My 98 ML320 idles fine in neutral but when I put it in drive, idle slows to approx 600 or less from 700 in neutral. Car has 201,000 Kms on it. Changed 1 engine side O2 sensor on left bank a year or so ago. Would a lazy O2 sensor past the catalytic convertor cause a rough idle. Any other suggestions would be helpful. Which other engine sensors control the idle speed under load. Other than the slow idle speed in drive, car accelerates fine and gas mileage has not changed.

Thanks for any advice and feedback.

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Vern Lowe

O2 sensors behind the cats do not control engine functions at all, only monitor that the cats are working properly. Have you done typical tune up maintenance (plugs, fuel filter, etc.)?

Did you pull any codes for misfiring?

DL

Changed fuel filter 2000 Kms ago. No codes showing. Does not misfire, just a slow idle in drive.

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Vern Lowe

Plugs are 4+ and were changed 52,000 Kms ago. Will try changing again.

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Vern Lowe

Take a close look at the vacuum distribution block mounted at the back of the intake manifold. It has a small pitot tube integral to it that affects the signal to the PCM It pulls straight up after removing the various lines from it and I am sure you will find it brittle and maybe broken like mine. A picture is attached.

Other things that can affect idle would be a partially stuck open EGR valve.

Crud build up around the throttle body blade can affect idle speed as well.

the power steering has a pressure sensor that signals the PCM for more idle speed. The AC is an electronic input I think,

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Try changing plugs to the OEMs. Many people have reported problems with ideling, misfires, etc. after going with the aftermarket Bosch +2 and +4 plugs. No one knows for sure why, but they cause strange problems after being installed, but often not right away. Search the forum for spark plug issues and you will see what I mean.