Usually a value over 0.5 should be enough at idle, if it raises to 3 when pushing the throttle...

In normal conditions it should be 1.2-1.5 at idle and 3 when driving(in both cases, when engine has been warmed up).

~Nautilus

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1990 260E Sportline (that's 300E 2.6 for our American friends) -> sold
2001 E320 4Matic Elegance -> my Dad's daily drive
2005 Seat Leon FR 1.8T