Need help diagnosing P0174 Fault

[su20]

I've been reading all the posts I can on this code, but can't decide where the most likely root cause is, I hope you guys can help...

2002 C4S 47k miles

My CEL lit up in late June, so I pulled the code with my Vag-Com (PC based software for Audi's, so it only can access the generic OBD codes on my 996, but it does allow data logging). It was P0174 - Fuel Trim; Bank 2: System Too Lean. I reset it, and started reading on the forum. I never noticed any problems with how the car was running, either at idle or under load.

In mid July it lit up again, by the time I got to checking it again, I had two codes:

P0174 - Fuel Trim; Bank 2: System Too Lean

P0507 - Idle Control System RPM: Higher than Expected.

I cleared the codes again, warmed it up, and tried to log MAF, short term fuel trims, and long term fuel trims, but I'm not sure I was using the logging function properly so I don't trust the data logs from this first check. I did notice, though, that the LTFT's were different, 6.3% rich on bank 1, and something like 3% on bank 2. I'm not sure if the difference points in any way to an O2 sensor rather than an MAF. But I found a thread on cleaning the MAF, so I got some electronics cleaner and did that. I ran it for about 10 minutes, re-checked the LTFT's and they were 3.1% bank 1 and 0.8% bank 2.

I drove locally for about 3 weeks, and the CEL has re-appeared, this time a new variation:

P0174 - Fuel Trim; Bank 2: System Too Lean

P0171 - Fuel Trim; Bank 1: System Too Lean

So I cleared them, warmed it up, and logged some more data:

I've seen Loren post a couple times that if you check the MAF readings at idle and at 2500 rpm, you can tell if it's working. In one post I think he put the values for both (actually idle and no load 3000 rpm) but the ranges are in kg/h. My logging software shows g/s. The values I got, averaging a group of readings:

741 RPM, 3.33 g/s

2540 RPM, 11.49 g/s

Assuming g/s is grams per second, these convert to 12 kg/h and 41.3 kg/h, which is just on the bottom edge of the 12-24 range at idle and below the 50-80 range at 3000 RPM (although I measured at 2500 RPM). Could this be the problem?

Also my LTFT's have now adapted to 2.3% on bank 1 and 1.6% on bank 2, much closer together.

I've done my best to visually check for intake air leaks, but I don't know if there is an easier way to do a more thorough check without special equipment. And I don't know how to get the oil fill tube off the engine block to check it, if that's even necessary.

I'm looking for advice on what to do next. Clearly I'm trying NOT to take it to a dealer for fear they'll just start replacing things. My dealer wants $308 for each of the four O2 sensors (Pelican has them for about $125). I could take it to them for a diagnostic, they'd do that and it might be worth it, then just replace whatever needs it myself.

I can log additional info if it would be useful.

I appreciate any advice you guys have.

Edited August 17, 2009 by su20

[Edward Liu]

I have a similar issue, did you ever find out what the issue was?

[Ahsai]

I have a similar issue, did you ever find out what the issue was?

Welcome! It's best if you can start a new thread or at least tell us the yr/model info and any error codes and symptoms. The more relevant info the better.