On short urban trips the particulate filter cannot burn through. The engine control unit then injects extra fuel late in the cycle to heat the filter, and part of that fuel passes down the cylinder wall into the sump. The oil level rises, the oil thins out, and it lubricates worse.
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The maker addresses this itself. Audi's 2015 service bulletin (01-15-18) tells the technician how to read the oil level on 2.0 TDI engines, because an overfilled level triggers an oil change message with no engine warning light and no stored fault code.
An oil level above the maximum calls for an immediate oil change, and it points to short urban trips. A diesel driven only in town is a bigger risk from both the particulate filter and the turbo.
A thinned, rising oil level is one precondition of a runaway engine. A diesel controls torque with the quantity of fuel injected, and it has no throttle. So if enough combustible liquid reaches the intake, the engine starts running on its own oil, and cutting the injection or switching off the ignition will not stop it. The revs climb on their own until the engine destroys itself.
A raised oil level alone is rarely enough for this. The classic trigger is a failed turbo shaft seal that feeds oil mist into the intake. But the two go together: thinned oil lubricates worse, and poorer lubrication damages the turbo bearing first.
If it happens while driving, the key does not help. Only cutting off the air does: cover the intake, discharge a dry powder extinguisher into the intake tract, or on a manual select a high gear with the parking brake on and release the clutch quickly.
What to check
- Pull the dipstick with a cold engine on level ground: the level must not be above the maximum mark
- Smell the oil on the dipstick: a diesel smell means it is thinned with fuel
- Ask whether the dash has ever shown an oil level sensor or oil change message
