Idle Speed Fluctuation A4 2.5 TDI

Hi everyone,
Unfortunately, my Audi A4, manufactured in 1999, with a 2.5 TDI engine, 110 kW, AKN engine code, still isn't running as it should, even after several attempts. I've been experiencing major problems for about a year now, and despite numerous investigations and attempts, I haven't been able to resolve the issue.

Problem:
When the engine is left idle for two or three days in cold weather, the idle speed fluctuates. It starts perfectly, then runs at a good 1050 RPM, which slowly decreases. You can then observe that it drops to approximately 950 RPM and begins to fluctuate between 800 and 1000 RPM. The longer it sits, the more pronounced the fluctuations become.
There are no error codes stored in the system.

Here's a quick summary of everything that has already been exchanged:
"Minimum 10 liters per minute (LPM) fuel flow, 2 turbochargers, electric pump (EP), fuel injector with needle lift sensor replaced, complete exhaust system including catalytic converter, timing belt, water pump with thermostat."

Thank you for your suggestions.
Best regards, Sebastian.

Hello,

The sensors in AKN motors are somewhat prone to failure.

Maybe someone is just trying to cause trouble.

What do the measured value blocks say about the idle speed control?
Then there's the famous two-mass flywheel. (Almost impossible to test).
A crankshaft sensor could also be the issue. However, it should trigger an error code in the control unit.
Or the M-STG itself is broken.

Hi,

and also occasionally compare what the temperature sensors on the engine are reporting and whether the values are reasonable.

Best regards, Rainer.

Thank you for the tips, I will try them out as soon as possible.

hello seba,

Rainer is absolutely right. Here are my experiences from last week:

Audi A4 Avant, engine code AKN.

Problem: After a highway drive, the idle speed would not drop below approximately 1000 RPM, and there were fluctuations in the idle speed, ranging from normal to as high as 1000 RPM.

Normally, the engine should be running at approximately 760-770 RPM when it's at its operating temperature.

VAG-Com connected: Error code 00522.

Display coolant temperature using VAG-Com measurement block 1, group 4.
And lo, it was revealed that the temperature fluctuated between approximately 80°C and -41°C, and that the idle speed increased as the temperature decreased, and vice versa.

I bought a new coolant temperature sensor (G62) from the parts store (complete with O-ring and plastic retaining clip, approximately €26).

After replacing the sensor, I checked everything again with VAG-Com, and everything was within the normal range.

Hello,

Today, during a short drive, I had the opportunity to observe the engine speed and engine temperature, although only through the air conditioning system. All the values were excellent.

The problem of fluctuating RPM only occurs when the engine is started, especially if it has been idle for a long time. When the engine is warm, there is no fluctuation, and the RPM drops normally to 760/780 RPM.

Next weekend, I will probably install a new control unit as a trial.

Have a nice weekend.

Next weekend, I'll probably try installing a new control unit as a test.

.

Hi,

'Then it won't start at all. The engine control unit needs to be adjusted to work with the immobilizer. I don't think it's the control unit. I also agree with the idea about the coolant temperature sensor. Does the sensor display the correct temperature value before starting the engine?'

Best regards, Christian.

Hello,

What do your camshafts look like? Your description suggests that they are worn out!


Serge.

Mine does the same thing, but only when it's cold. I start it in the morning, and when it's cold, it runs at around 950 RPM. I get out to close the gate, and then it sometimes drops down +/- 50 RPM and starts to fluctuate, going up and down between 900 and 950 RPM. It does this for about 30 seconds, then as the temperature increases, it drops further in RPM to +/- 850 RPM and then runs "stable" again without fluctuations .
The car has 10,000 km on it, and it has exhibited this problem before (although it mainly occurs with cold outside temperatures, even though it's parked in a garage). The camshafts are new (it also exhibited this problem with the old, worn camshafts, no difference). The temperature sensor is not new, but it shows correct values on the dashboard. Is there another sensor that provides the engine temperature to the MST (Motor System Test), or is it only the dual sensor with the clip?
This weekend, I'm going to connect to the car's diagnostic system (VAG***) early in the morning and monitor the engine temperature.