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

tbomachines · #1 07-09-2022, 11:44 AM

Thanks guys! He is chasing down vac lines (there are a lot of them on this engine, although theyve all been replaced with silicone at some point) and intake leaks. For me to buy this car I mostly want to rule out a cracked head, block, or HG as I don't have time to fix that. Sensors, fueling, and normal maintenance all to be expected in my book (its a 20 year old german sports sedan, I know what im getting into there). The 3.0 is known to be a really reliable engine, so fingers crossed. Only fuel that has been in it is 93, definitely no avgas.

Mxfrank · #2 07-09-2022, 11:46 PM

Different engine, but this may help:

Zulfiqar · #3 07-18-2022, 05:11 PM

Quote:

Originally Posted by tbomachines

Thanks guys! He is chasing down vac lines (there are a lot of them on this engine, although theyve all been replaced with silicone at some point) and intake leaks. For me to buy this car I mostly want to rule out a cracked head, block, or HG as I don't have time to fix that. Sensors, fueling, and normal maintenance all to be expected in my book (its a 20 year old german sports sedan, I know what im getting into there). The 3.0 is known to be a really reliable engine, so fingers crossed. Only fuel that has been in it is 93, definitely no avgas.

silicone vacuum tubing is usually a recipe for leaks unless you stick them with hylomar or tie them with zip ties.

I found that japanese vehicle factory vacuum hose is best or use the mercedes benz nylon tubing with the black vacuum hoses as end pieces to join them.

tbomachines · #4 07-21-2022, 09:58 PM

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.

Zulfiqar · #5 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 · #6 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 · #7 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.