How to adjust idle speed on OM603?
 

I think mine's a bit low... I couldn't find the idle speed adjustment nut on the injection pump. I guess I'm used to the layout of the old 617. Any ideas?

Idle speed is electronically controlled on this car, there is no adjustment on the injection pump.

There is, however, a resistor switch which can be changed to adjust the idle speed up or down. Look on rear side of the forward firewall, near the brake booster. There is a black electrical gizmo there - looks kind of like a rotary switch. It's not, so don't try to turn it. You can pull it out. It can be inserted in five, or perhaps seven (bad memory...) positions. Each one causes a different idle speed. Note where you started. Each step represents ~40RPM more or less from the baseline of 630RPM. There is a thread on this topic at http://www.peachparts.com/shopforum/diesel-discussion/27135-1987-300tdt-observations-questions.html?highlight=630rpm
Good luck.

This ?
 

ELR resistance trimming plug!

Its Here!
 

Driverside firewall on an 87SDL.

R values!
 

Chart from a 603 Manual.