Reading an old service manual for a 12-valve Cummins showed me how much fuel pressure they really need

I was flipping through a 1994 Dodge service book at a swap meet in Boise last weekend, just killing time. The section on the 5.9L said the injection pump needs a steady 5 to 7 psi from the lift pump, which seemed low. I always thought it was higher, like 15 psi or more. It made me double-check the specs on a job I did last month. Has anyone else run into issues because their aftermarket lift pump was pushing way too much pressure?

2 comments

2 Comments

henry1011mo ago

Yeah that's wild, I always heard you needed at least 10. Did the manual mention anything about what happens if you overpressure it long term?

christopher_west11mo agoMost Upvoted

My buddy ran his at 12 for months because he misread the gauge. The seals started weeping a tiny bit of fluid, nothing major. The real issue was the pump motor burned out way earlier than it should have. The repair guy said running high pressure just makes everything work harder and wear out faster. It's a slow death, not a sudden blowout.