Do you mean this part in the picture I posted?

http://www.peachparts.com/shopforum/diesel-discussion/324297-om603-head-removal-down-connection.html

I think it is attached to the cylinder block when the coolant is routed to warm the fuel, not the oil cooler. There is only 2 places that is can leak, unless it is cracked which is unlikely. The thread or the o-ring.

1) If the thread is damaged , I would suggest to JB weld or RTV it. Thread it back in. There is pressure from the coolant but JB weld should hold.
2) Sand down the inside and replace the o-ring where the fuel cooler fixture is inserted.

Make sure to install the clip back in.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
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