AUDI 80 ABK - Severe shaking/vibration at low RPM in all gears - FOUND

...and the CO level is very high! It's a 1993 model (manufactured in October) with 132,000 km on the odometer.
The oxygen sensor was measured with a multimeter, and it's fluctuating nicely between 0.1 and 0.9 volts.

Air mass sensor cleaned, tested while slowly moving the valve; no interruptions observed.
Throttle valve position sensor measured, OK.

Is there a known recurring problem? For example, some newer models have a four-sided ignition coil that malfunctions. This Audi has the older ignition module.

Vacuum leak test with brake cleaner: OK.
Exhaust manifold to oxygen sensor: OK.
Cat not clogged.

Hi,
It could be a temperature sensor. It's not uncommon at that age. You can simply simulate a warm engine by using a suitable resistor in the connector. Normally, the engine control unit (ECU) is also diagnostic-capable; unfortunately, in my case, it only works when the engine is not running...

Best regards from Berlin,
olli

Hi,
I once had a similar issue with an even older Audi 80 model. It had rough idling, unstable performance, and poor emissions readings. The mechanics struggled to find the problem. I eventually cleaned the air flow sensor myself, checked and resealed the fuel injectors, etc. In the end, it turned out to be the temperature sensor.
hg
Herbert.

That such a simple temperature sensor can make so much of a difference .
I will check it!

Heukera, the Audi is now running great . Finally, it's drivable again.
Exhaust fumes now smell like a properly functioning lambda control system!
The temperature sensor was working correctly. The problem was with the relay number 30, which is located under the steering column. It had a cold solder joint, just like in cases with AFN and 1Z relays, specifically number 109.
I noticed that all the voltages on the MSTG side fluctuate wildly during the motor-saw cycle, even though the voltage at the battery remains constant at 14.2V. So, I checked the supply voltage of the MSTG, and sure enough, it fluctuates between 5-14 volts.
Caused by relay number 30.

It's amazing that the good old, simple Digifant has stood the test of time so well!

Unfortunately, I don't have a 2+2 cable for the classic cars , so I can't perform a VCDS diagnosis.
Would that have made a difference for me?

Thank you for the feedback!