Low oil pressure question

[melissar]

Looking at a boat with older (1974) twin Lehman 120s and have a couple questions about oil pressure. Both engines are around 3000 +/- hours. The boat consistently runs about 30+ psi on the starboard engine, 20+ on the port side. (Gauges are inaccurate, these readings are taken at the engines by an experienced mechanical surveyor.) We have oil analyses that show nothing unusual from 5 years ago and 6 months ago and expect a current report this week (although for the current sample, only have 10-15 approx hours since oil change.) We don't expect the new analysis to show any changes. Current owner says boat isn't using oil and we have no reason to doubt that assertion. Engines run well, no knocking or pinging, no scary smoke or sheen, etc. I'd really appreciate thoughts from those of you more experienced than we are -- would you be confident, cautious or run?

 

[Ski in NC]

At what rpm and sump temperature are you getting those pressures?

 

[BIG CAT]

If the samples came back good and the oil psi was verified with the same known working gauge at the same oil temp. I would be looking into the oil psi relief asy.

 

[kchace]

The first thing I would have suggested is to use a mechanical gauge right on each engine to verify but it appears that's been done. 20 and even 30 psi is low at cruise speeds for a Lehman. Th fact that both engines are showing low is interesting. There are of course a number of engine related items that can cause low oil pressure. The only external thing I can think of that might be a contributor is very thin or possibly diesel contaminated oil. But to have something like the same thing on both engines is unusual to say the least.

The mechanic DID put a known good mechanical gauge on both engines, right?

Ken

 

[jleonard]

My manual says 35 psi at 2000 rpm.

 

[melissar]

Very good, working gauge used on both engines. Both engines have been consistently like this for at least the last five years. Temps both right around 165, and readings taken both at idle and about 1500 RPM with no significant difference.

 

[Lepke]

Low oil pressure is from bearing wear, oil pump wear, and rarely, weak pressure relief valve spring. The wear is caused by infrequent oil and filter changes, engine under a load before oil has warmed, and too high oil temperatures. Sometimes from excessive high rpms.

Most likely the engine went through periods of infrequent oil changes. Most of the pressure can be restored by changing bearings and oil pump. But there are machined places not usually served by replaceable bearings, like the lifter holes and rocker arms.

 

[melissar]

Also I'd add that the current owner purchased the boat in 2013 with the same PSIs that we got two days ago, has run it normally, changed oil and filter every 100 hours, and had no problems. The two oil analyses we have from 2013 and this February show virtually the same numbers on both port and starboard engines.

 

[sunchaser]

My thoughts - 45 year old engines, high sulfur fuel for many decades, possible wrong service intervals and the things Lepke mentions. Hopefully the boat has not seen paper towels or other experiments for oil filters.

Does the vessel suit you otherwise? If so have some rabbit hole funds available. How is engine access?

 

[melissar]

Never know about paper towel experiments (is that really a thing?!). The current owner is on top of it, and I believe the owner before him had most maintenance done by the yard where she was moored. But there's a lot of history we simply won't know.

With the exception of a few other issues that you would expect with a 40+ year old boat, she fits us like a glove. To a "T". Peanut butter to our jelly. Feels like home.

No one has a crystal ball and so yes, there will always be rabbit hole funds. It's a boat, after all. We don't want to be unduly naive, but we don't want to be scared away unnecessarily, either. I do have a message in to American Diesel to get some guidance, as well.

 

[melissar]

I am not the mechanic in the family, but wouldn't you expect to see metal in the oil if it were bearings? Especially over the course of five years?

 

[Martin J]

I had similar problems with my twins on one engine. Fair oil pressure when higher RPM's but entering a bay and easing back on the throttles. my port engine would drop oil pressure to 15-20 psi . It happened over one season. Had a chat with American diesel, and they suggested the oil pump. Whipped the engine up in the air and changed the pump. Viola! incredible pressure after that. Sounds like pressure is to low anyway, and at min recommended.

 

[BIG CAT]

I am not the mechanic in the family, but wouldn't you expect to see metal in the oil if it were bearings? Especially over the course of five years?

If you're pulling good samples. You will see a elevated reading in wear metal's. Copper,iron ect. If the samples are consistent with no increase in ppm. You would investigate further into the oil system.

 

[Ski in NC]

So no change in pressure from idle to 1500rpm? That is strange considering how the system works.

Worn bearings or oil pump mean low OP at idle, but it will rise with rpm.

Only thing the would explain low OP from idle up would be a weak regulator spring.

 

[jleonard]

What grade oil?
Not trying to start a war on oil grades, but perhaps a change might improve pressure.

 

[melissar]

Good samples pulled by professionals, so feel confident in method. 30 wt oil, although the Feb 2018 sample was 10w 40. Back to 30 again now.

Can anyone tell me what side the oil pump is on on these engines? Fore or aft? Can it be changed with the engine in place by just dropping the oil pan or does the engine need to be lifted?

 

[melissar]

Also, another question: I have read on several forums (possibly even here on TF) suggesting that low op isn't a concern on the 120s until it dips down as low as 15 or even 11 psi. Is it or is it not?

 

[Ski in NC]

Need to know pressure at 600,1000,1500,2000 with hot sump before any advice can be given.

 

[melissar]

Unfortunately I don't have the surveyor's report in my hand yet. When engine survey was done on Saturday, pressure was taken manually directly at the hot engine (165 deg.) and remained virtually the same from idle to 1500 rpm to my best recollection.

 

[FF]

Start to think of a rebuild when the lube oil consumption becomes too high.

 

[melissar]

So there isn't any unreasonable oil consumption. Spoke with American Diesel and they have some potential easy fixes, so will keep you posted.

 

[Aquabelle]

Is there a remote oil filter on the low psi engine? The extra hose lengths associated with remotes can create a differential pressure reading between engines. Also are the loil filters the correct type ?

 

[jleonard]

I'd like to see the list of potentials

 

[Lepke]

My Detroit mains idled at 10 & 15 psi before I overhauled them. And they probably ran that way for 20 years. If the engine maintains decent oil pressure at running speed, you're probably ok for a long time. If pressure starts to drop, it's much better to overhaul before failure. Less costly, too.

 

[FF]

DD are famous for low oil pressure at idle.

On coaches with 8V engines this is used to allow the coach air cond clutch engage.

When the oil pressure is below 5psi , GM knew the engine was at idle so the 80 hp air cond compressor could be engaged.

Worked for decades,

 

[Miz Trom]

MelissaR:

Have you received the report on the oil samples yet?


Pea

 

[Brooksie]

In case it wasn't mentioned b/4; engine oil cooler performance can have quite an effect on oil temp, thus viscosity, thus pressure, at running temps. Might be worth removing and cleaning the cooler from the offending engine. You could also get a larger cooler for just that engine. My Perkins, famous for low oil pressure, responded very favorably to a new and better oil cooling system.

 

[Barney]

So no change in pressure from idle to 1500rpm? That is strange considering how the system works.

Worn bearings or oil pump mean low OP at idle, but it will rise with rpm.

Only thing the would explain low OP from idle up would be a weak regulator spring.

I would back this diagnosis, the engines are old enough to be senior citizens, that little spring has been doing its thing , for a long time if you cant get a new spring you can always put a couple of washers under it to increase its pressure, like everyone did with side valve fords.
The other thing to check is lift the rocker cover and see how much oil is coming out of the relief valve hole ,if there is a good flow of oil just restrict the flow slightly with your finger see if the guage moves, use common sense in this method. if it burns your finger ??? !
Those old lehmans are absolutely brilliant engines so simple and absolutely back of the axe reliable ,good ones well serviced will do 10-12000 hours

 

[kolive]

I suspect the oil lines might be smaller diameter lines that could restrict the flow. There might be too many elbow fittings to affect a smooth flow also. Should be 1/2” lines. Try runnin a bypass line by removing the lines off the block and patching in a 1/2” line to bypass the oil filter and cooler and see what the pressure is. ��

 

[kolive]

My last post was meant to be tongue in cheek but it did not come off that way. My apologies for this. This turned out to be the solution that was suggested by Brian of American Diesel and happily worked out. Thank you Melissa for finding this through Brian. Your research skills are fantastic. New hoses plumbed correctly will be nice.