My 1997 Dodge 3500/Cummins 12V might, or might not have a problem with the ball-and-spring "relief valve" that is fitted in a drilled bolt that in turn fits inside a fuel banjo bolt near the front of the 5.9l. A hundred years ago on SOME Cummins or Dodge forum, someone suggested that might be a problem for me, and it clearly wasn't then, but again, might be now, since my fuel pressure dropped suddenly just recently and has been a bit flaky of late. I'd like to replace it just to eliminate that variable.

Fuel pressure near in-tank electric pump (mod) is 30+psi from direct measurement (deadheading the pump briefly.) But at the IP inlet, it's down to 10-12psi. As far as I know, there's no pressure-relief elsewhere in the lift system...is there? Per my mech dash gauge (plumbed into the IP inlet), it's been dead on 30psi for a long time, but within the last week or two slipped to 25, then 20, then 15, then...zero...

Anyway, maybe it's 'cause it's not actually called a pressure-relief valve, but I can't find that damned thing online or in the manual. Anyone?

I'm looking to buy the spring at minimum, or the whole damned thing (cross-drilled bolt, copper washer, internal washer at spring seat, spring, and ball.)

 

It's called the "overflow valve" - OFV for short. A failed OFV can cause loss of fuel pressure, though I would think it unlikely for your pressure to go to zero.

Pump problems can also cause low pressure. I imagine we're all curious about your in-tank pump mod, since these trucks did not come with in-tank pumps. NORMALLY, there is no other pressure regulator in the system - but your electric pump may have a relief that's failing.

A quick way to determine if its the pump or OFV is, with engine running, pinch the rubber part of the return line to the tank closed and watch fuel pressure. If pressure spikes, the OFV is the problem and the pump is at least semi-good - don't hold it pinched for more than a few seconds. If no pressure increase, you have a pump problem at the very least. This doesn't mean that the OFV isn't also bad, but you definitely have a pump problem that needs to be addressed before repeating the test.

 

There are several ways to degrade pressure at the IP inlet, aside from a faulty OFV. One, the fuel filter screen if clogged will not pass enough fuel to make power. When this happened to me I thought it was a boost pressure leak I couldn't find. Two, and obvious, a clogged fuel filter. Three, very obscure and not discussed here for many years, there is a short (~18") rubber hose from the hard fuel line to the inlet of the fuel filter. The inside lining of this hose can rot and come loose, which will impede fuel flow and drop pressure downstream. Be sure to replace it with a diesel-rated hose.