I HATE to interject something unless I am 100% sure about it, but with that caveat in place…
I believe that there is an electronic idle stabilization device on these cars. What happened in your particular case is not something that I am qualified to speculate upon. But hopefully this helps lead you in the right direction.
Your situation makes me a bit nervous – I was experiencing an intermittent tachometer for a while, then I went into the wiring harness connectors at all the control boxes & simply unplugged and then reinserted the connectors to clean any corrosion that could be causing resistance or an intermittent connection. It cured the problem, but now I am curious as to the condition of the wiring harness itself.
This makes me nervous, as my elderly father’s 1994 E420 went up in flames a few months ago due to an electrical fire that apparently started under the dash…
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Current rolling stock:
2001 E55 183,000+ Newest member of the fleet.
2002 E320 83,000 - The "cream-puff"!
1992 500E 217,000+
1995 E300D 412,000+
1998 E300D 155,000+
2001 E320 227,000+
2001 E320 Wagon, 177,000+
Prior MBZ’s:
1952 220 Cab A
1966 300SE
1971 280SE
1973 350SLC (euro)
1980 450SLC
1980 450SLC (#2)
1978 450SLC 5.0
1984 300D ~243,000 & fondly remembered
1993 500E - sorely missed.
1975 VW Scirocco w/ slightly de-tuned Super-Vee engine - Sold after 30+ years.