if you mean the switchover valve mounted on the inside left front fender it may be that the switch on the intake manifold has failed in the closed position.I think that would be an unusual failure but nothing is impossible.You will see the switch on the side of the intake manifold,it has 1 wire on it and is about a one inch hex,disconnect the wire and see if that makes a difference,if it does the switch is the problem,if not then the circuit that that wire connects to has become grounded somewhere.You will need a wiring diagram to locate the problem as that wire is connected to some other components besides the switch(at least it is on my 91 and i think they are the same).I hope for your sake it is that switch as it would be a simple fix,troubleshooting the rest of the circuit will be a bit more complicated. My$.02 Don

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97 E300D 157000 miles
87 300TD ?141k? miles