Welcome to the forum ... 
I would start with: 1) a valve adjustment and 2) a compression test. Even with the high mileage, no sense giving up on it before getting some data. A valve adjustment can make a pretty big difference if the clearances were way off. And compression numbers will tell you a lot.
Any trouble starting? Does it burn oil?
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1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)
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