Twice in 2 months (yes, rare) my 400E has developed a "lope" which cycles in rhythm every 2 or 3 seconds. It's noticeable when cruising, and has gone away both times when restarted after driving. The condition wasn't noticeable at startup either time, but developed after driving, and if I recall, both times I had just given it hard acceleration. New (reconditioned) MAF, o2, plugs, caps/rotors, coils, and coolant temp sensor (all done prior to the first instance 2 months ago). Fuel delivery? ECM?

I think it is vacuum leak. One of the hoses on the intake maybe cracked. I would start the car and remove the intake filter and bend the rubber/plastic hoses until the engine speed changed to find the bad hose. I miss my 400E.

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2010 ML350 Bluetec
2012 Mustang Convertible

hmm, even if intermittent?

And you're sure you're not getting any fault codes? Seems like that ought to be throwing a code.

My 560 does this sometimes when cold...doubt it's the same problem tho. I would check out the idle speed control valve.

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Current cars:
2000 ML55 AMG, 174k miles
2003 C240 T-Modell, 202k miles
1995 S320, 207k Miles

No idle speed control valve on a 400E.

Really? How is idle speed regulated?

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Current cars:
2000 ML55 AMG, 174k miles
2003 C240 T-Modell, 202k miles
1995 S320, 207k Miles

By a committee of electronic devices.