1991 420 SEL with 200,002 miles does the following:
There is the sound of something engaging near the passenger side of the engine bay at which time the RPM's drop to almost the point of stalling, the economy gauge rises to mid-way and a little puff of smoke comes out. At this time the idle recovers and settles until it happens again. Hot/cold, it does not matternor is it effected by A/C. I have changed or checked O2 sensor, idle valve, temp sensor, OVP.
Obviously something is triggering an overly rich mixture for a few moments but what is malfunctioning?