On your 86 300sdl, make sure you have the latest trap oxidizer, the 3 inch diameter one, not the large 9 inch diameter canister. That change on my 87 sdl gave me what seemed like 20 HP, and added 3 MPG to my fuel economy. The upgrade was free from the MB dealer. Other than that, make sure all the filters are clean, and you might pull your injectors and have them tested.

It doesn't apply to you but on my 240D, the fan is direct drive with no fan clutch. I've heard of guys putting a fan clutch from a 300D on their 240D to gain what must be about 3 HP.

Joe