Idle speed fluctuates in the semi-warm range

Hello!

Recently, I've noticed fluctuations in the idle speed of my TDI ACV engine. It only happens when the engine is somewhat warm. Also, briefly pressing the accelerator doesn't lower the RPM. So, this phenomenon only occurs at a very specific temperature. What could be the cause? Some value must be incorrect, or could it be related to the disconnected EGR valve??? Since my VAG adapter is unreliable, I can't read any error codes.

Greetings,

Peter.

Mc_Givertechnik wrote:

Recently, I have observed fluctuations in the idle speed of my TDI ACV engine. This only happens when the engine is partially warm. Even a brief press on the accelerator does not lower the RPM. Therefore, this phenomenon occurs only at a very specific temperature. What could be the cause?

Hi Peter,

According to experiences shared in various forums, engine temperature sensors sometimes behave erratically, providing incorrect values only within specific temperature ranges.
An increased idle speed is normally regulated when the engine appears to be very cold.
Or if the throttle position sensor is detected as defective... does the car react strangely to the accelerator pedal, or not at all sometimes?

Hello Peter,

...Even a brief burst of acceleration doesn't reduce the engine speed...

I suspect that what you mean is that the engine speed returns to a too-high value even after the foot has been taken off the accelerator.

This strongly suggests the temperature sensor, as Ulf already mentioned. Does your car also take a longer time to warm up when it's in this state?
In the event of a detected sensor failure, the control unit assumes that the engine is cold, initiates the glow plug system (which normally only activates below 10.5°C), and increases the idle speed.
It's possible that the transmitter is experiencing a short circuit or an open circuit to ground at certain temperatures.

Best regards, Rudi.

Hello Rudi, hello Ulf!

"The temperature sensor? I suspected something like that. Otherwise, the car runs pretty well, no noticeable issues."

Which sensor does the control unit obtain this value from? Is it only the water temperature sensor in the radiator, or somewhere else? The temperature reading on the Climatronic display shows correct values, so if it were coming from there, it shouldn't be the air temperature sensor.

Thank you, Peter.

Mc_Givertechnik wrote:

Which sensor does the control unit obtain this value from? Is it only the water temperature sensor in the radiator, or somewhere else?

Hi Peter,

The most important value comes from the coolant temperature sensor.

In this context, the other components likely only serve to justify a (programmed) suspicion of a defect; for example, a disconnected water supply valve won't necessarily improve warm-up performance even with a hot pump.