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Old 11-14-2018, 08:26 PM   #1
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A small oil pressure mystery

I did a speed test run before parking the boat for the season. Normally cruise at 1300 rpm, 33% load, 47 psi. The run up to speed noting figures every 100 rpm took maybe 20 minutes, ending in 3050 rpm and 99% load. I returned to 1300 rpm and the oil pressure dropped to 42 psi. Over the next several minutes it came back up to 47. Oil temps were up slightly during the run but not much (10 deg). I had noticed this the one other time I ran for a short time at full throttle as well. Oil was just changed, Valvoline Cummins Blue 15W-40, as called for by the book.

Foaming? Anybody else's engine do this? Cummins QSB5.9 380hp.
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Old 11-14-2018, 09:46 PM   #2
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My guess: you ran harder, oil get warmer so pressure drop a bit when you go back down, then oil cools down a bit, pressure get back to the same value. 5 psi difference looks tiny for me but I am certainly not an expert.

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Old 11-14-2018, 11:25 PM   #3
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Hi,

I do not know why, but I would not be worried. I often run 42psi in low rmp and faster run until 47psi (engine warm) my cm qsb 5.9.

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Old 11-14-2018, 11:58 PM   #4
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I'm not convinced that the oil heating by just a few degrees would affect pressure. It went from about 170 to 180. That can't have affected viscosity much. I should have pulled the dipstick immediately to see if the oil looked foamed.

Not worried, unless it is foaming which is generally bad. Even in that condition, oil pressure was 52 psi at high rpm. The curiosity is having it go to 42 back at 1300, then climb on its own to 47.
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Old 11-15-2018, 12:18 AM   #5
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Hi, you could ask this question at Seaboard Forum, Tony might know why oil pressure does this?

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Old 11-15-2018, 12:22 AM   #6
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It would help if you said what the boat is and if the engine is mounted relatively flat or on a steep angle.

The Cummins B series has had this discussed MANY times on B.D. forum. Many of them have a shallow oil pan which when overfilled will get the oil beaten into a froth by the crankshaft.
Yes the slight drop was most likely from foaming. When you ran hard likely the boat nose attitude rose more than you normally see so the oil piled to the back of the pan raising the level to get beaten by the crankshaft forcing air into it.
Once you drop back to your normal cruise the boat leveled, the oil level evened out and the froth disappeared.
I've seen it on my own engine, A Cummins V555 engine, untill I dropped the oil level by 2 litres. My dipstick was marked incorrectly plus my engine has a steeper mounting angle than is ideal.

If you normally do not see this don't fret. I suspect the foaming in your case is not extensive but if ever you see the pressure drop while actually running hard then be carefull. Foaming is not good but what you saw this time was not extensive so it disappears quickly as the air dissipates. You probably saw the result of minor foaming.

GO to Seaboard Marine: Tony Athens
sbmar.com
and do some reading about oil pan capacities and filling of the B series. He is literally the Cummins expert of the boating world and has helped hundreds of people. And not on just Cummins engines.
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Old 11-15-2018, 12:57 AM   #7
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Hadn't thought about the change in trim. Yeah, that might have an effect.
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Old 11-15-2018, 05:34 AM   #8
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Completely normal. A run up to high power setting will increase oil temp by quite a bit, and thus reduce viscosity.

I kind of doubt oil temp only went up from 170F to 180F. Usually at high power oil temp is more like 190-200F, but that depends on how long at power and where you are measuring the temp. I don't think the QSB from the factory has an oil temp sensor, so probably some sort of aftermarket device.

I've sea trialed many of these and the posted numbers are typical.
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Old 11-15-2018, 05:49 AM   #9
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Quote:
Originally Posted by DDW View Post
Oil temps were up slightly during the run but not much (10 deg). I had noticed this the one other time I ran for a short time at full throttle as well. .
What gauge did you read this on? Unless you have something special, the gauge panel reads COOLANT temp.
I'm with the others. Oil pressure lowered due to hot oil from the full power run. There is no thermostat in the oil cooler circuit.
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Old 11-15-2018, 05:57 AM   #10
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Slight digression question.

Your WOT is 3100, and you cruise at 1300. Is that the usual/normal/efficient cruise for your boat/engine?
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Old 11-15-2018, 06:56 AM   #11
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Hi,

It is, Cummins QSB Engine, I have asked about this issue and CM / USA answered me.


My answers:
Hi,

I have a Cummins 5.9 qsb 380hp marine engine 2009 and it reaches max rmp 3065 which is perfect my Nordic Tug 37.

I have read a lot of conflicting opinions on the engine to run at low rmp a long time, because the machine may damage the carbon and etc. Some say it's ok to run this type engine at low rmp if the coolant remains in the correct slot.

The time i run my engine is most often 850-1300 rpm since the boat is most economical in this rmp area and temperature is ok. Cummins runs fine and does not smoke any, exhaust pipe mouth environment does not show any black carbon.

What is the manufacturer's view of low load low rmp almost always, whether it is ok or damaging the engine?

It would be great to have an expert answer, all the web instead of rumors.


And Cummins say to me:
This is fine for our engines. It is not suggested to Idle (650-750rpm) for long periods. Generally speaking, you can idle for about 20 minutes or so at this range and be okay. If you plan to idle longer than 20 minutes or so it is suggested that you ramp up your RPM to about 800-1000. Working the engine under a light load/rpm is fine. This is not uncommon for some our engines (like Generators which normally work at 1800 RPM or less) and will not cause any undue harm.
Thank you for contacting Cummins.
Katelyn
Customer Care Representative


In fact I trust what Cummins says about other wise men ...
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Old 11-15-2018, 10:39 AM   #12
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If I need to monitor oil temps, I shoot the oil filters with my laser thermometer. They normally run 190 when the coolant temps are 175-180.

I once needed to bypass a leaking oil cooler to get home. The temps rose to 240 and held. No problemo!
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Old 11-15-2018, 11:06 AM   #13
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Completely normal. A run up to high power setting will increase oil temp by quite a bit, and thus reduce viscosity.

I kind of doubt oil temp only went up from 170F to 180F. Usually at high power oil temp is more like 190-200F, but that depends on how long at power and where you are measuring the temp. I don't think the QSB from the factory has an oil temp sensor, so probably some sort of aftermarket device.
The QSB does seem to have oil temp from the factory. The Smart Craft display does not display it, but I have the NMEA 2000 translator, and can display it (and many other parameters) on other display heads. I'm not sure where the sensor is, but the readings correlate well to IR gun readings from the pan and oil filter. The oil temp was still climbing and I would not be surprised if it would get to 200 on a prolonged run. It only got to about 180 on this limited run. If the reduction in pressure was due to oil temp, I'd expect them to somewhat track, and that was not my impression. It took perhaps 2 or 3x as long for the oil temp to return to the previous temperature, as it took for the oil pressure to return. If I do this again I'll try to correlate them.

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What gauge did you read this on? Unless you have something special, the gauge panel reads COOLANT temp.
I'm with the others. Oil pressure lowered due to hot oil from the full power run. There is no thermostat in the oil cooler circuit.
The QSB is an electronic engine, pretty fully instrumented, all data accessible via Canbus J1939 protocol. There are a number of NMEA 2000 translators available to make this data available to modern boat instrumentation, including one from Cummins/Mercuiser themselves.
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Old 11-15-2018, 11:17 AM   #14
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Slight digression question.

Your WOT is 3100, and you cruise at 1300. Is that the usual/normal/efficient cruise for your boat/engine?
As N.B. Sea says, these electronic engines don't seem to mind running at low settings. 1300 rpm is 33% load, 7.1 knots, 1.9 gal/h. 3050 rpm on the other hand is 99% load, 17.6 knots, 19.2 gal/h. A top cruising speed would be 2500 rpm, 13.3 knots, 12.1 gal/h. If I'm in a hurry to get somewhere, I take a jet plane - cheaper and much faster . The displacement mode cruise is a little better than half as fast, and over three times as fuel efficient.

Note that the % load is torque produced vs. torque available at that speed.
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Old 11-15-2018, 12:05 PM   #15
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It's normal for oil psi to be a little lower after being run at high loads. Even though the oil temp has dropped to the same temp as before the rest of the engine components are still at a higher temp than they were before. So the engine clearances have changed with the increase in temp.
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Old 11-15-2018, 12:21 PM   #16
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Quote:
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As N.B. Sea says, these electronic engines don't seem to mind running at low settings. 1300 rpm is 33% load, 7.1 knots, 1.9 gal/h. 3050 rpm on the other hand is 99% load, 17.6 knots, 19.2 gal/h. A top cruising speed would be 2500 rpm, 13.3 knots, 12.1 gal/h. If I'm in a hurry to get somewhere, I take a jet plane - cheaper and much faster . The displacement mode cruise is a little better than half as fast, and over three times as fuel efficient.

Note that the % load is torque produced vs. torque available at that speed.
Interesting. My Lugger LP668T engines rate totally different. Perhaps because of the low HP rating.

High Output Rating.............FWHP (kW)/rpm 130 (97)/2400 174 (129)/2400
Medium Duty Rating.......... FWHP (kW)/rpm 113 (84)/2400 154 (114)/2300
Continuous Duty Rating..... FWHP (kW)/rpm 105 (78)/2200 140 (104)/2200

So while they show 2200 as continuous duty, I usually cruise at 1700-1800 and achieve close to full displacement speed all things being equal.
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Old 02-13-2019, 12:57 PM   #17
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It's normal

Quote:
Originally Posted by DDW View Post
I did a speed test run before parking the boat for the season. Normally cruise at 1300 rpm, 33% load, 47 psi. The run up to speed noting figures every 100 rpm took maybe 20 minutes, ending in 3050 rpm and 99% load. I returned to 1300 rpm and the oil pressure dropped to 42 psi. Over the next several minutes it came back up to 47. Oil temps were up slightly during the run but not much (10 deg). I had noticed this the one other time I ran for a short time at full throttle as well. Oil was just changed, Valvoline Cummins Blue 15W-40, as called for by the book.

Foaming? Anybody else's engine do this? Cummins QSB5.9 380hp.
A momentary 10% drop after running at 99% load and then returning to 33% load? All engines will do this. It's normal and expected, and really, beneficial, because it indicates more oilflow at heavy load.

Thermal expansion is the primary reason, reduced viscosity at high temps is also a factor.

Under heavy load, (even though you won't see it in your coolant temperature), the oil-lubricated, load bearing surfaces inside your engine produce more heat, and their temperature rises. This heat does get dissipated through the cooling system eventually, but remember that heat and temperature are two different things.

As the parts that are under friction loads are heating up and expanding, this expansion creates more clearance for oil to flow through. (remember that as the components of your engine heat up, everything expands -- including the clearances). Given that your oil-pump wants to deliver a constant volume of oilflow, increasing the gaps that the oil flows into increases the volume of oil flowing and this is why the pressure drops.

Also, at the point(s) where the pressurized oil is hitting the hot bearings, the temperature rise is dramatic and oil will definitely thin out there, so this too increases flow which results in reduced pressure.

Either way, increased flow under heavy loading is a good thing.

After you back off on the engine load, the situation reverses. Metal parts cool down and contract and oil clearances return to normal and that's why the pressure goes back up.

Re: "Oil temps were up slightly during the run but not much (10 deg).."

Your lubrication system is also a cooling system. Just because the oil temp at the sender is (let's say) 160 degrees doesn't mean that the oil inside the bearings isn't much hotter. The oil carries this heat away from the bearings and (in most engines like yours) this is why there is a separate oil cooler. That 10 deg rise you measured at the oil temp-sender may have been 100 degrees inside the bearings.

Plenty more here:

https://www.google.com/search?q=%22c...%22+%22flow%22


Just curious, what was the oil pressure at 3050rpm? At high RPMs I generally see something like 60-70psi on my 6BTAs, a little higher for the 6LYA.
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