I have an idea, but I'd like to pick the brains around here for ideas/plausible confirmation if that'd be alright...

1998 Dodge Dakota Sport
3.9L V6 Magnum
RC Automatic.

Was about halfway through a ~2 hour trip and suddenly the check gauges light comes on and I spot my oil pressure reads absolute zero. *****. I pull over, shut it off, check my oil level and all, seems fine... start it up: Boop oil pressure is fine. Jump back on the interstate: about five minutes go by and zero oil pressure again... Again, I stop, shut it off, start it back up, oil pressure is fine. I'm beginning to think sending unit so the next time it does it, I slow down, but keep going... then a light clatter... Nope, it's not getting oil. I stop and let it cool for ~10 minutes and my brain running in circles (and naturally, where I am has no road shoulders and no cell service, I managed to find a barn cutoff to pull into). I start it back up and the oil pressure is back, I let it idle to get the oil worked back in and the clatter/ticking goes away and I began to try some tests.

Failure seems to be RPM/Torque related. Anything over 2k RPM or a medium-hard takeoff suddenly I get zero oil pressure until I turn it off and restart, OR I get lucky (~30% of the time) it comes back up when I come to a full stop and drop the RPM to idle 700. I managed to get home safely by only going 55MPH on the interstate with my hazards on (everyone behind me loved that I'm sure). This kept my RPMs around 1600 or so and it never dropped until the one time I had to give it a bit of gas up a sharp grade and it downshifted, RPMs shot up, and suddenly no oil pressure. That was a loooooong and stressful 1.5 hour crawl home.

My idea is it seems the pump itself may be fine since if I'm easy on it, or idle, the needle stays relatively in the middle of the gauge as it always has and slightly above middle at 1600+ RPMs. Extra torque or RPMs make it drop out, so I'm thinking the oil pump drive shaft is either worn, or the tip sheared and lower RPMs allow maybe a minimal amount to grab, but anymore makes it jump out. Or maybe the upper gear (under the dist from a quick google) could be worn? The only experience I have with oil pumps is on an old Cavalier RS which sheared the drive-shaft on the way to a job interview (ended up cutting a hardened steel allen key to replace it on the side of the interstate...)

Any thoughts, ideas, possible guides or suggestions would be appreciated. Sadly, I moved a few months ago and my lease specifically states no automotive work in the parkinglot so I'll have to limp it for a 2+ hour drive to my brother's place to work on it... Do not want... so knowing the best probable issue and already having what I need to fix before trying the journey would be appreciated.

 

can you get it to it if just revving engine in ur drive way , could be simple as a bad oil filter ...i wouldnt think the shaft broke but .....i have gotten a flat tire from a slinky before so any thing is possible ...i have had mine do some thing sim turned out there was a bunch of sludge and old timing tenshoner plugging pick up screen ...

 

I'm thinking that the drainback from the heads may be plugged/clogged. When you go over a high enough RPM all the oil is getting pumped into the valve covers, by the time you get it stopped it's drained back. A partial obstruction would do this. Checking would require that you pull the valve covers. You might get lucky by opening the oil filler and have someone run up the throttle for you, but it's a 50/50 shot that the driver side is the clogged side. The valves would still clatter because it's pressure through the lifters that keeps the valves from clattering, not oil on the valve springs. A worn oil pump would show a reduction in pressure across the board, and would be more likely to show low oil pressure at lower RPMs because at higher RPMs it's taking a lot more whacks per minute at pushing oil.

Marvel Mystery Oil is pretty good at de-sludging an engine, and keeping it clean inside. Any of the other engine cleaners is probably pretty good too, but Marvel is my go-to for stuff like this. Pour in a quart, run it easy for about 50 miles, then change the oil.

 

If not for the clatter, I would have said its the sender.

How is your oil maintenance? Do u run conventional or synthetic? Run low? Sludge can clog the pickup screen. Pull a valve cover to give u an idea of engine cond. Better yet, pull the oil pan. Might try running some motor flush thru it, but be aware, it can make things better or worse, depending on what u got.

 

Yeah, that sounds a lot like a clogged pickup screen. Pulling a valve cover will give you an idea of the condition of the rest of the motor..... If its pretty sludged up under there, I might be tempted to actually pull the motor, and do a thorough cleaning. Might end up being easier than trying to do it in the truck.

 

My oil schedule is pretty good, always around ~3000 miles and last change I ran seafoam through the crankcase for about 200-300 miles before the change (jesus the oil was almost like water when I drained it). I use a high mileage conventional/synthetic blend 5w30. It leaks a bit, so I don't really want to fork out the cash for full synthetic to keep topping it off (only 1-2 quarts between each change, so not horrible, but still.) I use the same oil in my Toyota since it burns about a quart per change it's just easier to keep one type of oil.
Filter I generally use genuine Mopar, as they're pretty cheap around here (if they're out of stock, I'll grab a Fram Ultra synthetic, currently it's Mopar though). I've usually ran seafoam (or gumout multi system) in the crankcase every ~3 changes to prevent sludge and when I did the timing chain about 3 years ago, it didn't seem sludged up at all looking in the pan (didn't pull the pan though). I would think if the pickup screen was clogged, it would read low, and get a *little* better at higher RPMs (since the pump would be spinning faster, creating more draw). I don't have the money, time, or resources to actually dig into the engine and pull it... at the same time, if I DO pull the engine, it's not going back in. I'll be finding a 318 and rebuilding THAT before putting it in. Buddy of mine has a Ram 1500 sport that the transmission just went out and he doesn't want to fix it. Engine just rolled over 100k and he has all maintenance docs, maybe I can talk him into letting me have it on the cheap.

To answer a couple questions:
Yes it will drop out if I rev it in park, especially if I bounce it for a faster rev. If I'm slow and ease it up it keeps pressure until about 2500 RPM.
Pressure is not low at all. At idle it's either right on, or slightly below 40psi (according to the gauge), when at ~1800 RPM it's slightly above.
Haven't pulled the valve cover, but looking into via the filler everything looks wet, but no immediate visual buildup anywhere and nothing carbon black, just that old oil brown.
I'll try keeping the filler cap off and letting someone rev it to see if the valve cover fills up.
If I can find it, I'll try some Marvels, for some reason automotive sections in stores are WIPED as of late, everyone doing car projects in the cooler weather?

Right now I don't have much free time, it'll likely be weekend after next before I can get any real testing in and provide more updates on findings.

 

If the oil isn't draining down, at higher RPM, the pump moves more oil, (at least, it is supposed to....) if it doesn't have capacity in the pan, pressure drops. Same goes for the pickup. If it is partially blocked, the pump can draw enough oil until the RPM get past the point where it simply can't draw enough oil to maintain pressure.... and pressure falls off.

 

Would that be a gradual, but quick drop, or instant? The drop to zero is instant, but if I shut it off and immediately restart (literally turn key to off, then immediately crank) the pressure is back to 40. Would enough drain down in ~1 second of non-rotation? I checked the oil level today, I'm actually over by about a pint (about 1/8-1/4" over the "full" line) It's never seemed to cause issue before, but I wonder if being slightly over would cause it... I figured the only issue that would cause is too much pressure and blow out a seal.

Edit: I'll try to get a video of the dashboard of what it does tomorrow so you get an idea of what I see.

 

I'm thinking pickup screen blockage. When you shut the engine off, whatever is there falls off, until it gets picked back up against the screen again

 

One would think if the oil simply wasn't draining back down fast enough, pressure would drop off like a stone, if it was the pickup being clogged, I would think that pressure would fall off gradually..... However..... (and you knew that was coming.....) it's possible that the pump just whips air into the oil, and that likely would cause pressure to drop off almost instantly.

I think you are going to have to at least pull a valve cover, and see what things look like. If nothing obvious presents itself there, pull the pan, and have a look at the pickup.