There's a good write-up in the "DIY Article" section that discusses locating vacuum leaks. Cars this age are likely to have old, hardened hoses that can easily cause this problem

The Idle Control Valve, also referred to here as IACV would also be worth exploring. The SEARCH facility has a lot of history on this device.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"