I'd recommend that you try to diagnose by loosening each injection line slightly to stop that injector from working, see if the idle is affected, and then tighten the line up and move to the next line. The lines that do not change the idle quality indicate where the problem is. What you describe is the classic symptom of leaking delivery valve seals (copper washers). The leak is internal - not visible on the exterior of the delivery valve / injection pump. If you can isolate the problem by injector line(s), then you can try moving the injector to see if the problem moves with it. If not, then that delivery valve is suspect.

If you cannot isolate by this method, then you can try rebuilding all the delivery valves with fresh copper seals. Research the procedure - you DO NOT want to mess this up, because then you may need to replace the injection pump.

If rebuilding delivery valves does not solve this, then having all the injectors tested and the pressures reset would be my next recommendation.

If the injectors are all proven good, THEN you may have a problem with the injection pump and should consider getting a rebuilt pump.

__________________
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