I read through so many threads on so many forums in my quest to find the cause of my 2010 A4's P2177 System Too Lean Off Idle code. Most of them were dead threads without a solution being reported, so since we finally figured out what was causing mine, I wanted to share the story for all the other searchers out there. If you want to skip the story and just find the answer, scroll to the bottom of the post.
My car is a stock 2010 A4, Premium Plus, manual trans. The P2177 code first appeared in November, 2018. It was intermittent, coming on a few minutes after starting the car, and eventually going away. The car didn't run noticeably worse with the light on vs. off. We knew that the P2177 code is a generic one that doesn't provide much info about the root cause. Since the car ran ok, we didn't do much trouble-shooting, and I didn't bother taking it anywhere because I figured an intermittent, non-specific code would just result in a shop throwing parts at it. Finally in September, 2019 the light came on and stayed on. My husband does almost all the work on our cars, so he tried a few things that were due for replacing since the car had 153k miles - O2 sensors and spark plugs. The PCV valve had a little oil leak, so he replaced that too (2nd time on that part). No luck, so we found a reputable local shop to take it to. We were hoping that the problem would be a normal Audi thing that an experienced shop would know to look for.
The shop was great to work with, but it took a long time to find the culprit.
They smoke tested the intake and crankcase – found nothing.
Try #1: evap valve was stuck and there were two new codes that I hadn’t seen before. P0172 too rich, and P0442 evap leak. This didn't fix the P2177 code, and was apparently just a fluke new problem since I last scanned it in November.
Try #2: They cleaned the MAF sensor and also tried putting a new one in on a test drive.
Try #3: Replaced the low pressure fuel pump, partly because I started noticing heavy misfires under high load conditions, like pulling up a hill at around 1800 rpm. New pump got rid of the misfire problem, but not the P2177 code.
Try #4: High pressure fuel pump pressure sensor replaced because it wasn't producing the proper signal. They ran into the problem of getting the wrong one and had to get the right one from a dealer.
Try #5: Replaced the diverter valve, because it wasn't operating as expected. This made the car idle noticeably smoother, but didn't eliminate the code.
Try #6: Replaced a loose hose from the intake to the PCV
Totally unrelated – my alternator clutch pulley went bad during this whole deal, and the shop replaced it with one they scavenged off another car’s bad alternator. For free.
Try #7: Inspected the fuel injectors and did a spray test. One of them looked cruddy and had a bad spray pattern, so they manually cleaned it and got it to look like the others.
At this point, I decided to dive in and do my own data analysis, so used VCDS to log a bunch of laps over the weekend to try and figure out what to do next. Without clearing the code and resetting the fuel trims, I saw 25%-32% long term fuel trim off idle, with the short term trims +/- 10% and the long term idle trim +/- 1%. The MAF sensor correlated pretty well with the absolute and normed load values. There were some misfires recorded, but nothing really bad. Then I cleared the code and trims and ran the same lap again. This time it misfired noticeably until it went into closed loop and put in +25% short term trim, which then transferred over to long term trim. When I did this on a cold start, it ran horrible, so bad that it put the control system into some kind of weird mode and never transferred the short term trim to long term. That meant every time it went into the decel fuel cutoff mode, the short term trim went to zero and had to go back in once I got back on the gas.
I didn't find any particular cylinders misfiring more than the others. The specified and actual fuel pressures overlayed and correlated well with the fuel pump percentage. Charge air pressure correlated with normed load values. Because the issue existed at idle and low speeds/RPM, it was pretty clear that it wasn't a MAF sensor or anything correlated with high loads. Lambda jumped to 1.28 as soon as it went to closed loop, but then went back to 1 once the short term trim came in. The car thought it had a leak, based on the leakage recognition parameter. I uploaded three snapshots of recorded data from three different laps in case people find those interesting.
We capped off the hose from the PCV valve to the intake to check for big crankcase leaks. The trim stayed the same, so no problem there. We figured it was either a sneaky intake leak or bad injectors, because the fuel system looked good through the pumps. We even checked for the brake booster leak we saw in a Youtube video. Even though the problem was obvious in the data, the CEL didn't come back on for several days, but that had to be due to some funky logic for what it takes to set it.
Try #8: I took it back to the shop and asked them to send the injectors out to a shop with a high pressure flow bench. That technician couldn't believe the car ran decent, since the flow rates were so bad. They were 89/119/92/87 mL over 30 seconds to start with, and he got them back to 121/123/121/120 mL. On the next test drive, the long term trim was down to about +1%. The shortage in flow rate basically added up to the +25% fuel trim, so we were pretty confident that was the real root cause. The fuel mileage got a whole lot better, too. So don’t settle for an external cleaning by a shop without a flow bench. Send them out for real testing and cleaning. Or just replace them if you buy the cheap Bosch ones without the logo stamp. The cost for testing and cleaning was $220 plus the shop’s labor for removing and reinstalling them. We could have done it, but didn’t have time to mess with all this stuff, and our shop had been great and didn’t charge us for all of the work that they had done trying to figure it out.
So in the end, fuel injectors not flowing at the right rate. I’ve always used top tier gas, so maybe just an issue with high mileage direct injection injectors. Regular doses of Techron and Seafoam are the plan from here.
Maybe this will help someone, or maybe it's just an interesting story.
Reply With Quote
Originally Posted by keiffr
Thank you for posting!
Bookmarks