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.

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TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth