have you changed the fuel filter?
good flow to the pump?
bleed to the pump?
bleed injectors?
made sure no leaks to suck air and no seeping at injectors?
baring real mechanical problems ...most diesels that simply fail to make rpm or make rpm under load are fuel starved.
have you serviced the air filter? checked for birds nests?
serviced the inline pump cam box.. oil or diesel in it?
any time i run one out of go juice I change the filter(s) and check flow from tank and at each location before the pump
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