Funny,
I've been having a similar problem all winter. I thought it was a "cold" issue, but now that it has warmed up it's still doing it. The longer the car sits the worse it gets.

So tonight I checked the priming pump and found that when I pushed it diesel squirted out around the push cap. Well I pumped it about 20-30 times and then tried to start after a single GP cycle and it started. It hadn't ran in about 4 days so maybe air in the system is the problem.

More work for tomorrow :-)

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Will W - 1984 300SD federal