I'm not 100% sure on that particular car, but there should be a 2-wire connector on the VERY BACK SIDE of the injection pump that controls idle speed. This is virtually the same engine as the OM602 as used in the '87 190D 2.5T's. From this connector on the back of the IP, trace the wires to find what I believe is called an ELR (don't know what it stands for). This will be a round knob that kind of looks like the adjustment knob on an air pressure regulator. Pull out on the knob, then rotate it to control idle speed. The knob is marked with a series of numbers, and there is an arrow to show which number the ELR is set on.

The ELR should be mounted on the firewall somewhere. On my cars (W201's), it is over near the battery.

Hope this helps.

SteveM

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'93 190E/D 2.5 Turbodiesel 5-speed (daily driver)
'87 190D 2.5 Turbo rustbucket - parts car
'84 Dodge Rampage diesel - Land Speed Record Holder
'13 Ram 2500 Diesel
'05 Toyota 4Runner