Wave-like acceleration: Causes in TDI engines (Articles)

If the TDI engine behaves as if the accelerator pedal is being repeatedly released and pressed again, even when the accelerator is fully depressed, then when reading the diagnostic data, you will likely find only fluctuating values for parameters such as boost pressure, air mass, and fuel injection quantity, rather than a clear indication of the actual cause.

Most likely, these will be normal pressure fluctuations caused by other processes that enable or amplify them.

This can happen, for example, when the soot reduction system is active during driving (instead of the driver's preferred setting). Torque limiting (see /viewtopic.php?t=8670).

Then, any change in the airflow will result in a change in the amount of fuel injected.
The airflow into the intake manifold is, in turn, highly dependent on the boost pressure. So, any change in boost pressure, when the soot reduction system is active, results in a corresponding change in the fuel injection quantity, which in turn causes a (more or less noticeable) change in engine power.

Control intervention -> reduction of boost pressure -> lower injection quantity (= acceleration decreases) -> reduced turbocharger drive reinforces the initial control intervention -> the boost pressure can drop significantly below the target value, which triggers a control correction to increase the boost pressure.
Control intervention: Increase in boost pressure -> larger injection quantity (= acceleration increases) -> more turbocharger drive reinforces the initial control intervention -> the boost pressure can increase significantly above the target value, which causes a control correction to reduce the boost pressure -> see again above at control intervention: reduction in boost pressure, etc., etc.

That completes the loop, and we have a self-reinforcing oscillation of boost pressure and fuel injection volume – something that could potentially happen in any TDI engine.

In engines that are functioning correctly, potential "torque surge" is generally prevented by interrupting the feedback loop between air mass and fuel injection – by using a lower, fixed torque limit or the driver's desired power output to determine the maximum fuel injection amount, regardless of (moderate) boost pressure control.
In this way, the EDC can quickly and smoothly adjust the boost pressure without being affected by feedback effects.


Possible causes for the camel driver effect (in approximately decreasing order of frequency) include:
a) faulty mass airflow sensor
b) Extreme pressure surges, for example, caused by sticking solenoid valves or severe binding in the charging mechanism, and strong control corrections can cause the soot limit to be activated during pressure dips.
c) Blocked intake and/or exhaust passages result in low mass airflow (MAF) values.
d) The boost pressure is regulated at too low a level (e.g., due to a faulty boost pressure sensor).
e) Ineffective cooling system, e.g., due to a blocked air intake, under unfavorable conditions (e.g., (Heat + high altitude) results in low LMM values.
f) Chip tuning, which is designed to ensure that the soot reduction system (at full load) is constantly active.

In case a), with aging LMMs (Linear Measurement Modules), the measured value usually decreases first in the uppermost measurement range.
Then, initially, the characteristic oscillations can only develop in the higher speed range.
As the "camelback" effect (a phenomenon related to engine performance) becomes more pronounced at higher mileage and extends towards lower RPMs, the likely cause is an aging (or becoming dirty) mass airflow sensor (MAF).

However, not all TDIs exhibit this rocking motion when the particulate filter regeneration is active, because the feedback loop can also be mitigated by designing the charge pressure control parameters of the EDC with a certain tolerance for small deviations between the target and actual values.
These regulatory errors are no longer (or very slowly) corrected, to the point where the overall dynamics of changes in boost pressure and injection volume are no longer sufficient to trigger a self-reinforcing oscillation.


These representations are only a thought model that, according to the author's knowledge, corresponds to the experiences of practice.
Corrections and additions are explicitly welcome!