78 mpg... that's just astounding!

I've never seen that kind of mileage from a 617-powered car, even on ebay.

But yes, you'll probably find that the clear pre-filter is visibly clogged. I always carry spare fuel filters with me. The car usually goes from driving fine to sluggish very quickly when a filter clogs.

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1992 300D 2.5T 249k - Parked with a bad transmission
1981 300SD 142k - Daily driver