Leaving aside the issues I have with fuel/air mixture (rough idle), it appears to be worst under load (in Drive), say at a traffic light stop. Idle drops to 500rpm or less, and the engine seems very strained. If I slip the transmission into Neutral, it is much better; idle rpm is then ~650-700.

Is there an adjustment on the transmission to cure this, so the vehicle doesn't feel so strained at idle? Or shouldn't I worry about it?

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Simon Shearer

1986 420SEL
1997 SL500

2001 Victory V92 Deluxe 1500cc Cruiser
2007 Yamaha 1300cc Tourer (wife's)
2010 Lexus RX 350 (wife's)
2002 Kia Rio (for daughter #1)
2002 Hyundai Accent (for daughter #2)

With fuel mixture issues I wouldn't mess with the trans. I would correct the mixture issues and I bet the straining would stop.

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I could be wrong but I think idle for these V8's is like 550+/- 50 rpm. There should be a sticker under the hood above one of the headlights saying so.

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my 91 420sel idles at 525 in gear and it's smooth. if the economy needle is not pegged to the left then you have a vacuum leak somewhere causing the rough idle. you can also try cleaning the idle air valve to see if it improves the idle