Thank you.

I will poke around the O2 sensor (I think I have a spare one lying around), and will google up the EHA and see what I find.

There is an occasional high-idle issue, where the car idles at about 1000 when warm (no AC engaged). Probably not related though.

EDIT: I think I have a EHA off my wife's '88 300SE lying around as well, I was trying to fix a rough start issue. Will it work on the 560, or will it have different settings?

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1990 560SEC
1988 300SE
1978 928 euro S engine, big brakes, and more
1996 Suburban TX shark rig
1988 Suburban Pacific steelhead rig
1992 300 TE (recently totalled )