Low oil pressure when hot - camshaft plug has not fallen out.

[pea42]

OK, one problem so far: low oil pressure when oil is hot.

V1902

I had thought the problem was the missing camshaft plug, so I took the gear case off and looked.

There is a plug in the end of the camshaft.
I looked really closely and the "plug" in the end of the camshaft is actually some kind of set screw that threads into the end of the camshaft. Parts books shows a set screw at the back end of the camshaft, but nothing at the front end.


So, why would the oil pressure drop when the engine is hot?

It's a new rebuild with new bearings and all parts within spec (according to machine shop).

I hooked up a mechanical gauge and the readings were about the same
When cold, mechanical gauge read about 5 psi higher than electronic gauge
When hot, the readings were about the same, with the mechanical gauge showing maybe 2 psi higher

I ran the engine for about 5 hours, varying between 1800 and 2400 rpm over that time, split over 3 days.
When engine is at running temp (180*F on water temp gauge and about 230*F on on the crank case and oil filter , using IR thermometer), oil pressure is low. like around 20 psi at 2400 rpm

Cold, right when starting: 50 psi

As temp goes up, oil pressure goes down.
oil is RotellaT 15W-40
Temp at the gauge and oil filter taken with IR thermometer
No change in oil level (increase or decrease)

psi,temp (IR thermometer on lower crankcase and oil filter)
49,135 F
41,140 F
40,170 F
20,240 F @ 2400 rpm
15,237 F @ 1800 rpm
10,230 F @ 1400 rpm
08,210 F @ 750 rpm (idle)


Started off at 1100 rpm until water temp reached 140
Then increased to 1800 until temp @ water temp gauge reached ~180
Then increased to 2400 rpm for 20 minutes
Then decreased to 1800 rpm for 5 minutes
Then decreased to 1400 rpm for 5 minutes
Then decreased to 750 rpm for 5 minutes (idle rpm)

Thanks

 

[85Hokie]

My engine - 900 hours, D750A, starts cold with 45+ PSI, running at 90% WOT will hold at about 40 psi once hot, back to normal idle right around 20, if I drop the idle where you can hear each cylinder fire is is under 15.

Oil - once it gets "hot-hot" will all have the same viscosity, or very close to it.

"As temp goes up, oil pressure goes down." - this statement is very true, WITH all situations and ALL oils, period. COLD less viscous oil will always bump the pressure up at first, then as it gets hot, the pressure will drop.........for this very reason, hydraulic activity (pressure) is less after running the machine for a good while.

"It's a new rebuild with new bearings and all parts within spec (according to machine shop)."

WHAT specs ? You have those specs handy on what they built it to, plastigage readings by chance?

 

[pea42]

Hi Hokie

I don't have the values with which they compared their measurements. All I know is that, when I picked up the motor, they said it was in really good shape and everything was 'within specification' and they did some 'minor honing on the cylinders'.

I'll give them a call on Monday.

 

[Daren Todd]

You could have an issue with the pick up tube for the oil pump.

Had a couple Mitsubishi diesels that had great oil pressure when cold, but would drop considerably when they got up to operating temperature.

We ended up finding a hairline crack in the pick up tube going to the oil pump.

In this case the crack was where a solid clamp supported the pick up tube inside the oil pan area. Figured it was caused by vibration since it was solid mounted to the block.



Sent from my iPhone using Tapatalk

 

[pea42]

I fixed the problem of low oil pressure when hot: Replaced fuel pump and oil pressure relief valve (spring & ball). Don't know which fixed issue. Since I had gear case off, it was easy enough to replace the oil pump.


First, my questions and below, the background info.

I've read and we all hear that diesel engines need to be run up to operating temp to boil off water, burn carbon, complete combustion, etc... Never turn on for a few minutes, the shut off.

What exactly does "operating temp" mean, and which temp?
Coolant temp at the gauge?
Oil temp in the block?
Cylinder temp at the combustion chamber?

What should the oil temps be, is 200 - 230*F normal combustion chamber and oil temps? Don't have any reference for these and all literature I can find refer to water temp @ gauge, which is not indicative of the actual temps where combustion occurs.
My test results show that combustion chamber temps reach max. temps even when water temp is 160*F (but climbing), but oil temp does not reach 200*F until after about 15 minutes. even then water temp is below the thermostat temp (180*F)

The part that confuses me is, with a 180*F thermostat, I thought I had to get water temp gauge up to 180*F, but I found the engine reached its max temps (combustion chamber and oil temps) well before the water temp reached its max. In fact, most of the time, the water temp never reached 180*F.


-----------------Background information and test results --------------

All oil temps were made with IR thermometer and they are the exterior surface temps, not the actual fluid temps or internal temperatures. I checked the IR thermometer against the thermostat housing and they matched the water temp gauge. so the IR thermo is at least as accurate as the water temp sensor of the thermostat housing.

Temp measurements made at following locations:
1. Lower thermostat housing (this temp corresponded with water temp gauge
2. 1" below the glow plug of each cylinder (I refer to this as the combustion chamber). This was the hottest area and was an area that only has about 3/4" radius. Outside that radius, temp drops by 30 degrees. Almost identical on all cylinders.
3. About 2/3 way down from the top of block to the bottom of oil pan, directly below cylinder #1
4. The connection between the oil filter and the hose that carries oil from the block to the filter (I have a remote oil filter installed)

Mechanical oil pressure gauge was +/- 2 psi of the electronic gauge.

thermostat is a 180*F thermostat
Oil is RotellaT 15W40
Engine: Universal 5444/M50 (kubota V1902 block)
These engines do not have an oil cooler.
Freshwater cooled, with 3" heat exchanger
There does not appear to be any overheating issue (coolant temp over heating)
Raw water temp when engine was in water: 85*F
Raw water temp when engine was in shed: 70*F

I didn't see any difference between raw water temps, but in shed, I could not put a load on engine.
It did seem to take about the same amount of time for the coolant temp to reach 170*F whether in shed or in boat with load (5 - 10 minutes, did not monitor it closely)

Low oil pressure readings: 20 psi @ 2400 rpm, 5 psi @ 730 rpm (idle)
This was when the oil temp was around 230*F
Water temp @ gauge as 185*F

The only consistent pattern was when oil temp went higher than about 200*F, the oil became thin enough to cause pressure drop to almost nothing at idle and barely 20 @ 2400 with full load in water.

I think I'm over propped. Tied at the dock, max rpm was 2400 while in fwd and temps slowly rising to: water @gauge, 190*F, oil in block, 230*F. The prop is a 2 blade 18 x10 michigan wheel. I ordered a campbell sailer 3 blade. not sure when it'll be here.

Anyway, neutral max rpm is 3000.

I fixed the low oil pressure problem. I pulled the motor out of boat and took it back to the shed to replace the oil pump and oil pressure relief valve. This is a spring and ball in the gear case where the oil filter screws on. When oil pressure gets too high, it pushes the ball against the spring, opening a passage way back to the crankcase, releasing pressure.

Put it all back together and now oil pressure when hot (oil temp ~206*F, water temp 175*F) is about 35psi @ 2300 rpm, 38 @3000 rpm. These are in fwd, but no load because the engine is in the shed. Idle (760rpm) is about 13psi.
1000 rpm brings oil pressure up to 20 psi.

This is much better than before new pump and pressure relieve spring/ball replacement and meets the oil pressure specs in the manual.


In my tests, the oil temps in the block and the input hose to the oil filter were between 205*F and 230*F, depending on how hard I was working the engine the two locations were always within a few degrees of each other. The combustion chamber temps were always around 230*F, regardless of water and oil temps.

For example, when boat was in water, in fwd, max temps I got before stopping tests were water at gauge: 200*F/oil entering filter 230*F (Combustion chamger was about 240*F). That was after about an hour of gradual increasing rpm until I hit max 2400.

In shed with no load: water temp at gauge 175*F/oil entering flter, 205*F, temp at combustion chamber: 230*F

Interestingly, the combustion chamber temp quickly reached the 220*F range even when water temp was < 160*F, but rising.


So as far as "operating temp" for health of engine, I reckon it should be measured at the combustion chamber and, maybe also the actual oil temp. Hotest oil temp was 1) in middle block (about 2/3 distance from top of block to bottom of oil pan) and where the oil entered the oil filter. These temps were about the same. There was about a 20 degree decrease in the oil temp after it passed through the oil filter.

It didn't really matter what the water temp gauge read (150 - 180), the combustion chamber quickly went to 210 - 230*F
A) combustion chamber temp quickly gets to ~220*F and stays there.
B) oil temp slowly rises to about 10 - 20*F below combustion chamber temps.
C) Exhaust side of head is aobut 15*F cooler than fuel side (glow plug side)
D) Exhaust gas usually was about the same as combustion chamber temps, except when heavy load, then exhaust gas was up near 450*F to 500*F range, with combustion chamber temp around 240*F.


In all my tests, the coolant temp (measured at the lower thermostat housing and water temp gauge) was usually between 160 and 180*F. Also, because combustion chamber and oil temps rise quickly, then level off before coolant temp gets near the thermostat temp, coolant temp is not a good indicator of engine temperature until after engine is hot; that is, when coolant temp stops increasing and levels off between 170 and 190*F, depending on the load.