W140 idle speed
 

Can anyone tell me how to increase the idle speed of my w140 S280 which has the M104 engine. The problem is when I select drive, the idle speed goes down to 500rpm, and the engine struggles a bit.
Many thanks,
Lester

Not familiar with the 280, but if it's like other W140's, idle speed is computer controlled...not adjustable.

W140 Idle speed
 

Hi Lester,
I would bet that you have a dirty throttle plate. Not knowing what year your car is, I can't tell you how or what to remove to see the throttle plate - (there is an early and a late 104 engine, and the air intake ducting systems are different. ) But - if you can remove what you need to in order to see the
throttle plate, have someone step on the throttle all the way ( engine OFF )
and look at the edge of the plate and the side walls of the throttle housing
to see if there is any carbon build up. If there is, the amount of air coming in at idle is restricted and idle will be too low. Take a can of carb spray and a
rag covered screw driver tip and clean the throttle housing and plate. Try not to use too much spray as car will be hard to start afterward. But once the throttle housing is clean, you should notice an increase in idle speed.
( please do this at your own risk - no responsibility assumed. )

If the car is new enough there is a correction program in the diagnostic computer that can set the idle speed higher. It would probably have to be a ME system not a HFM, which means 97 or later.