[W126 Coupe] Idle speed, idle control unit, idle air valve

[Frederick Auld]

Dear All,

Firstly, thanks to Talbir (Feb 2003) and others for supplying useful
information on the diagnosis of idling problems in their 560SECs (to find
it, search on "006 545 84 32").

Since purchasing my 1989 560SEC, I've noticed that idle speed is somewhat
higher than my 1986 500SEC.  It would initially rev to 900 and fall back to
650 after a minute.  After the engine was warmed to operating temperature,
it would shoot back up to 1000.  Shunting the transmission between D and R
at 1000rpm can't be good.... So... this morning I decided to diagnose the
problem.  To my surprise, idle speed on the 560 this morning shot up to 1250
(ish), so felt lucky to have researched the subject on this site only the
night before (uncanny how that happened).

Step one was to install the 500's idle control unit.  It's located behind
the passenger side floor carpet, beind the plastic panel.  On the 560, it's
part number 006 545 84 32.  On the 500, (from memory) replace 84 with 10.  

This initially dropped RPM down to around 550, but only for one minute
before strangely returning to more than 1000 (I am guessing this was because
the 500's engine was last revving at 550 when last switched off, and somehow
the idle control unit remembered this so continued with 550 until it
realised that its host had changed).  Swapping beteen the two idle units
made no difference to idle speed.  

The second trial was to replace the idle air valve from the 500, which
fortunately appears to have solved the problem permanently.  However, it
still revs to 900 when cold for around 30 seconds, then falls away to 650
rather quickly, and eventually down to 550.  This is not consistent with my
500, which settles gradually to 550, without the 560's step-jump from 900 to
650.  Perhaps this is the difference bewteen the 500 and 560's idle control
units (slightly different part numbers).

So what's wrong with the 560's idle air valve?  Upon closer insepction, the
500's air valve contained a rubber thing inside (peering inside one of its
pipes), while the 560's did not appear to have this.  I squirted some WD40
into the 560 unit, and then pretty sure I heard a clicking noise inside the
unit, as if something had released... To experiment, starting the 500 with
the 560's air valve has NOT replicated the problem (that is, idle speed is
around 500-600 rpm) on the 500 !!  However, idle quality is inferior...
Seems more lumpy and uneven.

Regards,

Frederick