Audi 2.5 Problems: Common Issues & Solutions

"Hi, my Audi A4 2.5 TDI 132kW, manufactured in 2002, is acting up. On longer drives, it switches to limp mode after about 30 minutes. After I turn it off and restart it, everything is normal again. This happens repeatedly, and sometimes it can drive 500km without any issues. The car has hardly been driven in the last 6 months. The fuel consumption is also a bit higher. The error memory shows the following:"
- Engine control unit for fuel injection pump, part number J399, no communication.
-Engine control unit locked 035.
- Intake manifold pressure occasionally exceeds the specified limit.
-Needle piercing mechanism for G80.
Intermittent interruption/short circuit after applying positive voltage.

"On Friday, I visited two specialized workshops. One wanted to replace the turbocharger and mass airflow sensor, with a cost of 2300 euros, which made me skeptical because the turbocharger was only replaced 50,000 km ago. Then, I went to another specialized workshop for a second opinion, and they wanted to replace the fuel pump after reading the error codes. However, since I needed my car today, I didn't do anything. Today, I was on a business trip, driving 700 km, and the problem occurred again twice. Could it also be that, because the car hasn't been driven for a long time, it's simply clogged up?"

Does anyone have any idea what the problem is?

Hello!

Here's a question beforehand: Could it be that the battery was discharged at some point after a long period of inactivity? Are there any other errors found in other control units?

-A problem with the intake manifold pressure can be caused by a stuck variable turbine geometry (VTG) unit.
-Nadenhubgeber von def. Needle lift distributor.
-J399: Fault in the fuel pump control unit (it shouldn't even start with this!).
- Engine control unit (ECU) locked due to faulty reading coil/WFS control unit/key (transponder).

All of these errors, taken together, seem like a bit too much and are implausible to me.
First, query all other "StG" entries, noting ALL errors! Then, delete them and check again after the test drive to see what, when, and how each entry was recorded.

Best regards from Munich!

Hi,
The battery was never discharged, and the engine started immediately after the long period of inactivity, despite the freezing cold (with a puff of black smoke, like an old Eastern Bloc truck).
There are no errors in the other control units.
I took it to my usual mechanic's shop today; he also suspects the turbocharger. He'll put it on the engine dynamometer tomorrow and try to find the cause of the problem.
He said that if it's the turbocharger, he can flush it with a special cleaner. Has anyone had experience with that?

Hello!

Yes, the turbocharger, or more likely its variable geometry turbine (VTG), is probably the cause of the intermittent limp mode.
Cleaning is always a good tip, but flushing it out?? More likely, it's better to disassemble it, clean it, and then reassemble it .

"However, the errors 'J399' and 'NHG' are not related to the turbo... This is giving me a bit of a headache."

Do the errors reappear exactly the same way after deleting the files?

You need to try to reproduce the error before you can know what's causing it. I suspect it's either your turbocharger or its control system.
First, clear all errors, then test.
Try shifting into a higher gear while climbing a steep hill, then accelerating fully (from approximately 1500 RPM to around 3000 RPM). If it then goes into limp mode, it's likely due to the 'Turbo' system. It doesn't necessarily mean a new turbocharger is needed; you need to investigate the cause. It could be a stuck or malfunctioning VTG (Variable Turbine Geometry), a faulty vacuum sensor, or something else.
Get yourself a helper, and then take a look at the following:

When the engine is off, check the VTG (Variable Turbine Geometry) on the turbocharger. Make sure the rod is all the way down; press it down with your finger and check if it was in the lowest position. Then, the assistant starts the car, and the rod should move smoothly upwards until it hits the top stop (you can press it upwards with your finger to make sure it's fully engaged, but be careful, as the engine may be hot and you could get burned).
After that, you can report back.

Thanks for your advice. I'm eagerly waiting to hear what the mechanics at the workshop say tomorrow after they run the engine on the test stand. I hope it's just the turbo that needs cleaning. It could also be the injector nozzle, since the fuel consumption is also higher. Roughly, how much does a new injector nozzle plus injector nozzle control valve cost?