Check the white plastic line from the center of the manifold (92 350 SD, 603 engine) or the rear of the intake manifold (617 engine) for leaks. The rubber bits that connect the plastic line are old and hard by now, plan on replacing all of them in the engine compartment.

Also check by blowing down the line that it holds pressure. If not, check the switchover valve on the firewall for leaks. It goes (on the 603) from manifold to switchover to ALDA (aluminum squarish box on the injection pump) to the blue vac amplifier on the driver's side fender for the tranny.

I'd bet you have a fault in the pressure sensing lines, so you don't have pressure on the ALDA and no extra fuel. That 603 shouldn't have any trouble at all holding any highway speed you want at any altitude!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!