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Old 08-12-2018, 12:16 PM   #1
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Low oil pressure question

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?
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Old 08-12-2018, 12:27 PM   #2
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At what rpm and sump temperature are you getting those pressures?
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Old 08-12-2018, 12:54 PM   #3
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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.
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Old 08-12-2018, 01:30 PM   #4
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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?

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Old 08-12-2018, 02:00 PM   #5
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My manual says 35 psi at 2000 rpm.
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Old 08-12-2018, 03:19 PM   #6
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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.
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Old 08-12-2018, 03:20 PM   #7
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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.
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Old 08-12-2018, 03:37 PM   #8
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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.
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Old 08-12-2018, 03:41 PM   #9
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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?
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Old 08-12-2018, 03:48 PM   #10
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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.
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Old 08-12-2018, 04:09 PM   #11
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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?
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Old 08-12-2018, 04:20 PM   #12
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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.
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Old 08-12-2018, 04:48 PM   #13
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Quote:
Originally Posted by melissar View Post
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.
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Old 08-12-2018, 05:14 PM   #14
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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.
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Old 08-12-2018, 07:29 PM   #15
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What grade oil?
Not trying to start a war on oil grades, but perhaps a change might improve pressure.
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Old 08-12-2018, 08:41 PM   #16
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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?
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Old 08-12-2018, 09:52 PM   #17
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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?
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Old 08-12-2018, 10:43 PM   #18
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Need to know pressure at 600,1000,1500,2000 with hot sump before any advice can be given.
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Old 08-13-2018, 10:45 AM   #19
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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.
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Old 08-14-2018, 07:48 AM   #20
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Start to think of a rebuild when the lube oil consumption becomes too high.
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