Get a new seal ring (a really big O-ring) for the IP (goes between IP collar and the hole in the engine that accepts the IP). This is "A Good Idea" so you don't have to remove the IP again in order to replace that seal after you start the engine and find out the seal is bad. Clean out the channel and collar well and lubricate the seal with engine oil.

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Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC