On my 83 300SD there is a small electronicly controled vaccum valve on the firewall above the injection pump - it's not hooked up but what is it for??? I can't find any loose lines and I will try to get a digital camera out here to take a few shots of it - any ideas??

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1983 Mercedes 300SD
1987 Mercedes 300SDL
2001 VW Passat 2.8 AWD
2007 OM642 Jeep WK 4x4