TDI Passat: Faulty MAF Sensor & Head Gasket

Hello everyone,

I'd like to tell you about what has been happening to my Passat TDI (AFN engine, Passat Variant model year 1999, 160,000 km) over the past few months.
It started in January when I noticed that the car was no longer reaching its top speed. With a lot of effort, it could still manage 180 km/h. After that, it stopped. My mechanic didn't have any equipment to read the error codes or perform road tests, so eventually I went to VAG (a car dealership). The error memory was clear, but the road test showed: the mass airflow sensor (MAF) was still okay, but the boost pressure increased too much at the beginning (over 2.3 bar, if I remember correctly) and then dropped because the electronics reduced the amount of diesel being injected. I left the car with VAG for a day. The result was about 100 euros for their assessment: either the turbocharger or the control valve for the variable turbine geometry (VTG). However, they didn't want to replace any parts, as they said it wasn't that bad yet.
What I also noticed after that was that even slightly reducing the throttle at top speed (175-180 km/h) still provides a small boost. Also, the fuel consumption at top speed was always around 10 liters per 100 km. Before, it used to be 12-13 liters!
Okay, a few weeks ago, I noticed that the coolant level was below the minimum mark. I topped it up, and after a couple of hundred kilometers, it was low again. Also, the engine performance has decreased further. I had the turbo pressure checked, and it wasn't okay in one cylinder. => My suspicion was a blown head gasket. My mechanic replaced it. In one cylinder, they saw that it had been slightly washed out by water.
The performance still wasn't quite 100% after that. So we went to the Bosch service center and had them perform a diagnostic test. The result: in 90% of cases, it was the mass airflow sensor (MAF). Because as the engine speed increased, the measured amount of air eventually decreased, or didn't increase proportionally. We replaced the MAF sensor, and since then, it's running like new again, and it hasn't used any coolant either.
Conclusion: While the engine power was still somewhat acceptable before, who knows what further damage might have occurred if I had waited. Therefore, it's always a good idea to pay attention to how the engine is performing.

Best regards, Stefan.