After the leak-down test, another way to test the injection pump would be to remove all the hard injection lines, put some rags around the top of the IP to catch the fuel from the next step, and have a helper crank over the engine while you watch the fuel output from the delivery valves. Each should "spit" a little or a lot of fuel up, depending on the accelerator position (idle or off idle up to floored). Should look like a little display fountain throwing fuel up in the air and inch or two. Any obvious difference between #2 and the others will let you know your next step.

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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