Sounds like a worthwhile try but I still think it's the IP. Your problem is EXACTLY what mine was. I just got back from the injection pump shop and the guys told me that the plungers and barrels were bad. He showed me the plungers. With the naked eye they looked fine. With a magnifying glass you could see many small scratches or lines that he called scoring. He said that when the engine was warm and with the thinner diesel, these scoring marks allowed fuel to leak by which prevented IP from making enough pressure to pop the injectors. Especially at idle with the lower volume of fuel. When cold or with thicker WVO, it couldn't bypass through the scoring and would run great. Let us know what you find.

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'84 300SD, '87 300SDL, '81 300SD