Audi A4 B7: Limp Mode & Smoke - Troubleshooting

Good evening, everyone.

After my Bora was running again, my supervisor approached me and asked for advice regarding his Audi A4 B7 with a 2.0 TDI engine and 140 horsepower.

His problem: The engine goes into limp mode under load, and there's a white-grayish smoke that increases with the engine temperature.

We took a short test drive, and it's clear that the boost pressure is significantly too high, and despite adjusting the solenoid valve, it doesn't decrease. While stationary, we ran the "VTG gymnastics" sequence several times. The turbocharger itself responds very cleanly. During another test drive, the boost pressure again exceeded the target value considerably and then triggered emergency mode. There's a corresponding error entry in the fault memory: "Boost pressure regulation limit exceeded."

He had already taken the car to an Audi authorized service center, where they checked the turbocharger. According to their assessment, the bearing play is perfectly fine. They only replaced the vacuum hose (leading to the VTG valve) and pointed out that the smoke was likely caused by oil in the charge air hoses – they suggested he should "simply let the engine rev freely." This isn't really possible with the limp-home mode activated.

All other components were in perfect condition. The temperature sensors were all within the expected range, the particulate matter (PM) sensors showed almost no deviation from 0 +-0.2 mg/H, and the diesel particulate filter (DPF) was only 20% full. Otherwise, the car runs very smoothly, and there are no unpleasant noises or anything like that.

Has anyone ever experienced something like this before, or are there any known common weaknesses in this type of engine?

Tomorrow, I'm giving him a vacuum pump so he can re-check the vacuum system and the variable geometry turbine (VTG) separately.

Thank you and best regards,

Timm.

A normally functioning valve timing gear (VTG) can still seize up at different temperatures.

Has he driven a lot of short distances?

We got a "used" Galaxy TDI from 2005 for my girlfriend this weekend. The previous owner also had limp mode issues. After driving the 300 km to get it home, and clearing whatever was causing the problem ("Rohr frei" - likely referring to cleaning out the exhaust), everything seems fine again, even under heavy load in 6th gear on a steep incline.