[willistwillis]

Hi guys, new member here. Still getting the hang of it...I posted this in the "general" forum by mistake before realizing it should probably be here. So, I have the apparently common no-low-end-power situation on my '09 335d that I bought a couple months ago. I would very much appreciate some advice from someone who's gone through this.

The car has about 120K on it. So far as I know it's unmodified. Up until recently, it ran great and had tire-spinning low-end power. A couple weeks ago I hammered a merge lane, then immediately had to slow down. When I went back to the accelerator, I had no power and got the half-motor icon in the dash which I take to be limp mode. At 2,800 RPMs or so, the power comes on strong but the car then usually goes back to limp mode at that point until re-started.

The codes that come back are P0299 (turbocharger underboost), P244A (DPF differential pressure too LOW), and P14A3 (DPF pressure implausible to atmosphere). FWIW, I'm pretty sure I had the latter two codes all along and prior to this episode.

I've done searching and reading ad-nauseum. If I've missed anything applicable already written, I apologize. Based on all I've read, here's what I've done:

Vacuum System. Vacuum seemed to be the most common culprit. I verified the vacuum pump is working. Replaced all vacuum lines on the passenger side. Tested the vacuum reservoir. Replaced the two vacuum converters in the vacuum block with new ones from Tunemyeuro. Applied vacuum to the EL valve in the vacuum lock - it held vacuum. Each of the converters and EL is getting vacuum. I applied vacuum to each of the three output destinations from the vacuum block. The switchover (big silver cannister) held vacuum and I could see the rod move. I also could move it by hand and I observed it moving when a helper started up the car. The wastegate held vacuum and I think I could hear it move. The bypass held vacuum and I think I heard it move too. I did away with the EGR cooler spur of the vacuum system in the course of all this. I also swapped-in the EL from the EGR cooler as a way of maybe ruling out the bypass EL, but no difference. None of this work improved the problem.

Boost Leaks. I checked the red charge pipe junction and inspected the intercooler for signs of boost leaks, but I don't see any evidence. I didn't pressurize it somehow for testing, but I never heard any sounds of air escaping while underway. Also, with my scanner hooked up, the boost pressure on the live data doesn't increase at all when accelerating at low RPMs, which I would think it might at least a little, if boost were actually being applied to even a leaky system. So I don't think this is the problem. I also replaced the rubber gasket between the intake pipe and the large turbo, and that PCV hose I broke that leads into the intake.

Other. I removed and cleaned the MAP sensor. It wasn't particularly gunky but I cleaned it anyway. The boost pressure shown on the live data (pretty much normal atmosphere) seems plausible under the circumstances, so I don't think that's it. I smell a very, very, very small amount of exhaust in the vicinity of the small turbo when a helper gooses the throttle good. So I removed the banjo fitting for the exhaust pressure sensor that's right there to check it. The fitting was gunk-free and the hose in good shape. I cleaned it all and reinstalled it. Again, the live data shows exhaust pressure slightly higher than intake pressure, so I don't think that's it. I inspected the hoses for the other exhaust sensor that's mounted to the back of the vacuum block. The hoses are connected and appeared to be OK. I didn't remove them because of the PIA access. I guess this sensor could cause the P244A or P14A3? But I'm pretty sure I had those codes before this incident, so I'm unsure of the importance.

I've read that the small turbos don't often fail, but I haven't tried to access it to see if the vanes spin. It looks like I'll have to take off heat-soaked rusty bolts, which I'm not wild about. Is there another way to verify the small turbo is actually spinning? Watching live data while driving around I can see that boost is commanded (as is partial wastegate closure) but none appears.

Any advice or suggestions are appreciated, before I give up. Thanks in advance.

Willis