G4 AHF Engine: Jerking at Low RPM

Hello everyone,

My sister's G4, with a 1998 AHF VP TDI engine and MSG 038 906 018 J, has been experiencing the following problem for some time, which seems to correspond to the "jerking" issue mentioned in the forum guide for VP engines; however, the tips provided there have not yet resolved the problem.

When driving at low RPM (regardless of whether it's 1st, 2nd, or 3rd gear), approximately between 1100 and 1300 RPM, the engine starts to jerk violently. As soon as you give it a little more gas, the problem disappears immediately.
Secondly, the engine jerks and vibrates when idling, especially after driving at higher speeds on the highway and then switching to idle, for example, in a parking lot. If you turn the car off and immediately back on, this second problem disappears. This second jerking is different from the first one, which occurs at low RPM; it's similar to what you'd experience with a gasoline engine where a cylinder has failed. The vibrations are transmitted throughout the car, including the steering wheel, etc. However, it's not a limp mode or anything like that, because you can simply drive away and have full power until the next traffic light, where the jerking starts again immediately.

The problem (both issues combined) occurred approximately at a mileage of 140,000 km. The timing belt was replaced as recommended at 90,000 km. I have now visited several Bosch service centers in W and D: The first time, "something was adjusted on the engine control unit," which reduced the intensity of the jerking, but it did not disappear. At the second Bosch service center, now at a mileage of 160,000 km, the fuel injector was disassembled, and a "broken tab" (I will provide the invoice with the exact diagnosis tomorrow) was diagnosed and repaired. At the same time, I also had the timing belt replaced again.

Despite this, the problem still persists. Additionally, the second Bosch service technician read an error code: "00539 - Fuel temperature sensor (G81) 29-00 - Short circuit to ground." Consequently, during the second visit, the fuel temperature sensor, which is presumably located somewhere on the valve cover, was replaced.

Today, I borrowed a VAG-COM device (after reading extensively here about what it can do) and logged the measuring blocks 3.8 and 11. These logged data represent the exact situation of the jerking at low speeds (the jerking on the highway couldn't be reproduced due to weather conditions). The driver gently presses the accelerator pedal and maintains the speed at the indicated RPM.

At the same time, I found the aforementioned error code in the error memory again today and cleared it, but it reappeared 5 minutes later. I also logged the fuel temperature, which was normally around 30-31 (degrees?), but occasionally spiked to over 40 for a short time before returning to the average temperature (is this normal?). However, there doesn't seem to be a temporal correlation between the two.

I've already tried the DIY troubleshooting steps (adjusting the idle setting in channel 1). The displayed 5-digit value could only be changed so that the emissions reading varied between 7.2 and 9, but the problem persisted. Additionally, the pump was also properly adjusted again, according to the second Bosch service center in Germany.

I now hope that someone here can give me the crucial tip as to what is causing the jerking. The Bosch service wants to replace the entire mass airflow sensor next (another €400), which seems unreasonable to me, especially since it was supposedly correctly adjusted, repaired, and cleaned. Incidentally, the mass airflow sensor has already been replaced (and the values seem to be okay). I have also ruled out a faulty throttle body through the disconnection test; even a tractor jerks .

The graphics are located at /viewtopic.php?p=101954#101954.

Thank you for your help.