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.

Did you check with Cummins to see what they say the oil pressure should be ? I have a 98 but it's a 24V and my oil pressure on the dash gauge is 60 cold and when running down the highway , I haven't put a mechanical to compare it . I found it odd , they must have changed something because the gauge in my 2000 don't think reads as high as the 98 but the needle goes to a similar spot ??? It's been a long time since I've run the 2000.

It is not an actual gauge, the sender is just an open/close switch. The gauge reading is software based on engine temp and RPM.

Which is why I noted they are on the same spot on the gauge but the reading numbers are different and suggested finding out from Cummins as to what the actual oil pressures are supposed to be.

Well, it's time to bring this thread to a close. First off, what did I do. I regasketed the oil cooler/oil filter adapter and that seems to have raised the oil pressure regardless of how you are measuring it. I carefully looked at both gaskets and can see nothing wrong. But it is what it is. As to those who mentioned the inaccuracy of the IP gauge, that is the understatement of the century. I installed a permanent second gauge plumbed into the spare port at the top of the oil filter adapter. These are the results.

At an ambient of 75 degrees just after startup:

@1000 rpm, IP gauge = 60 psig, aftermarket gauge = 75 psig
@2000 rpm, IP gauge = 70 psig, aftermarket gauge = 100 psig (100 psig range so this may be higher)


At operating temperature (have no idea what the engine temperature is, just that it stabilized):

@idle, IP gauge = 20, aftermarket gauge = 37 psig
@1000 rpm, IP gauge = 20 psig, aftermarket gauge = 40 psig
@2000 rpm, IP gauge = 40 psig, aftermarket gauge = 75 psig

One thing that I found interesting was that when I let the clutch out and and the engine speed momentarily drops below idle, the IP pressure gauge goes to 0, the bells start chiming, and the aftermarket gauge still shows slightly less than 30 psig.

So, what have I learned?

1) You can perform a self test on the instrument cluster using the trip odometer reset button.
2) All of you guys are right, the IP oil pressure gauge is barely connected to reality.
3) If you do a comparison between the IP gauge and a test gauge, single point comparisons can be misleading. I wire-tied a mechanical gauge I had to the left windshield wiper arm and drove the truck like that for a couple days. Amazing what you find over a range of operation conditions.
4) There is a TSB covering this very problem, TSB 08-22-99. I don't think mine has been reprogrammed as it doesn't have the update decal on the PCM. And before someone asks, I'm not driving 150 miles to have this done when I have the aftermarket gauge.

So, that's that, I consider this matter closed. Thanks to everyone that commented.