There is a vertical adjusting screw that uses a locknut which a 9/16 deepsocket fits over. Below that vertical idle adjustment screw, I see a small plunger and it is rusty.

I tried turning down the idle all the way and I still cannot manually turn off the engine.

Tachometer just stopped working but it was at around 950/1000rpm when last working. However it idled normally then after you stepped on the gas it would go to the high idle.

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'84 190D 2.2 OM601.911 144K