Does your fuel shut off lever not work?. This is acuated by vac., usually, when you turn key. When you depress manually, you bypass the vac system. Check to make sure where your problem is. Where did you dump in Seafoam? The only way to shut off a diesel, is to starve fuel, or air. Plugging intake will work, as you have found out. The brown/black vac line on my '84 CD, goes from firewall to IP, just in front of oil filter housing and above RD bolt. Haven't a clue as to it's function. Odd this should happen as it did, yet sometimes you look at an old rubber connection funny, well...

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83 SD

84 CD