I have a 1989 560SEC with 177k miles that i just bought 4 weeks ago, car has a wierd bouncy idle in both 'P' or 'D' sometimes it will idle smooth as silk can't even hear the engine, and then out of no where the rough idle again. When in drive it shakes a little more, even when you take off from stop its hesitant and shaky but has alot of power if you step on it and it makes some wierd exhaust sound along with the engine kind of like a muscle car this happens in both 'p' or 'd' too not that loud though. Car always starts right up even in cold so i don't think its the sparkplugs, while checking today I noticed a vacuum hose disconected right under the air cleaner, this hose connects to the ignition control module (posted pic to show) could this be the cause??? I tried to put it back but the thing broke and when i tried to remove the air cleaner one screw was extremely tight that it got jammed by the 10mm key... can't remove it now.

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May be thermovalve, on the head if your car has one.

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Present cars:
My car: E-class 420CDI, 2008, W211, V8, 180 000km

Wife's one: C-class 220CDI Sport Coupé, Euro, 2002, W203. 345 000km

Son's one: GLK class 220CDI, 2009, W204

Sold E class 260E, W124, 1988 beloved car sold after 489 000 kilometres of reliable services (engine M103, clutch and 5 speed manual gear box all original).

E-class, W210 320CDI, 2000[/B], 225 000km, Sold