I just had a thought when I was writing the last post and I thought I'd run it by you. Let's assume for a minute that my problems are entirely due to an air leak in the fuel line before the IP.

Facts:
1) the problem was more pronounced when running on diesel on a hot engine.
2) the engine would cold start on diesel long after warm the idle on diesel was bad.

What these two operating conditions have in common is that the fuel is at the same temperature as the coolant (or at least somewhat close).There are no thermal gradients accross seals and the thermal stress on metal sealing rings is reduced compared to hot engine, cold fuel. Fuel hoses should be less affected by thermal expansion than metal seals. Following this theory the culprit would be a metal seal (copper or aluminum) at a banjo bolt or the lift pump.

The only seal where I can't see the fuel afterwards is the seal on the banjo bolt that connects the fuel line to the IP (two seal rings that look like aluminum - non-magnetic). I also used to open this bolt to bleed air out of the system when changing VO filters as it was the only point in the system that both systems had in common. Do the seal rings in a bonjo bolt fail if you open and retighten them too many times?

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84 300SD with Greasecar vegetable oil kit. Boston, MA