First check the vacuum lines for an abandoned connection. Then check the vacuum hose coming off the vacuum valve at the IP. If there's oil in that vacuum line, then the valve is suspect. While the engine is running apply suction on this line to try to shut off the engine. If you can't, replace this valve. Be very careful to do this correctly that it hooks onto the IP, otherwise you won't be able to turn off the engine and it will rev to the max and blow. If you can shut the engine off with suction, check the 2 vacuum lines that connect to the vacuum valve at the key switch. You may have to replace this valve. Good luck.

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85 300CD Signal Red/Tan sold
83 300D Manganese Brown 109K
97 E420 Midnight Blue 197K sold
98 BMW 328i Vert White 100K, sold
95 BMW 525i White 125K, sold
93 BMW 525iT Red 193K, sold
95 E320 Green Wagon 125K, sold
94 E320 White 127K, sold
85 300SD 156K Grey (Annie), sold
84 300D Lapis Blue 170K (Judy), sold
99 ML 320 Black (lease), 1998 C230 White (lease)
00 Honda S2000 Red (lease)
86 Mercedes 300E (sold)
84 Porsche 911 Red (sold), 1965 Porsche 911 White (sold)