Performance Issues: Troubleshooting Guide

Hello,

I drive a Golf IV with a 90 HP TDI engine. For some time now, I've noticed that I occasionally experience a loss of power. Sometimes it pulls without any problems, but other times it pulls well up to 120 km/h, and then it takes a long time to reach 140 km/h. After that, the acceleration disappears; even when I'm giving it full throttle, it slows down. If I start pumping the accelerator, it sometimes seems to recover, but not always, and I never reach the top speed anymore. It's not acceptable that I have to constantly pump the gas (which I don't think is good either)!! What could be the problem? I have an appointment with VW soon, and I don't want them to tell me some nonsense and start replacing parts that end up costing a lot of money and don't fix anything. Oh, and the Golf was manufactured in 2000, and I was promised some goodwill because it's not yet 3 years old. Thank you!

Hi.

Loss of power at higher RPMs is often caused by excessive boost pressure, which then triggers the engine's limp-home mode.
However, with a car manufactured in your year, it's usually no longer possible to deactivate it by repeatedly pressing the accelerator pedal.

If there's no noticeable (hard) jerking, I could still suspect the mass airflow sensor (MAF) as the cause. However, it usually malfunctions constantly, not just occasionally, which would then point to a problem with the boost pressure.

If it's producing a lot of black smoke during problems, it could again be due to an EGR valve that is intermittently sticking open...