Engine Temp. vs Coolant Temp. "Operating Temperature"?

[P69]

After having installed the engine back in the boat (engine was rebuild completely), I've been chasing a  low oil pressure problem (which I have fixed) when the engine was "up to running temperature" (i.e. hot)

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.