[Ted Toal]

In my 2006 330i E90 N52B30 non-turbo, my check engine light came on and I read the codes: P0172 and P0175 - both banks running rich. It seemed to be getting bad mileage too, and smelled of gas when first started, though it ran fine. I brought it to Midas, they replaced the MAF, cleared the codes, and said if the light came on again it might need O2 sensors. It continued to run excellently and gas mileage returned to normal, but the light DID come on again within 100 miles and I decided to try to diagnose it myself (so far to no avail). I started out replacing the upstream O2 sensors but could not reset the codes, then realized it would not likely be O2s because both probably wouldn’t go out at the same time. I bought a new reader that could show live data (I now realize there are many different types of codes and reader interfaces and mine only connects with KWP-2000 format). The live data showed long term fuel tuning in both banks to be around -20 to -25, which accounts for the two codes. Short term fuel tuning WAS near zero. Upstream O2 sensors were flickering around 0 and downstreams were around 0.6 I think. My best understanding is that the upstreams being around zero is correct and expected if the computer is accomplishing its goal of getting the mixture right. At one point I also got a MAF code (one time only), and I disconnected the MAF and worked on the connector pins to be sure they were making contact, and then the MAF code was gone. I tested fuel pressure and it was normal. I thought perhaps the injectors were worn and replaced them, and then the short term fuel tuning went to -15. I put the old ones back and it remained around -15 and since then has crept up to -30. I have a feeling the problem is unrelated to the injectors, but it’s just getting worse. Upon first starting, at idle, it smells strongly of gas and it runs very rough, but I see that it’s running open loop then, and as soon as it switches to closed loop (30 seconds), the idle smooths out, hinting that the strong negative fuel tuning returns the engine to the proper mixture. It continues to run great on the road. I got a couple of cylinder misfire codes (#2 and 4), which happened when I was starting it over and over and looking at live data, and it seems the problem is worst at idle and the repeated restarts may have made the cylinders misfire. I removed the plugs for those two cylinders, and one was wet with fuel and the other was somewhat sooty, which I take as indicators that it truly is running rich at idle before closed loop starts, and that this is not simply a sensor problem saying that it’s running rich. I’m at a loss as to what could be causing this based on what I read about the causes of running rich (on all cylinders apparently, or maybe just those two cylinders, one was left bank and one was right). I keep suspecting that the new MAF is bad so I replaced it and also retweaked the connector pins but that made no difference. I’m wondering about the MAF live data, which shows 18 g/s at idle. That seems high from what I read, but I haven’t been able to find a spec on this particular engine. I looked for bubbles in the coolant, and there were bubbles, fine foamy bubbles coming up. I can’t tell from what I read whether I would expect large bubbles if there’s a gasket leak, or if small foamy bubbles could indicate it. I’ve had to add coolant a couple times recently, so I think there IS a coolant leak, but I noticed a lot of gunk and wetness around the coolant temperature sensor, so it looks like it might be leaking there a little. What should I do next? Vacuum test? Compression test? Pressure test on coolant? Replace EGR? It really seems like the thing is getting excess fuel from somewhere but I can’t imagine where that would be. The 0172/0175 codes are stable, they don’t ever go away, and NO other codes are happening. I would think I would get other codes indicating something else that would help me isolate the problem. I’m also wondering about cheap Chinese MAFs. I don’t know what kind Midas put on, but the one I bought was about $25 so it’s probably a cheap one and maybe worthless?

Late addition: freeze frame and live data.

The freeze frame:
DTCFRZF: P0172
FUELSYSA: CL
FUELSYSB: CL
LOAD_PCT: 67.5
ECT(°C): 90
SHRTFT1(%): -25.8
LONGFT1(%): -10.9
SHRTFT2(%): -19.5
LONGFT2(%): -11.7
RPM: 1984
VSS(km/h): 22
SPARKADV(°): 17.0
IAT(°C): 20
MAF(g/s): 38.67
TP(%): 27.1

And now live data at 60mph, 45mph, idle:
DTCCNT: 2,2,2
FUELSYSA: CL,CL,CL
FUELSYSB: CL,CL,CL
LOAD_PCT: 45.1,34.5,46.7
ECT(°C): 97,87,92
SHRTFT1(%): -26.6,-20.3,-20.3
LONGFT1(%): -13.3,-14.8,-15.6
SHRTFT2(%): -25.8,-14.1,-14.8
LONGFT2(%): -11.7,-12.5,-12.5
RPM: 2055,1446,730
VSS(km/h): 92,65,0
SPARKADV(°): 23.0,24.0,-6.0
IAT(°C): 13,15,16
MAF(g/s): 27.05,12.97,7.49
TP(%): 23.5,18.4,15.7
O2S12(v): 0.810,0.895,0.905
O2S22(v): 0.815,0.910,0.905
LAMBDA11: 1.024,0.940,0.941
O2S11(mA): 0.047, -0.285, -0.277
LAMBDA21: 0.980,0.983,0.947
O2S21(mA): -0.094, -0.082,-0.250

1. Things that seem strange to me:
Idle load 46.7% ???
2. Short FT more negative in bank 1 than bank 2 at 45 mph and idle
3. Idle rpm 730? Tach shows about 500.
4. Throttle position 15.7% at idle??

Another thing of note: I was concerned that my OBD tool would not reset the codes, but then learned that the codes are not regular DTCs but pd DTCs, meaning they cannot be reset by the tool, but instead are reset by fixing the problem triggering the code AND THEN doing a Drive Cycle.

I’m wondering if maybe the injectors WERE a problem and I was misled by strange highly negative short and long fuel tuning parameters after such a change, and maybe I should have done a drive cycle before dismissing them as not the problem?? Maybe that would have cleaned up the fuel tuning values??