CEL - O2 sensor and turbo overboost - BEW

Hi there!

Been chasing down this issue for a while now doing things like smoke testing the vacuum lines (no leaks there), put a new O2 sensor in (same codes came back). I guess I need to dive in deeper, so I'm looking for advice of what to do next.
I haven't cracked open any EGR stuff, so EGR delete is an option. I've seen the threads on pulling the turbo apart to unstick vanes, which looks daunting.

Any other low-hanging fruit I should attack before heading into EGR or turbo?


Here's the VAG-Com errors. Two other notes:
- The CEL seems to come back on 5 to 50 miles after codes are cleared at exactly the moment I take my foot off the accelerator while driving 50-60 mph.
- The car drives great!!! Tons of power, 40-45 mpg consistently, idles smoothly. If I wasn't planning on selling it I would probably leave it just the way it is forever. Absolutely no driveability issues whatsoever except the classic 4th-gear-hill-climb-boost-goes-to-zero-shut-engine-off-while-driving-turn-back-on-problem-solved issue. That happens once a year max.


18628 - Oxygen (Lambda) Sensor B1 S1
P2196 - 000 - Signal too High (Rich)
Freeze Frame:
RPM: 1953 /min
Torque: 0.0 Nm
Speed: 51.0 km/h
(no units): 255.0
Temperature: 36.9 C
Absolute Pres.: 1009.8 mbar
Voltage: 1.825 V
Lambda: 0.0%
16618 - Boost Pressure Regulation

P0234 - 000 - Limit Exceeded (Overboost Condition)
Freeze Frame:
RPM: 2142 /min
Torque: 170.0 Nm
Speed: 97.0 km/h
Load: 0.0%
Voltage: 1.825 V
Bin Bits. 01101000
Absolute Pres.: 1734.0 mbar
Absolute Pres.: 2142.0 mbar
Lambda: 0.0%

Excellent step-by-step HOW-TO in my signature on how to scientifically track down under/overboost issues.

Thank you for the reply.

I take it you are referring to the Diagnosing and Fixing Limp mode for A4 1.9TDI, right? The other link is broken it seems.

Looking at the Diagnosing and Fixing Limp mode for A4 1.9TDI link, I should test the MAF and turbo actuator first. That's what I'm seeing at least. Should give me a couple of things to do!

Thank you.

Are there any clues I could gain from keeping a Vag-com running while I am driving? I don't own one but I can borrow one from a generous Vag-com owner.

OVErboost is typically one of two things: the VNT actuator/VNT mechanism (part of the turbo) is sticking and not coming to the rest position on its own when vacuum is released, OR, the N75 is unable to release the vacuum quickly enough.

On the BEW in particular, I have seen several times a condition where the vacuum line to the VNT actuator rubs against the air inlet tube, and wears thin enough that it sucks itself shut when vacuum is applied, but not before it also pulls the VNT actuator down. So what happens is, vacuum gets "trapped" after that pinched section of hose, and holds the VNT down, so even when the N75 vents the vacuum, the VNT itself is still in the boost position.

You'll have to fish the vacuum line from the VNT up and out to check it. It has a connection spot along with the intake port control and EGR cooler flap control connections up near the firewall. So if you find a bad spot, you can just replace that one section down to the VNT.

Ok cool. I'll check that this weekend.

Yay!!! Turbo issues fixed with new N75! I used the MightyVac to put vacuum to the vacuum hose going from the N75 to actuator and saw that I had lots of actuator movement when in the 5-20inHg range. Replaced the N75 and I get some nice VCDS graphs of Boost Expected vs Boost Actual now!



However, I still have an O2 sensor issue. I'll start a new post on that one.