What would cause fouling across entire cyl bank (non-merc tech question) - Page 2 - PeachParts Mercedes-Benz Forum

Zulfiqar · #16 07-25-2022, 12:05 PM

Quote:

Originally Posted by tbomachines

Checked everything for vac leaks that I could think of again, and nothing to be noted. I replaced the MAF today with a brand new OEM unit and it seemed to run smoother,

I was able to hook up VCDS (VW/Audi diagnostic software) and pull data logs from about a 5 mile drive around town. I logged short term fuel trim for bank 1 (the side that appears to be functioning properly) and bank 2 (problem side). From what I see, the O2 sensor reads mostly correct for a period of time, and then it basically flatlines at 1.0 lambda with small variations around it. This means its keeping 14.7:1, but during WOT it should be reading <1.0. The 2.0 spikes are from liftoff when the injectors are basically shut off. Bank 1 registers these spikes, but bank 2 keeps it at 1.0.

What would cause the O2 to be functioning properly for a short amount of time (roughly 3 mins, car was already up to temp), then practically go comatose? I am not fully convinced that the o2 is the culprit here, since its already been replaced 3x. I did get a P0153 pending code for low refresh/update below threshold.

Charted on a graph, Blue is bank 1, red is bank 2.

Check the cat on bank2 - its probably clogged. A clogged cat shows lean activity too. Are you getting lean on both pre and post cat oxygen sensors on bank1?

tbomachines · #17 07-25-2022, 05:09 PM

Quote:

Originally Posted by Zulfiqar

Check the cat on bank2 - its probably clogged. A clogged cat shows lean activity too. Are you getting lean on both pre and post cat oxygen sensors on bank1?

yeah i assume the cat is toast now. Post cat o2s are coded out (tuned ecu) so no reading on them. If I need to dig more I can take a voltage and duty cycle readings with my DMM. I could always gut the cats if it is an issue -- the WOT trims are not too bad though, so unless they creep up, I am going to leave it as is.

Zulfiqar · #18 07-25-2022, 05:54 PM

Quote:

Originally Posted by tbomachines

yeah i assume the cat is toast now. Post cat o2s are coded out (tuned ecu) so no reading on them. If I need to dig more I can take a voltage and duty cycle readings with my DMM. I could always gut the cats if it is an issue -- the WOT trims are not too bad though, so unless they creep up, I am going to leave it as is.

an oldskool fluke graphmeter can scope an o2 sensor too, with a DMM you will be seeing average value and that too delayed quite a bit.

you can get a cheapie DIY DSO scope and check the O2 sensor that way, its pretty reliable for such simple jobs. the sensor only shoots a max of 0.94V DC.

For testing the cat, you can make a port tester with a busted or dead oxygen sensor, gut it and weld/jbweld a pipe nipple on it then stick it in the hole for the primary sensor. Connect an oldskool compound gauge to that and check the pressure at idle and throttle burst.

tbomachines · #19 08-21-2022, 04:15 PM

Well I put about 500 miles on it and the sensor is giving me errors and car is running like garbage again. This time I am getting zero lambda reading back, which is odd. Voltages are consistently reading normal values at a regular rate. It tends to give lambda correctly up to a couple minutes then it zeroes out and seems like its an open circuit. I am not sure if the car is going into open loop mode for that one bank or something, doesnt really make much sense. I am thinking maybe I can do an ECU swap, this one had been professionally tuned in the past but its got 100k on the clock with it, so it would be strange for the ECU to crap out. Other thought would be to test wiring to the ecu pinout for continuity, but also seems unusual considering it is working fine. Still no coolant or oil burning.

I should have mentioned this is a 6 wire (wideband) sensor not the old fashioned switching narrowband.

tbomachines · #20 05-07-2023, 10:53 AM

Some updates on this one. I replaced all the O2s, cats, and pretty much everything that could be replaced on the car related to fueling. Originally I think the poor running was due to bad/worn injectors and a leaking pcv system.

Ultimately, PO had replaced all the leak-prone vacuum lines with silicone, but I think he had hooked them up slightly wrong. After doing the PCV, I think I made the problem worse and accidentally tee'd in the wrong joint. This car has 2 EGRs, and the vac tee caused them to get uneven vac distribution. Strangely never got any emissions or egr related codes, but did cause uneven fueling as I think the EGRs (called combi valves in audi world) were opening separately and at inappropriate times. I broke out the manual last night and line by line, tee by tee got the vac back to factory spec. It seems to be running great. I think I still have a leak somewhere since the car adapts to the rich side, but its more even and within range of ECU adaptation.

At this point, half the car is new so hopefully itll take many more miles. I am cautiously optimistic at this point, its been a tough 6 months and Ive been traveling quite a bit for work, which requires 120-150mi drives to the airport and back. Its a 6 speed with a buttery smooth revvy 6cyl and a resonator delete, so when it runs, its a hoot.