Automatic Gearbox: Engine Changes Gears Itself

Hello everyone,
I have a Hyundai Starex (year 1999) with an 80 horsepower diesel engine. During longer trips, especially in the mid-range (around 2000-2500 RPM), the engine starts to sputter slightly. It accelerates and decelerates without me touching the gas pedal. Usually, when the engine speed goes above 3000 RPM, the sputtering stops.
The fuel injection pump is the prime suspect, but what could be wrong with it? Perhaps there's an electronic component malfunctioning, but that's not something that's apparent from the user manual.
Does anyone else have a similar problem and know the solution?
Sure, here's the translation:

'Thank you in advance.'

Hello Womro,

Does the car actually become faster in case of a fault, or does only your instrument cluster display an increased RPM? If the RPM in the instrument cluster increases while the speed remains the same, and the engine noise also changes accordingly, I suspect that the clutch is slipping.

Hi Marko,
The clutch can definitely be ruled out. The jolt forward reminds me of the acceleration of a jet taking off. I can't achieve that kind of acceleration with the accelerator pedal.
If you disengage the system during this phase, the engine speed will fluctuate between idle and approximately 4000 RPM.

A truly interesting bug!

So: If, in the event of a fault, the acceleration is significantly greater than what can normally be achieved with the accelerator pedal, then this (just thinking out loud) means:
1. More energy is converted.
2. Consequently, more fuel (diesel or oil) needs to be injected into the combustion chambers than usual.

I immediately wonder if your device produces soot in case of a malfunction. If so, is it blue or black soot?


The engine starts to sputter .
The sudden forward surge reminds me of the acceleration of a jet taking off. I can't achieve this kind of acceleration with the accelerator pedal.
Nice description for 'slightly crazy' .


To what extent does the engine still respond to the accelerator pedal in the event of a fault?

Is this dependent on temperature? Is it dependent on load? Is it dependent on speed/gear?

It sounds a bit like the jerky behavior of a 1Z engine, but at different RPMs and much more extreme. A sticking/difficult-to-move fuel metering unit in the pump and an unfavorable control strategy of the engine control unit could cause something like this, but it shouldn't only occur during longer drives in that RPM range.

But I can't really think of anything suitable to explain the stronger acceleration than 100% pedal. Could you qualify the acceleration a bit more? Is it closer to 110% pedal or closer to 200%?

Regards,
Private.

My suspicion is that oil is being drawn in, either through the crankcase ventilation system or the turbocharger.
My daddy had this on his Renault 11 (God rest his soul, that Renault!).

Regards,
Hermann.

So that the topic can be concluded and other users can also benefit from it.
The car had black rust and was shaking uncontrollably.
It turned out that the control line for the fuel injection pump had a loose connection at a connector. (Unfortunately, the good advice from the mechanic came at a high price.)
Hyundai even recommends removing these stackers and soldering the wires.
If this action doesn't help, the fuel injection pump will be removed and refurbished as a last resort – I hope I don't have to deal with that.