The way you describe slowly losing power, it could also be an injector letting air into the fuel rail. As more air gets from the cylinder to the fuel rail, more and more cylinders shut down.
If you think this is a possibility, unplug the HFCM and remove the underhood fuel filter lid and fuel filter. Then have someone turn the engine over while you watch inside the underhood fuel filter housing for air bubbles.
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Diesel Technician Society
