What would cause fouling across entire cyl bank (non-merc tech question)
 

Hey folks, I'm considering buying a 2003 Audi A4 from a friend. It has the 3.0L V6 (naturally aspirated) 30V engine. He has been meticulous with maintenance, and generally speaking the car doesn't show it's 174k mileage at all.

Before I purchase, trying to determine whether there is a head issue at all. O2 sensors on one bank have fouled twice within the last 10k miles, and the flex pipe for that side also corroded out. So there is something that is passing through the exhaust and gunking it up. He pulled the spark plugs and there is white discoloration on the tip of all 3 plugs in that cylinder bank. Another friend of mine (does head design and failure analysis for a major OEM) said it was likely oil or potentially a lean condition. The car uses a small amount of oil (quart every 7k or so), no coolant, and has been that way since he got it at 100k. No coolant loss or signs of contamination in coolant/oil.

The part that is bugging me -- a head crack, gasket failure, etc would be extremely unusual to happen across all 3 cylinders in a bank. Most of the other causes I can think of (clogged injector, for example) also would not occur across an entire bank, but only impact one cylinder. AFR/lean burn conditions would likely impact both cylinder banks. Only thing I can think of might be a bad or leaking fuel rail, or possibly a leak in an intake gasket that would primarily impact one bank due to the location. Plugged cat??

Btw only code is the O2 sensor, which we know needs to be replaced. The O2 is in all likelihood a symptom and not a cause since it has been replaced more than once.

Any ideas? Car runs fine until the O2 sensor starts to pull timing and mess with the fueling sicne it is gunked up. It hasn't been driven much in the last 2 years.

Pics of the plugshttps://uploads.tapatalk-cdn.com/202...901ff17015.jpghttps://uploads.tapatalk-cdn.com/202...3b4a6928c4.jpghttps://uploads.tapatalk-cdn.com/202...0d99d7bf7d.jpg

That looks like quite lean.

I had a similar case on a Toyota 3MZ V6 with recent work done to engine, except it was opposite. I was having black plugs and stuck lean codes on the AFR sensor for bank1.

comes out that the mechanic who did the job left the AFR sensor loose and it was sensing lean due to the air being sucked in from the threads. (It was the rear top sensor on a sienna meaning you work with braille only) You may have a sticky injector causing the ECU to pull back fuel on that cyl bank.

Try to plot both pre and post o2 sensor graphs of that bank and see which is which along with the current and learned fuel trims (short and long term) you can then try to simulate the opposite condition with either a spray like carb cleaner or induce an air leak from the evap system and see if that forces the system back to normal.

In my above experience with the toyota, I was seeing rich signal at the post cat sensor and controlled signal at the AFR (literal AFR nor an O2 on toyota) with very rich fuel trims for that bank - confirming that the AFR sensor was lying.

AFR trouble codes are not necessarily just for the whole engine - its dependant on ecu design, meaning if the ecu controls the 2 banks individually or combined (like old GM engines) you also may be having a clogged catalytic converter issue on that bank, it can severely overheat the sensor and also cause this. An air intake manifold leak can happen on one bank only too and then only that specific bank will throw a code.

Thank you! I was leaning toward an intake or PCV leak after doing some more reading. O2 testing probably next to determine STFT and LTFT, but will have to replace the sensor again first, which is a PITA.

Would a smoke test flush out air leaks?

Apparently there is a vacuum leak on that bank causing a dangerously lean condition.

When using the term “foul” I automatically think a rich or oil soak condition, but that picture indicates a dangerously lean condition.

Hope this helps.

If it was an air leak or vacuum hose loose or split, wouldn't a carb cleaner spray like Zulfigar mentioned cause the engine revs to change if sprayed close to leak?

That's what I did to find an intake manifold leak on a 99 Aurora. Otherwise, if the O2 sensor is bad as stated couldn't that cause it? O2 sensor frequent failure would most likely be a different problem ???

What sort of gas was he using? 100LL avgas would foul the plugs and screw up the cat system.

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.

Different engine, but this may help:


https://www.audiworld.com/forums/att...ting-valve.jpg

Well I bought the car. We ruled out any head issues, so I am comfortable taking it from here. Zero vacuum leaks found, we sprayed pretty much every line with carb cleaner and no surges. I have a couple of check valves on order, in case maybe one of those is leaking.

I drove it about 75 miles home last night and it ran beautifully. I am wondering if it had a batch of bad fuel. It was mostly sitting (albeit started periodically) for about 18 months. A tank or two of fuel was added during that time, but I wonder if it went bad. The filter was changed about 10k ago, so thats good. Does not totally explain the single bank of fouling, but perhaps one of those injectors is on their way out and exasperated the problem, theyre all original at 175k. I will be hooking up VCDS and datalogging things when I get a chance. With a full raw log I should be able to find deviations easily once I get back to my desk.

I also picked up a MAF, since the original has never been replaced. At this point, might as well have a spare if it isn't the problem. I have had cars in the past whpse MAF is within threshold but is still reading just poorly enough to throw off AFR or wreak havoc with the O2s without throwing a code. Also grabbed a FPR since theyre pretty cheap, although in my experience a failing FPR will typically run rich.

Not too many other things it could be at this point. Its running really well now, ill probably pull the plugs after every couple hundred miles and see if there is any one cyl off. Ive alsp got a monitoring gauge I am installing to keep an eye on momentary data. Thanks for the help everyone.

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.

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.

That's a real puzzle and quite a way out of my experience.

Only thing I can think of is a wiring/connector issue or the ECM that interprets the signal.

If you don't solve it soon I think you could make a couple of long cables up and criss/cross the two sensors so they each feed the ECM input of the other. Don't know if the ECM smooths out the input readings before it adjusts the fuel amount for each side, but if the bank 1 input showed the same signal corruption it might be the ECM or the wiring harness for bank 2's sensor.

If the bank 2 sensor input connected to the plug for the bank 1 sensor ECM corrupted then it would seem to be the bank 2 Oxygen sensor or it's cable.

Hope this makes sense. May not due to my lack of experience with this type failure.

replaced all of the fuel injectors today, new O2 in, reseated all of the vacuum lines I could find...and its running beautifully. My data logs are showing no fuel trim imbalance. Long terms at idle are very close to zero on both sides, which means no vacuum issues whatsoever, and its balanced between banks finally. On load, the stable side is reading slightly lean (4.7 max, around 3.9 avg) and the bank that was giving me issues is at 1.3, which is great. Both a tad lean so it could be fuel pressure, but unless they creep up toward 10 I am going to sit back.

My guess is there was a combination of crappy fuel at some point, clogged, weak, leaky, and poor firing injectors (more on one bank vs the other) and it was enough not to throw specific codes but to screw up AFR on the one side without a complete burn. That coked the sensor and exascerbated the readings. I also replaced all of the PCV lines yesterday...the new valve honks like a goddamn goose (no joke, thats what it sounds like) but it is functional. The old valve was silent but not sure how well it was working.

Now its time to put miles on it -- will certainly continue logging to monitor...if anything for fun juicy data to sift through (profesionally work in data science engineering so I love this sorta stuff). Couldnt be happier at the moment, hope it stays this way and remains stable.

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.

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.

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.

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.