I agree with JEB. As long as the pavement was dry (not snowy), and you were going at least ten miles per hour, engine warm, it should push-start fine in first gear. You need to spin the engine at probably 6-700 rpm to get it started. With a diesel, I'd think that the only real issue would be maintaining that engine speed long enough for it to catch and start. How long did it sit before you tried push-starting it?

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mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)