I tested the vacuum on my other 92 300D (the swimming car), and in OFF, it lets vacuum straight through to the shut off valve.

The car with the problem dumps the vacuum. So I've ordered the valve from Brumos in Jacksonville for $29. Doesn't seem to be available at aftermarket places.

The FSM says you don't have to remove the instrument cluster to get the ignition switch out, unlike the post at http://www.peachparts.com/shopforum/showthread.php?t=216041
says.

We'll see.

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1983 240D Silver/Blue "Sadie", unknown miles. 100k miles on WVO single tank, converted to 2 tank about 10k miles ago, FPHE, Injector Heaters, Aux Fuel pump. Alcohol/Water injection. Frantz oil bypass filter
1992 300D 2.5 Turbo Green/Tan "Jade" 267K miles Stock.
1992 300D 2.5 tan/tan "Rocky" on 2 tank WVO. Pressure actuated Turbo "rat's nest" surgery completed. 197k miles