Coolant Temp Sensor Failure: Jerking/Hesitation?

Hello everyone!

My Audi (270,000 km) was at the VAG workshop due to engine misfire issues. They replaced the coolant temperature sensor, and the problems seem to be gone now, but could the sensor really have been the cause?

The symptoms were most likely related to misfires and occurred during strong acceleration, starting around 2,500 RPM. The term 'bucking' describes it quite well.
It started with the car not reaching its maximum speed. The acceleration was good (as far as I can tell) up to about 180 km/h, then there was a sensation, like driving through a large puddle, with a slight braking effect and a speed reduction of about 5 km/h. After that, the acceleration resumed up to about 180 km/h, and this pattern repeated itself.
A few days later, the engine started to buck. This didn't happen when accelerating gently, but it was most pronounced when accelerating at full throttle.

Due to the initial lack of power, I simply tapped the diesel filter (it had been in for 40,000 km, using biodiesel!), and I also mentioned this to the workshop. Since I had already purchased a new one, they installed it there; then they also (or perhaps before?) read the error memory, checked the intake hoses, read the measuring blocks, checked the injection timing, inspected the valve spray adjustment, and finally replaced the temperature sensor in the cooling system.

(I'm listing all the points here because they all appear on the invoice... Is it actually common practice to calculate each measurement separately using the diagnostic computer )

Okay, here's the translation:

Now, to my question: Could this sensor have been responsible for the symptoms? I know that the coolant temperature influences the target value for the injection timing (to compensate for the longer ignition delay in a cold engine), but that doesn't explain the rough running, does it? (I'm not even questioning that the sensor was faulty.)

Regards,
Udo.

Hi Udo,

It also seems unlikely to me that a faulty coolant temperature sensor could cause the engine to stall.
Unless, perhaps, there's a glitch that causes the injection start to fluctuate, for example, switching back and forth between the values for a cold and a hot engine, or that somehow confuses the engine's computer.

But then, wouldn't you likely be able to notice a temporary change in the sound of the machinery (louder / hammering)?

Hello Ulf,

'The symptoms are definitely gone now! Could a clogged fuel filter cause those intermittent issues? It would certainly explain the original lack of power, but what about the stalling? If the ESP system gets an air bubble, could that lead to the short-term interruptions I described?'

Well, it's working perfectly again, but I'm a little concerned about the price of the diagnosis (~150 EUR).

Regards,
Udo.