MKB 1Z Error Codes: Slider Position Potentiometer - G99

Hello everyone!

'For some time now, I've been occasionally getting an error message in my Audi 80 1.9TDI Avant (manufactured in November 1993): 'Potentiometer for slide range - G99 - signal too high'. Sometimes it says '...implausible signal', but more often it's a kind of 'jerking' with increased black smoke from the exhaust. The problem usually appears when accelerating, but it can also occur at a constant speed. The jerking is hard to describe, like the engine momentarily cuts out or doesn't get enough diesel. The increased black smoke, in my opinion, probably indicates too much fuel. What do you think?'

The aforementioned Poti G99 is supposedly located on the fuel injection pump, which was already replaced with a new one after the error appeared, but the error still persists!?
Otherwise, my AGR valve broke, and I tested all sorts of sensors with a multimeter, but couldn't find any faults. I also tested the potentiometer mentioned, using the 'self-help' guide, but didn't find any reason for complaint (maybe there's a loose connection?).
I once cleaned the mass airflow sensor (MAF) and drove the car for a while without it connected, but I couldn't tell if the jerking was caused by the missing MAF or if it was the old problem recurring.

Does anyone have any idea what's causing this error and (even more annoyingly) the stuttering? What else can I check? Should I clean the contacts (which ones, though??) or suspect a broken cable, etc.?

Okay, one more thing: Could the 'Poti' mentioned above possibly be the so-called 'rule enforcer'? I've come across this term while reading some threads, but I haven't been able to understand it fully yet...

Thank you in advance!

Hello,

Yes, that potentiometer is the standard control element, which in newer fuel injection pumps is no longer implemented as a potentiometer, but rather as an inductive sensor.
Reason: The potentiometers were likely sensitive to water or biodiesel in the fuel.

Therefore, here are my questions for you:
- When was the last time you changed or drained the fuel filter?
Do you ever use biodiesel?

As short-term measures, I recommend the following:
- Drain the fuel filter (there's a drain screw at the bottom; let it run until diesel comes out, as water is heavier than diesel and will collect at the bottom).
- Switch gas stations.
- Pour diesel system cleaner into the fuel tank (effectiveness questionable, but it might dissolve some deposits in the ESP).

Best regards, Rainer.

Hello Rainer,

During troubleshooting, I also changed the filter and used LiquiMoly diesel additive... And not just once.

I've never used biodiesel, and I usually go to different gas stations.

As you can see, this appears to be an extraordinary problem.

Sure, no problem. Here's the translation:

'Thank you in advance.'

Hello,

"Ugh, that's bad. A loose connection could be the cause of the jerks, and also for the excessively high voltage reading (e.g., if a cable is intermittently disconnected, the voltage reading for the control unit could drop, it could try to compensate, and then the connection might re-establish).

Unfortunately, I can't offer any better advice than to carefully check the entire cable run and the connector from the pump to the control unit.
Unfortunately, I don't know if that potentiometer is available as a spare part.

Best regards, Rainer.