -22 as soon as it goes OL. My gut tells me there's a bigger problem.  Is it common for the FPR on these to go bad? For $95 might just have to replace it as I don't have a good way to measure fuel pressure. Anyone have any other ideas? Is there any way I can test for a stuck or sticky injector. Once I get it back together I'll post a picture of my Torque screen, see if anything jumps out to those with more knowledge of what "normal" values are for this car. Thanks

[CT10LGT]

Well, just figured this problem out. For reference, I pressure tested the entire intake, checked the fuel press (when running and when engine off to see if it bled down quick), replaced the upstream O2 (even though it appeared to be reading correctly). As a last resort I replaced the injectors (in case one was not always closing) and replaced the exhaust OCV (because I had a spare set and it was the solution to someone elses similar but not identical problem). Nothing helped. STFT droped as soon as it went closed loop and then eventually pulled the LTFT to -25 and then you get the CEL again. So I started pulling sensors and actually Ohm'ing them out. Found the intake air temp portion of the MAF was showing "open" all the time. Bought a new one ($80) from RA, swapped it in, problem solved. So, the morel of the story is don't always believe the OBD2 data. In my case Torque was reporting the intake temp was "normal" however Torque must be mapping the "intake temp" to some other temp sensor because the one the ECU uses, the one built into the MAF was open. Open temp sensor means "as cold as you can measure" thus the ECU thought, damn it cold out, better squirt lots of gas in this dense air, then the O2 sensor goes, whoa, there is too much gas, start trimming it back, right up to the point where LTFT is at -25 and it trips the CEL. All while Torque is reporting all sensors reading "normal" values. Good news is it's got nice new injectors now, should be good for another 100K miles.

[m sprank]

Never used torque.  Good to know though.  Glad you figured it out. 

[Scubaboo]

Interesting!  Thanks for sharing the solution. 

[silverton]

MAF replacement is pretty standard for p0171/2