Finally, it works! (Or: It's working!)

A4 2.5 TDI, manufactured in 1998, with 120,000 km mileage.
So far: poor cold start, occasional power loss, oil leak, high fuel consumption.
Solution: New timing belt (the old one was stretched, causing incorrect injection timing), new glow plugs (5 were defective, 3 threads stripped when removing them), the crankcase ventilation filter was clogged, resulting in overpressure in the crankcase, and millimeter-thick layers of tar and oil were removed from the intake manifold.
I have deactivated the AGR system (using the vacuum hose).
Effect: Improved airflow in the lower RPM range, reduced turbo lag, resulting in sufficient power even below 1500 RPM, and hopefully better fuel economy (as I often drive at partial load, on Swiss highways, etc.).

hansemann2,5 wrote:

3 threads stripped out when unscrewing

Hi,

It happened to me too! How did you fix it?

Best regards, Christian.

Hello,
I didn't make that myself.
The tap was thoroughly lubricated with grease, properly aligned, and carefully re-cut, and it's finished.
The next change is likely to cause bigger problems...