My '98 12 valve has always had low oil pressure (confirmed with gauge). So I decided to run this to ground. I pulled the oil pressure regulating valve and spring out of the oil filter housing last night. Looked down the bore expecting to find a complete 360 degree seat for the valve spool. What I saw was what appears to be a seat less than half of the circumference. It looks more like a stop for the valve spool than a seat. Is this what it is supposed to look like? Am I misinterpreting how the valve works, ie, there is no seat and it is more of a shuttle style valve? I have the FSM and it is no good as far as anything detailed. Anyone have any input?

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About 25 hot at around 1800 rpm. 15W-40 Valvoline diesel. 180,000 miles.

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That is correct and that is not what the mechanical gauge said, 25 at about 1800. I'm convinced that I have low oil pressure, the question about the oil pressure regulator is just going down the possible reasons, easiest ones first.

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Low instrument cluster oil pressure reading. That's how I can say it has always had low oil pressure. First order of business, check with a known good mechanical gauge. I have done that. Simple as that. And before you ask, yes the instrument cluster gauge agrees with the mechanical gauge.

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My 1998 truck has the sender for the analog dash gauge. It may have some other function, I haven't checked. As to what oil pressure the truck had when new, have no clue, What it has now is about what it had when I bought it used at 150,000 miles.. I did buy my 2010 Cummins new and it caries 56 psig hot at about 1800 rpm with 5W-40 Valvoline full synthetic diesel oil after 54,000 miles.

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So it's not a gauge? Interesting. I'll have to research the FMS on this one.

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One of the first things I did was the free length check on my relief valve spring based on the criteria in the FMS. It failed and I bought a new one. All of the relief valve components, plug/O-ring/spring/valve have been discontinued by Chrysler. I had to source the spring through a Cummins dealer using the ESN. As to the oil pressure sender/gauge, I went through the operational description and the wiring diagrams in the FSM. Unless I am missing something it reads like a conventional resistance based sender with an analog gauge. Could not find a switch, per se, anywhere in the wiring diagram or mentioned in the text. Don't know what to say.

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This is my first time inside a Cummins and I have to admit that had I known at the start what I think I do now, I'd probably done some things differently. I guess that's how we learn. Thanks for the comments!

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Well, I only know what I see. And what I see concerns me. I don't call that a hunch, I call that a concern. I had a low oil pressure concern on a 383 once that I ignored because it met the minimum oil pressure specifications. When the pickup fell off due to a crack that progressed through the pickup tube....that did not end well. I don't ignore low oil pressure concerns any more.

I don't have an answer for you as I've never looked inside one of those housings, but I am pretty sure the modelled oil pressure started with the 24 valve engine. Not positive though.

I just pulled the oil filter adapter off this afternoon looking for a gasket problem. My question has been answered. What you see down the bore is NOT a seat for the relief valve plunger, it's a stop. And the diagrams in both the Dodge FSM and the Cummins repair manuals about how the valve functions are grossly inaccurate.