Sounds like you're leaking vacuum somewhere, or the shutoff valve is shot. This same thing started to happen on my W126. But be VERY aware of the possibility of a runaway engine. I replaced the shutoff valve and the engine subsequently ran away, cracking that beautifully made #14 head. That is one scary thing to watch..

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1987 300SDL 271,000
1999 E300 Turbodiesel 155,000ish. (Dad's)