On my 1987 300TD the engine is rough at idle when in "drive" and smooth at neutral. I suspected motor mounts but by inspection and shaking the engine they seem fine. The car has 104K miles. If when in "drive" at idle I put on the parking brake and give it just a little fuel the idle smooths out. I would like to increase the idle but not sure how. Help please.

Hello, There are several things to check. Yes, motor mounts new check the flex disk, also check the shift linkage bushings.
Those are parts that are wear items. Also does the rpm on the tach seem to drift?

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92 300D 2.5L OM602 OBK #59

With the engine idling, disconnect the ELR actuator on the tail end of the IP. If rpms don't drop, you have an idle control problem. It might be a bad connection or a faulty OVP relay. Faulty OVP relay usually means no tach, AC or cruise control. If rpms drop... I've never had to deal with that There's an ELR trim knob near by the brake master cylinder. Try a higher number setting. Pull the knob out, turn to another setting then push the knob in.

Sixto
87 300D

What does ELR stand for and what does it do. I changed the adjustment nob from a 4 to 5 as suggested. It may have helped a little. Will have to drive the car more.