Oil Pressure Driving Me Crazy - Not Cam Plug

[jlm46]

Hi all, looking for advice, I searched all the threads here about oil pressure that I could and haven't found a solution. Unfortunately it's not a missing cam plug, and it's not the oil pressure sensor.

I have a d950 engine that has had low oil pressure when hot since I got it. I added a mechanical gauge to verify and confirmed the low oil pressure. Oil pressure is 42psi when cold at idle, but when hot it drops to around 20psi at 2400rpms, drops to 10 at 1500 rpms, and will trigger the buzzer at idle - if I bring the speed down slowly and only go to idle after its been at 1500 for about 5 minutes, then I get 10psi at idle and buzzer does not come on (is this a clue?). I'm running fresh 15w-40 conventional, it was worse with 10w-30

Over the winter I decided to rebuild it for that and other reasons. Rebuild went great in my amateur view, engine starts good, runs good, doesn't smoke or burn oil, but still has low pressure when hot, unchanged after rebuild.
During rebuild, among other things I did the following:
-replaced oil pump
-replaced pressure relief valve
-verified cam plugs are not missing
-new main and rod bearings including the front main, plastigage in spec
-cleaned strainer which had a lot of goop on it and put on new o-ring
-new front cover o-rings

I can't figure out what else could be at play, only other thing i saw someone mention was that if the camshaft or block was worn where it rides since it has no bearings that could possibly affect oil pressure, but I only saw one person mention it in a thread.

Any ideas? It's very frustrating to have to slow the speed over 5-10 mins to avoid dropping below 10psi, especially after what I thought was a successful rebuild.

Video of good pressure at cold idle:
20250721_133918.mp4

Some photos from before reassembly in case they have any clues:

 

[JohnDB]

I'm completely ignorant about your engine model, you've covered almost all the bases but 2 things occur to me:
1. Did you replace the oil filter with genuine Kubota? Not saying that any other brands are unreliable, but to save argument over aftermarket brand x vs brand y vs brand z.
2. Is it possible you have the wrong dipstick>>>overfilling engine oil>>>oil beaten to death by the crank lobes>>>aeration>>>air sucked in with oil by the pump>>>low oil pressure? Did the amount of oil to fill the engine match the capacity listed in the manual (tho occasionally the manual is wrong).

Interesting problem, looking forward to other's responses.

 

[jlm46]

Those are really interesting ideas JohnDB thanks for giving me something else to think about. I checked the part number stamped on the dipstick and it shows up as a d850 dipstick, and the d950 dipstick has a different part number. I wonder if it was replaced with the wrong one at some point in its history.

The engine in my application sits at an angle so if overfilled I could easily see the back of the crank churning in the oil and aerating it....I wonder if that could be why my pressure comes up a little after I reduce RPMs for a few minutes, I had always thought it seemed strange that the oil could cool by any significant amount in that time in a way that would boost the pressure back up.

I did not take note of the exact amount of oil that went in during the initial fill after rebuild, and when using the drain on the bottom of the oil pan it doesn't get it all out because of the angle of the engine, and I just always trusted the dipstick. Heck for $25 I'm going to go ahead and order the d950 part number and see if it shows any difference in the height of the full mark vs. the one in there now. Will comment again on how it turns out.

 

[JohnDB]

If the engine is on an angle then maybe the dipstick - even the correct one - is part of the problem? Others on this forum may know more about this sort of application. Could be in a boat? It would help to know how the engine is angled, how many degrees side-to-side? Front to back? Photo of dipstick location in relation to how the engine is tilted.

 

[jlm46]

Yes it is indeed on a boat, and the engine is angled about 7 degrees with the back being lower to match the angle of the prop shaft. No side to side angle. This d950 engine was used in thousands of these boats (and the d850 before that in older models) and I was unable to find much chat about oil pressure other than people with oil sensor and cam plug issues. This forum has much better info on this engine, and the boat groups related to this tend to have more basic info.
Dipstick location is fairly centered on the pan, perhaps a little towards the front.
I ordered the correct part number dipstick and I'm going to compare the two to see if the marks line up the same or not and will go from there.

 

[Daren Todd]

I had a Mitsubishi diesel that started showing low oil pressure when hot after a rebuild.

Sent it back to the engine shop since it literally had about 5 hours on it.

The pick up tube for the oil pump had a hairline crack in it.

When cold, oil pressure was good. As it warmed up, the oil pressure dropped.

The crack was expanding when it warmed up.

 

[85Hokie]

Jim,

I have the exact SAME problem on a similar engine ( D750-AH )

I have 40+ psi cold, and a high idle it will be 18-20 hot - at low idle, it will drop off to damn near zero.

I thought I had a hydraulic fluid getting into the oil - but that would not drop the viscosity as much as it was when I emptied the oil out.

I changed the lift pump and changed the oil and filter again.

I am using Rotella 15-40 T6.

I cannot verify the oil plug on cam however.

I cut grass with this B7100 that has about 1000 hours on it - runs strong and little smoke or blowby.

My solution is start it - cut with it and thus idle down to the off position!

 

[jlm46]

I had a Mitsubishi diesel that started showing low oil pressure when hot after a rebuild.

Sent it back to the engine shop since it literally had about 5 hours on it.

The pick up tube for the oil pump had a hairline crack in it.

When cold, oil pressure was good. As it warmed up, the oil pressure dropped.

The crack was expanding when it warmed up.

I was worried about something like that, i took off the pick up and cleaned it very thoroughly when i did the rebuild, but i wasn't looking for a crack...if it were obvious maybe i would have seen it, but i didn't think to examine it very carefully. In this application getting the pan off is much harder than in the tractors, the engine needs to be raised off its mounts so I would have to wait till winter if all other avenues fail. Thanks for the details of your case, it certainly sounds possible.

 

[jlm46]

Jim,

I have the exact SAME problem on a similar engine ( D750-AH )

I have 40+ psi cold, and a high idle it will be 18-20 hot - at low idle, it will drop off to damn near zero.

I thought I had a hydraulic fluid getting into the oil - but that would not drop the viscosity as much as it was when I emptied the oil out.

I changed the lift pump and changed the oil and filter again.

I am using Rotella 15-40 T6.

I cannot verify the oil plug on cam however.

I cut grass with this B7100 that has about 1000 hours on it - runs strong and little smoke or blowby.

My solution is start it - cut with it and thus idle down to the off position!

Wow, interesting to hear that I'm not the only one, I think your model engine is very similar to mine. I wish I could find out what's causing it, its hard for me to avoid idle because on a boat I need idle speed for docking, so its problematic to have to wait a while to get down to idle without setting off the buzzer.

 

[JohnDB]

Yes it is indeed on a boat, and the engine is angled about 7 degrees with the back being lower to match the angle of the prop shaft. No side to side angle. This d950 engine was used in thousands of these boats (and the d850 before that in older models) and I was unable to find much chat about oil pressure other than people with oil sensor and cam plug issues. This forum has much better info on this engine, and the boat groups related to this tend to have more basic info.
Dipstick location is fairly centered on the pan, perhaps a little towards the front.
I ordered the correct part number dipstick and I'm going to compare the two to see if the marks line up the same or not and will go from there.

View attachment 159051

In addition to suggestions others have made, is the dipstick tube original (not the sort that normally goes down through a hole in the block) or the right length for that dipstick? Therefore oil level too low in relation to the oil pickup, or too high in relation to the crank counterweights? Exacerbated by the inclination of the engine. Someone with this model engine may help here. I try to avoid assuming anything when it comes to marinised engines.

 

[lugbolt]

did you check every single clearance? Cams to block, rod bearing clearance, main bearing clearance, etc?

 

[North Idaho Wolfman]

I'm spitballing here:
Measure the height of oil pan and I will look and see if I can find a fill amount in quarts for fill.
Then you could put that amount in it and check against the dip stick to see if the level is right.

 

[D2Cat]

There is a cam plug on the backside also, I'm quite sure.

 

[jlm46]

did you check every single clearance? Cams to block, rod bearing clearance, main bearing clearance, etc?

Since there are no bearings in the cam to block, I was not aware that checking them was a thing, so that is one thing I didn't do that could be a potential cause of my issue. But of course it's a lot of work chasing a potential wild goose. I did check all bearing clearances and with the new bearings they were within spec. The engine only has 1400 hours so it'd be surprising since I only saw one person online mention the cam to block being an issue, but the engine was mistreated by prior owner so it's hard to say. Included photo of the cam I had taken during rebuild in the original post, but can't make much just looking at it.

 

[jlm46]

There is a cam plug on the backside also, I'm quite sure.

Yes confirmed that plug is in as well, and had taken some photos during rebuild where we can see the back of the cam, added to original post. I wish it was something simple like that!

 

[Yooper]

Head scratcher for sure! One thing you could try as an experiment is straight weight high viscosity oil such as 50w ‘racing’ oil. Maybe there is an oil formulated for diesel in a straight weight high viscosity?

 

[jlm46]

Head scratcher for sure! One thing you could try as an experiment is straight weight high viscosity oil such as 50w ‘racing’ oil. Maybe there is an oil formulated for diesel in a straight weight high viscosity?

Thanka great tip, this engine doesnt really get run in the cold so will definitely try a straight weight oil to see if it helps.

Continuing from JohnDB's tips, i took the dipstick and marked the tube on the outside approx where the full line is when dipstick is seated, and its definitely above the top of the pan...and the crank counterweights on this engine come to within about an inch of the top of the pan if I recall...it could be churning in the oil, Im going to bring down the oil level to half way between F and E on the dipstick and see if any difference. Wouldnt it be nice if it was that simple...i somehow doubt it lol, but desperate at this point!

(This second pic is not my engine but same model)

 

[BX25D Rookie]

I had a Bayliner 17' bow rider boat. Long gone now.
It had the marine version of the Ford Pinto 4 cylinder overhead cam engine.
2.3 Liter if I recall correctly.

Marine duty is way more brutal on an engine than tractor usage.
If I ran that 2.3 Liter wide open traveling on one of New York State Finger Lakes lengthwise,
you might be at wide open throttle for 45 minutes or longer.
The oil got hot and when you arrived at the other end of the lake and throttled down to idle,
the hydraulic lifters would clatter/chatter due to low oil pressure.

Since that Bayliner had unlimited cooling capacity due to the cooling system actually was the lake water, several minutes of idling and it cooled off, the oil pressure went up and the hydraulic lifters became quiet again.

I would start with a good straight weight motor oil, perhaps 40 or 50 weight, and see what happens.
That particular marine application may need a engine oil cooler installed.
If possible, run it hard, wait for the low oil pressure condition, and measure the engine oil temperature.

 

[JohnDB]

... i took the dipstick and marked the tube on the outside approx where the full line is when dipstick is seated, and its definitely above the top of the pan...and the crank counterweights on this engine come to within about an inch of the top of the pan if I recall...it could be churning in the oil, Im going to bring down the oil level to half way between F and E on the dipstick and see if any difference. Wouldnt it be nice if it was that simple...i somehow doubt it lol, but desperate at this point!

View attachment 159085

That oil level looks way too high for my liking. If you lower it, need to ensure it doesn't compromise the oil pickup from the sump when the boat is operating (motion from waves etc).

 

[North Idaho Wolfman]

How much oil are you putting in it?