Intermittent power loss on a Passat TDI PD 115PS

Hello everyone!

My Passat, manufactured in 1999 (with parts from the year 2000, according to the mechanic), and equipped with the 'Pumpe Düse' engine with 115 hp (code AJM), no longer delivers its original performance, despite numerous visits to the workshop. Specifically, when driving uphill, it often experiences misfires in the range of 2000-3000 rpm, which are so strong that it feels like someone briefly touched the brakes. Strangely, it seems to only go about 20 km/h slower when I can maintain a constant speed, but it suffers a roughly 50% power loss when I need to briefly release the accelerator and then want to accelerate again. The mechanic's diagnostic tool shows no error codes. The vacuum pressure of the 'pre-charging pump' is at the lower limit.
Please provide the text you would like me to translate from German to English. I will then provide the translation.

Hello,

Are you sure the diagnostic device didn't show any errors (some technicians might quickly delete unwanted errors)?
Did the workshop perform a test drive? (You can find the data for this in the error database under "target values").

The pump in question is a tandem pump (vacuum and fuel). More important than the vacuum would have been the fuel pressure in the first place.
Are all the filters working properly?

How many kilometers are on your engine? Have you done any modifications or tuning? Do you use biodiesel?

Hello Michael II,

Thank you for your reply. The last mechanic I went to is genuinely motivated and I'm sure he wouldn't try to cheat me. Furthermore, I personally rode with him, so unfortunately, it seems like the diagnostic tool isn't reporting any errors. Apparently, this device is not querying the pump, which is why it was specifically checked.
The filters have been replaced, and all the pipes have been cleaned.
The engine has approximately 72,000 kilometers on it. No modifications of any kind, and never biodiesel.
The engine's performance is noticeably declining month after month. For example, today it could only manage 110 km/h on a hill, whereas it should be able to do 160 km/h. In addition, the occasional misfires are annoying, and I only dare to overtake on the highway now! The fuel consumption is also getting lower, so I'm starting to suspect that I might have a fuel supply problem.
A month ago, I was at an intersection, about to pull out, when the preheating indicator came on and the usual beep sounded. The engine was running, but I couldn't move forward due to a lack of power. After restarting, it worked again as if nothing had happened.
Should I take my car to a tuning shop, hoping they have better diagnostic equipment than the VW workshops?

Best regards,

Armin.

The diagnostic devices will be the same, as there isn't much of a selection, but what's more important is who operates them and how the measurements are taken.

Perhaps you could get the measurements taken that are described in the error database for your engine (while adhering to the conditions), and post the results here.

By the way, the VAG-COM + adapter is also worth its money... you can do things like this on your own with it, and you get roughly the same output as the VAG 1551 (and even more).

Best regards,

Jan.

Hello,

If the preheating indicator light is on and everything is okay after a restart, it indicates emergency mode. In all the cases I'm familiar with, an error code is stored in the engine control unit.
A decrease in fuel consumption doesn't necessarily indicate a problem with the fuel supply.
As I said, in this case, a properly calibrated measurement run is really the only thing that will help. Then, you can further narrow down the source of the problem (e.g., injectors, mass airflow sensor, etc.).

Make sure to carefully inspect the mass airflow sensor.
It must be tested with a diagnostic device at 3000 RPM and full load.
See you, Thomas.