Pull the valve cover and look for a worn out cam lobe -- this is more likely from no oil changes than a broken ring -- broken ring will make it smoke pretty bad from excessive blowby, and it would both use oil badly and foul plugs rapidly. More likely is a burnt or non-opening valve -- this will cause low compression and oil fouling.

Worth a try, a cam swap is easier that fixing a broken ring.

You can remove the injection line and just plug the hole on the fuel distributor, it won't affect the other lines.

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!