I have 1999 e320 with 146,000. The car runs good and drives very smooth. However I do have an idle issue. it seems to want to idle around 600 I have adjusted the idle at the throttle and had it to 900 it idled good for about a day and a half and is now back down to around 600 even though the adjustment I made to throttle has not been changed. I am rather stumped on this so any advice would be appreciated!

 

there is no idle adjustment so not sure how youre adjusting the idle. could be many other things causing the problem, you should go to a dealer and ask them. you also can have the idle bumped up at the dealer. you can only raise the idle a few 100 rpms but you most likely have another issue. fuel pressure, mass air flow, air filter, correct spark plugs and gap?

 

I did notice that the throttle didn't have the usual screw that I am used to seeing on most cars so maybe, more correctly obviously (now anyway) I didn't correctly adjust the idle. What I did was turn the a plastic screw type of thing that the throttle cable passes through. I do understand that the issue could have many causes, I posted hoping that it was a known issue with an easy fix lol. It seems not! You confirmed what I had already suspected that the hunt is on to find the source of the issue. Not a fun position to be in. Curiously though except for proper gap and plugs as well as air filter (which I checked and is clean) wouldn't the other issues throw a check engine light? I currently do not have a lit check engine light.

 

you could be turning the plastic screw and actually are pressing the accelerator slightly by tightening the cable, but not actually changing the idle speed, most likey you dont have a sprk plug issue but ive seen many crazy things that make these car do weird things., i would try a dealership and pay them and hour to hopefully get you in the right direction for your issue.