So I have low Oil Pressure, now what?

[chphotographic]

Hi all,

Long story short I have low oil pressure. I have tested this with a mechanical gage. *The problem is real. *At start up the pressure is normal. *At cruising RPM as the engine heats up the pressure drops from 60 PSI to 45PSI. *Then once I bring it back to Idle the pressure drops to almost Zero, (the alarm does NOT go on). *

The way I understand it worn bearings are letting too much oil pass, and pressure is not able to be maintained when the oil heats up and the pump is spinning slowly. *I don't think diesel is leaking into the oil, (the colour and smell are ok-- I think) but it could be I guess.

The thing is I am at least 2000 miles from any chance of rebuilding the engine. *I can get some simple truck parts along the way but only simple stuff. *

What can I do to nurse this girl across the line?

Info:
Engine: Kubota v1505-b, 37hp, 4 cylinder, 1992, 5000 hours,**
Oil: when I bought the boat there was a bunch of 30 and 40 weight synthetic oil on board, *I did a change 30 hours ago to cheep 15w40 Pennzoil. *(the problem started before the oil change)
I have only had the engine for about 50 hours of use. I know that it was well maintained for the first 20 years, but the last few years the owner was totally clueless so who knows......
*

[a64pilot]

I would first cut the oil filter open and look for brass colored metal.
Assuming there is none, then I’d run straight weight 40W oil, maybe 50W if you can get it and if it’s needed.
Ideally you need or determine if the oil pump is good and if the bypass is stuck open, but doesn’t sound like that is an option.
If you find a lot of brass looking metal in the oil filter, that may indicate a spun bearing, and then almost certainly it’s going to completely quit soon, very soon.
Does it have any kind of hollow knock when idling, hot, that isn’t there when it’s cold?

If not and there isn’t any metal, run the heavy oil, do not run multi vis oil and baby the thing, run her easy and don’t let it overheat. Treat it like it’s an old man, don’t make it work hard, I’m not normally a fan of STP or motor honey, but it may be indicated here to thicken the oil.

This want a sudden thing was it, low oil pressure since you had it?

[Compass790]

Yr oil pressure doesn't sound that bad to me, pretty good for a 5000hr engine. Try what A64pilot suggests. Mains/big-ends probably worn but may go for many years yet. Did it suddenly change or always been that way? It's completely normal for the oil pressure to drop when oil warms up as it thins out & resistance to flow decreases. Marina neighbours 20hp Kubota is 80psi cold & 60 hot.

[Compass790]

Oil pressure alarm in our Yanmar YSM 8 goes off at 1.4 psi or 0.2 kg-cm psi ( data frm w/shop manual) so obviously they dont think you need much pressure at idle. BTW the Kubota figures I gave you are for a low hours engine at cruising revs

[chphotographic]

Thanks all, I'm going oil shopping now. this drop in pressure happened the first time I ran her for 4 hours straight. before that I only used the engine for getting on and of the pick, and the like.

[Wotname]

Originally Posted by chphotographic

Thanks all, I'm going oil shopping now. this drop in pressure happened the first time I ran her for 4 hours straight. before that I only used the engine for getting on and of the pick, and the like.

It takes quick some time (say 30+ minutes) for oil temperature to get to hot so it is quite possible it has been like this for some time. You had never got it hot enough to be an issue until now.

There is good advice upthread. Use heavier oil and don't idle!

I won't worry too much unless there are bad noises coming from the engine.

FWIW, my 50 year old 4 cylinder diesel tractor engine runs at almost 0 psi at idle when hot; my solution to not to let it idle . Only needs a few extra RPM to get it up to at least 15 psi.

[wholybee]

Before you switch to a high viscosity oil, consider this:
Oil pressure is measured right after the oil pump, and a high viscosity oil will increase oil pressure there. But, it will also reduce oil flow to the rest of the engine, where you need it. You might have a situation at idle with nearly no oil at all getting the the parts that need it. If it were me, I wouldn't change the grade of oil. Lower pressure and higher flow is better.

[Wotname]

Originally Posted by wholybee

Before you switch to a high viscosity oil, consider this:
Oil pressure is measured right after the oil pump, .......

This is engine dependant, many (most?) are measured after the oil filter.

[Compass790]

Originally Posted by wholybee

Before you switch to a high viscosity oil, consider this:
Oil pressure is measured right after the oil pump, and a high viscosity oil will increase oil pressure there. But, it will also reduce oil flow to the rest of the engine, where you need it. You might have a situation at idle with nearly no oil at all getting the the parts that need it. If it were me, I wouldn't change the grade of oil. Lower pressure and higher flow is better.

Yes this seems to be an argument that has merit too. Hard to get a definitive answer but if the poster wants high oil pressure high viscosity monograde is the way to achieve that. Experts? seem to be divided but my vote goes to multigrade as look how much faster yachts ( sailboats in American english ) go with technology improvements & technology bought in multigrade.
Oil arguments can go on forever. LOL

[sailorboy1]

if the oil was really too low, the engine would already be dead

[hpeer]

I found this....I think he is taking gas engines but I’m guessing it is similar for diesel.

“How much oil pressure is enough? Keep in mind that what counts is flow. A time proven rule of thumb is 10psi/1,000rpm. This assumes the normal range of engine bearing clearances and it is well proven that 10psi/1,000rpm will supply enough oil flow when normal clearances and parts are used. Since we have no convenient way to measure flow, we use pressure as a surrogate. If the bearing clearances are larger, we need more flow to keep an adequate film of oil between the bearings and the shaft. Do we need more oil pressure? No, but it will take more volume to produce the same pressure because the resistance is lower. So, we need a high volume pump to maintain oil pressure. This costs hp – more power is needed to drive a HV pump. The reasons to choose large clearances are beyond what I want to go into now.”

http://kennedysdynotune.com/oil-pressure-tech/

I also found a statement that most oil lamps are set to come on at 4-5 PSI, DANGEROUSLY LOW. So I think that kind of provides a range of acceptable. 10PSI/1,000RPM but never below 5PSI. If it dips that low at idle then pick up the idle a bit until you get pressure.

[a64pilot]

You need flow, and sometimes pressure, modern engine bearings need a pressure film to ride on.
It is possible to build engines of course that have no oil pump, like a lot of lawnmower engines for example, they are just splash lubricated.
They do this by having very low bearing loads, or said another way their bearings are oversized. Almost all engines used to be this way. Of course it limits the power you can get from an engine.
Aircraft engines were I believe among the first full pressure lubricated engines, done that way to get as much power out of the size and weight that you could, and for this reason the first US WWII tank engines were aircraft radials. US just didn’t have large high power pressure lubricated engines suitable.
Old engine designs often were splash lubricated engines and added the oil pump, which gave you an engine that would run almost forever and on low oil pressure, early engines ran very low oil pressure. I think the Ford 8 and 9N tractor engines are an example of that, they live longer than their owners. Likely a reason our old design Diesels have a reputation of living long lives, their bottom ends are overbuilt. But that is expensive, heavy and inefficient.

More modern engines were designed from the beginning for high oil pressures as its most efficient, smaller lighter bearings and rods and crankshaft can handle higher power with high pressure.

All this is to try to explain that the answer of how much oil pressure do you have to have is it depends. Old designs can run for a very long time with little oil pressure, just some flow, where a modern design may soon fail with low pressure. I’m calling low being around 10 PSI or less just to put s number on it.

Also the whole interior of your motor is actually oil cooled, the water only really cools the head and a little of the cylinder, so oil flow is very important to carry away the heat.

Usually an oil pump has a pressure relief valve, often called the cold oil relief valve as of course cold oil is thicker and pressure higher.
The flow rate of the pump is the same, until the relief valve is opened, then of course there is less flow cause the relief valve is dumping the excess.
However running s heavier grade of oil should not reduce flow unless your getting the pressure high enough to open the bypass valve, something that is unlikely in an old worn engine.


Brand new air cooled aircraft engines usually run 50W oil. The reason is that they are built very “loose”open tolerance that are similar to a worn out liquid cooled engine.
Primary reason is no matter what, you can’t have an aircraft engine seize, one way to make this very unlikely is to run very loose tolerances, which means you need thick oil.

Modern automotive engines have gotten so precise in their machining and can hold such tight tolerances, some like Toyota run 0W oil, 0W-20 actually.

So within limits of course you want to adjust your oils viscosity to maintain the pressure, this is the reasoning behind multi viscosity oil, in theory it will self adjust its viscosity with temperature to maintain a steady oil pressure. Not so high when it’s cold, and not so low when it’s hot.

[a64pilot]

Now just me, but I don’t think multi viscosity oil is warrantied in our application.
Now if you live in Wisconsin and crank your truck up to go to work every morning with sub zero temps, than absolutely multi vis is the way to go.
However I don’t know about you guys, but I don’t like it when the temp regularly goes much below 80F, if it does, I move. Endless Summer

For those like me, I don’t think multi vis is necessary. There is no free lunch in this world, everything has limitations, noting is perfect, to include multi vis oil. One of the first things to wear out in it is the viscosity improver.
I prefer a quality, high detergent single grade oil myself

[Lepke]

Before I overhauled my Detroit mains, idle pressure with warm oil was about 5 psi. At rated hp (1800) about 25 psi.

a64pilot has it right. The heavier oil will flow fine, especially when warm. As long as you don't have metal in your filter. It's a good idea to cut open oil filters as a regular part of maintenance. Also if a longer spin on filter is made to fit the threads and seal diameter, it will reduce any restriction in the filter and clean the oil better. Many filter manufacturers have online filter specs.

[a64pilot]

Originally Posted by wholybee

Before you switch to a high viscosity oil, consider this:

Oil pressure is measured right after the oil pump, and a high viscosity oil will increase oil pressure there. But, it will also reduce oil flow to the rest of the engine, where you need it. You might have a situation at idle with nearly no oil at all getting the the parts that need it. If it were me, I wouldn't change the grade of oil. Lower pressure and higher flow is better.

Your right in an engine that isn’t worn out.
Example is don’t run 15W-50 oil in a new engine that the spec calls for 5W-30, unless there are specific reasons to for the reasons you state.