I just replaced my idle control unit, which fixed my high idle. It was 1400, but is now a nice steady 650. However, it still has the same "stumble" it has always had when first started. The car starts instantly (even at -5)and for about 30 to 45 seconds, idles at just above 500 rpms, then the rpms will start dropping and for 30 seconds to a couple of minutes it will drop so low it almost stalls (sometimes it does). Then the rpms start to increase until it is idling normally (650 rpms). (Before I replaced the ICU it would increase until it idled at 1400)

Can anyone give me clue as to what this could be. Cold start valve, maybe?

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2000 Mercedes S500
1990 Mercedes 560SEL
1970 Triumph Spitfire