You can’t jump to the conclusion it has low compression without a compression test. I’d wager it has a fuel leak and has air in it. The IP never goes bad really and the fact he called it a “fuel pump” like in a gas car tells me he probably doesn’t know s*** about diesels (the mechanic).

Why is your vacuum system FUBARED? Anything is fixable. Most likely it isn’t routed correctly and you need a diagram. I’d check this car out and see if it’s rusty and In better shape than your car. I remember you saying you’ve got lots of issues as far as low power, rust etc. might be able to make your car the donor to fix this car.

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Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k