I have question:
I have 91 SDL 350 with 961 motor, I replaced nozzles (from Mercedes source), replaced glow plugs (all reads about 1 ohm), new (but aftermarket ) glow plug relay.
So an engine starts right the way even to day at 25 F and runs smooth for first few1 min. I like to warm up for few minutes before driving it, so after 2-3 min it will run like "misfiring", unstable, and getting smoke blue smoke (diesel ) not a lot but enough to see. When I started to drive, it will smooth out and putting good power. While engine still lower than 80 F, at stop light it will "berb", like one misfire every 10-15 sec, even oil gauge will flux. (My rmp gauge is not working).
Oil pressure at hot (just above 80F) right at 2 mark or just little below.

All nozzles are put in spec about 2,000 psi. Al tested and properly torqued.

What would it be a problem? Is relay is not staying long enough. At least it's felt like that.
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91' MB 350 SDL, with 961 engine and #14 valve head
09' ML320 Bluetec