#293 holds the pipe firmly against the backside of the Water Pump allowing the
O-ring #287 to seal the pipe to the Water Pump Opening.
If #293 is loose #287 doesn't seal the end of the pipe.
'Hence your leakage.

Thanks!
I now know about #287 and can order Two for replacement!
Seal Ring #012 997 5148
(The same O-ring seals the fuel preheater exchanger to the Block on the opposite side of the engine!)

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'84 300SD sold
124.128