With the fuel consumption that high, either the rack inside the pump is jammed or the cold start valves are leaking into the manifold. Check the fuel pressure. I've seen the wrong type fuel delivery pump used on these to save a few bucks cause massive cold start valve leaks.The cold start valves will open by themselves at 18-20 psi.
Other than that the best approach would be to pull the injection pump, injectors and cold start valves....then send them out to be tested/rebuilt before the excess fuel causes engine wear. You really don't want to let the car run for long like that.I had my injection pump done when my hiway fuel mileage dropped to 8 mpg. Now I get 15 to 16 mpg from it on the road.
Regards,
Randy D.

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W.R.Durrance
ASE Master Tech
Mercedes Tech 20 Years
68 250SE
68 SAAB 96 Ice Racer
69 300SEL 6.3
70 300SEL 3.5
81 300TDT