EGR Related Intermittent High Idle 1993 300E

[SFMerc1993]

Hi All

I'm chasing down an intermittent high idle problem on a 1993 300E. I want to determine whether the EGR change over valve may be responsible.

Symptoms:
- Intermittent high idle (1000rpm in D, occasionally suring 1000 - 1500 rpm in P)
- High idle coincides with loss of CC
- High idle appears triggered by cold start, usually disappears on warm re-start
- High idle reappears midway through long (80 mile) journey, removed again on re-start when engine left off for several minutes
- Error code 5 & 6 showing (LED blink box)

Work done so far:
- Engine wire harness replaced
- Throttle body replaced (used late model, wires in good condition)
- New MAF, IAT, OVP. Oxygen sensor 12k old
- EGR valve cleaned and appears to work (cannot blow through)
- Clogged EGR (hot inlet) pipe replaced with new, rubber version (about 30k ago)
- Head gasket replaced 40k ago

Other
- At 'good' idle, the revs are constant-ish (700 plus of minus 30rpms - usually a drop followed by bounce back
- Car otherwise drives beautifully with good power, kick-down etc
- Removing vacuum line/oil filler cap causes revs to drop
- The vacuum line into the EGR from the ETA is sometimes a constant suck, sometimes a pulsating suck. It doesn't seem to correlate with the high idle

The appearance of the high idle with the cold start and then engine at normal running (as well as code 5) leads me to believe something is wrong with the EGR system. I need the car to pass smog, so a few more goes before I hand over to the pros.

Any advice appreciated. Thanks! --Tom

[SFMerc1993]

This turned out to be the idle speed control module under the passenger side footwell.