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Thread: What is normal oil pressure (hot) for a newly rebuilt engine?

  1. #1

    Default What is normal oil pressure (hot) for a newly rebuilt engine?

    I had my Gray Marine 4-112 rebuilt last year. Right away I complained of low oil pressure when the engine is hot. The rebuilder kept recommending heavier grades of oil so now I am running 40 weight ND. After an hour at 1,200 RPM, my normal cruising speed, the pressure goes from the 35 it is when cold to about 20 pounds as read on a new electrical gauge. He says that is fine. It seems to me that an engine with 50 hours on it should hold pressure even when hot. So I am trying to find out what is normal and who is right on this issue. I have searched and searched and can't find an answer so I come to this group.

  2. #2
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    Default Re: What is normal oil pressure (hot) for a newly rebuilt engine?

    Carpe the living sh!t out of the Diem

    I'd rather look back at my life and say "I can't believe I did that" instead of being there saying "I wish I'd done that"

  3. #3
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    Default Re: What is normal oil pressure (hot) for a newly rebuilt engine?

    I can't quote you correct values for the oil pressure. But there is no question that the viscosity of the oil will drop significantly from cold to normal operating temperatures, and as the result the oil pressure will drop. Oil pumps are typically some type of positive displacement pump like a gear pump or a vane pump, which means that for a constant engine speed the oil flow is constant. With a constant flow, the pressure is strictly a function of the backpressure of the system, which is a function of the oil viscosity. Higher temperature => lower viscosity => lower pressure. As long as you have oil flow to all of the correct places the system is working.

    Cheers,

    Bobby

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    Default Re: What is normal oil pressure (hot) for a newly rebuilt engine?

    As hard as it always is to believe, even brand new gauges are faulty. I'll try to get a spec for you, but try to hook up a temporary mechanical gauge and check it with that, be sure to bleed it at the gauge. Also, engine with poor oil pressure sound like crap, so if yours is running fine with no rapping, you're likely fine.
    Last edited by Big Fish Billy; 10-23-2013 at 02:36 PM.
    "If you can't find the time to do it right, where will you find the time to do it over?"

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    Default Re: What is normal oil pressure (hot) for a newly rebuilt engine?

    Here is a site you can join and get the information you need: http://www.oldmarineengine.com/discu...3432/4164.html
    "If you can't find the time to do it right, where will you find the time to do it over?"

  6. #6

    Default Re: What is normal oil pressure (hot) for a newly rebuilt engine?

    Thanks for the replies. I had read the manual and noticed that pressure under 20 was a cause for concern which is why I posted this thread. What the manual does not say is what RPM is considered normal. Perhaps my engine, which is set up for 1200 RPM normal operation, is lower than normal. This is why I am asking others what they run as I am sure there are people with wooden boats that have this engine.

  7. #7

    Default Re: What is normal oil pressure (hot) for a newly rebuilt engine?

    Quote Originally Posted by Big Fish Billy View Post
    As hard as it always is to believe, even brand new gauges are faulty. I'll try to get a spec for you, but try to hook up a temporary mechanical gauge and check it with that, be sure to bleed it at the gauge. Also, engine with poor oil pressure sound like crap, so if yours is running fine with no rapping, you're likely fine. Also it take a long time for rings to wear in, especially the good quality ones. I've rebuilt several engines, one time I used real high quality chrome rings...at 50 or so boating hours a year, it took the 7 years before I finally stopped using a little oil and they were broken in. So first thing, hook up a mechanical gauge.
    I ordered a mechanical SW oil pressure gauge today. I can't see my electrical gauge showing a drop in pressure if there isn't one but no point having that be a question mark so out it goes and in goes the mechanical one as soon as it shows up.

  8. #8
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    Default Re: What is normal oil pressure (hot) for a newly rebuilt engine?

    A new engine should not need #40 oil to maintain a minimal pressure when warm. The book listed above says 40psi with #30 oil

    I hate to say it - but I think you may have a clearance issue. (cam bearings, rod bearings, main bearings, or the oil pump itself - all these were replaced with new, right?)
    Prove the gauge first.

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    Default Re: What is normal oil pressure (hot) for a newly rebuilt engine?

    I came up with the same numbers as The Big Fella up there, 5-15 at idle, 20-40 at operating speeds...as hard as it is to believe, the manufacturer may know more than us....hopefully the mechanical will give you a few more pounds of piece of mind........
    "If you can't find the time to do it right, where will you find the time to do it over?"

  10. #10

    Default Re: What is normal oil pressure (hot) for a newly rebuilt engine?

    Quote Originally Posted by Canoeyawl View Post
    A new engine should not need #40 oil to maintain a minimal pressure when warm. The book listed above says 40psi with #30 oil

    I hate to say it - but I think you may have a clearance issue. (cam bearings, rod bearings, main bearings, or the oil pump itself - all these were replaced with new, right?)
    Prove the gauge first.
    New crank and new rod, cam, and main bearings. No mention on the statement of work about the pump itself other than it was cleaned.

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    Default Re: What is normal oil pressure (hot) for a newly rebuilt engine?

    Quote Originally Posted by allene222 View Post
    I ordered a mechanical SW oil pressure gauge today. I can't see my electrical gauge showing a drop in pressure if there isn't one but no point having that be a question mark so out it goes and in goes the mechanical one as soon as it shows up.
    You don't necessarily have to swap the electrical gauge out and the mechanical gauge in. Many boats have a mechanical gauge mounted in the engine compartment allowing you to confirm the pressure if there's a questionable reading on dash-mounted electrical gauge.

  12. #12
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    Default Re: What is normal oil pressure (hot) for a newly rebuilt engine?

    Quote Originally Posted by allene222 View Post
    New crank and new rod, cam, and main bearings. No mention on the statement of work about the pump itself other than it was cleaned.
    If the engine needed all the other bearings, the pump is suspect....
    But it could be as simple as a stuck (open) pressure relief valve. The manual suggests the relief valve setting is 40 psi, but it may need more rpm to achieve that.
    Why is WOT at 1200 rpm? (The 112cu. in. model shows 31 horsepower at 2200)

  13. #13

    Default Re: What is normal oil pressure (hot) for a newly rebuilt engine?

    Quote Originally Posted by Canoeyawl View Post
    If the engine needed all the other bearings, the pump is suspect....
    But it could be as simple as a stuck (open) pressure relief valve. The manual suggests the relief valve setting is 40 psi, but it may need more rpm to achieve that.
    Why is WOT at 1200 rpm? (The 112cu. in. model shows 31 horsepower at 2200)
    If WOT is wide open throttle then that is not 1200 RPM. At 1200 RPM I get about 6 knots, or more or less hull speed. However, the engine has lots of power. What I know is this. The boat was specified by Lapworth with a 4-91 engine with 25HP and a 14x8 prop. Mine had a 4-112 installed at some point which is rated at 31HP as you state. I contacted Martec and they sold me a 14x10 prop. I just run it at 1200 RPM because I was told that is where it should run and it has no problem there. It is nowhere near wide open. In the slip in gear I was able to get above 1200 with only 3 cylinders running (stuck valve -- another story). I was easily able to get 1400 in the slip after freeing the valve as part of the test of oil pressure. I am sure it would go higher but at some point I guess the transmission might slip. At 1400 I had 21 pounds of oil pressure btw.

    The summary is I have too much HP and perhaps too much pitch. But I was able to get back to my slip last year with gusts over 50 knots where most boats were not able to get in so I can't complain about the power. If I need more power, it is there.

    I have no idea if the boat is set up correctly given the excess HP but that is the way it is.

  14. #14

    Default Re: What is normal oil pressure (hot) for a newly rebuilt engine?

    Quote Originally Posted by Dan Richard View Post
    You don't necessarily have to swap the electrical gauge out and the mechanical gauge in. Many boats have a mechanical gauge mounted in the engine compartment allowing you to confirm the pressure if there's a questionable reading on dash-mounted electrical gauge.
    The boat came with a mechanical oil pressure gauge but the tack drive broke so I install all new gauges. It will be trivial to switch out the electrical and install the mechanical. The plumbing is already there. The sending unit is actually mounted on the back side of the "dash" and is kind of in the way so taking it out and routing the tubing over to the new gauge should be easy.

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    Default Re: What is normal oil pressure (hot) for a newly rebuilt engine?

    Quote Originally Posted by allene222 View Post
    If WOT is wide open throttle then that is not 1200 RPM. .
    That is just the rated horsepower at 2100 rpm at full throttle, assuming one could load it that much. Your set up sounds OK.
    (The engine may run a long time the way it is, but if it was mine I would want more pressure. Low oil pressure is always an indication of a clearance issue, and it could be clearances in the pump).

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    Default Re: What is normal oil pressure (hot) for a newly rebuilt engine?

    I would want to know that the clearences in the existing oil pump were well within tolerence if your mechanic decided it just needed a clean. As the oil pump is the heart of the system, its paramount it has to be right. Maybe your oil pump was replaced at some point, but if your other items had worn that they needed replacing, i would want to check that oil pump.

    I would not have thought you would have to run on 40 grade unless your rings are real loose.Have you done a compression test on the bores?

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    Default Re: What is normal oil pressure (hot) for a newly rebuilt engine?

    The rings have nothing to do with oil pressure, they keep compression chamber and oil separated. Bearings clearances hold oil pressure.

  18. #18

    Default Re: What is normal oil pressure (hot) for a newly rebuilt engine?

    Next step is to put the mechanical pressure gauge in and run my test again. I will report back. I will also check the pressure relief valve and try the washer trick as it is easy although I can't see how it would help unless the valve is stuck open.

  19. #19
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    Default Re: What is normal oil pressure (hot) for a newly rebuilt engine?

    If you run out of options, you should be able to change the oil pressure be adding a washer under the plug on the side of the block for the oil pressure relief valve (check the manual). If is like a Gray109 (still a Continental block) there is a threaded plug on the side of the block. Remove that plug (the spring for the oil pressure relief valve is in there) and add a thin washer between the plug and the spring. This will increase the oil pressure. -- Rule out the other causes first.

  20. #20
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    Default Re: What is normal oil pressure (hot) for a newly rebuilt engine?

    Hi Allene222
    You have not told us much about your hull - length, beam shape, and most importantly approx. displacement nor an adequate description of the prop and it's mounting. Post pics please.

    Now, a diesel rated at 31hp @ 2,200 rpm should not have a 'normal' running condition of only 1,200rpm for it's good health.
    Your 'running' oil pressure is low because your operating revs are too low.
    As pointed out in a Post above, the oil flow is from a positive displacement pump; the flow from such pumps is virtually linear with respect to engine rpm; i.e. double the engine revs will give double the oil flow (almost). Then, with no other change to the system, double the flow results in double squared i.e. 4 times the pressure drop in the system. Your oil pressure gauge should then read something like 80 psi rather than 20 psi when hot EXCEPT that long before that pressure, your oil relief valve will be in play preventing/bypassing excessive pressure. As also suggested previously, check your relief valve operation/setting.

    Your current 14" dia. x 10" pitch prop giving 'hull speed' at 1,200 engine rpm sound very much to me that you have far too much prop.
    Does the engine exhaust exhibit black smoke ?retired design engineer

    If the craft's designer recommended a 14"x8" prop then with such a prop you would probably get something like 1,600 to 1,800 engine rpm at hull speed and thus a "happier" engine, more oil pressure and smoother running. Only thing worse for a diesel than lugging at low revs when it could be spinning more happily at higher revs is when you are at the other end of the scale and have it is revving it's head off with no or very low power demand (this later condition results in glazed bores).

    Have encountered a similar problem with a 32' steel chine yacht with a Perkins 4-109(?) and Borg Warner 1.9:1 gearbox. Engine laboured, made lots of noise and vibration and blew black exhaust smoke; not sure about oil pressure. But upon full review of the whole drive system incl gearbox ratio etc and propeller, we found the prop had been taken off a small plywood crayboat [lobster boat to you in USA] and was supposed to turn faster with LOTS more hp and drive the smaller, straighter crayboat at planing speed. The 3 blade prop had large "clover leaf" like blades probably in excess of 6" wide each which would have had a blade area of something like 65 to 70% [long time ago so relying on ageing memory] of the 14" diameter circle.
    Our local prop manufacturer advised that for a non-planing yacht, as opposed to the planing ply crayboat, we needed far less prop blade area and a finer pitch.
    The new 3 blade prop he made had an individual blade width of about 65mm [2 1/2" in USA], finer pitch and the correct hand rotation for the gearbox and the engine revs went up to something like 2,000+ rpm [Perkins are rather high revving anyway], the black smoke disappeared, the engine ran more smoothly, vibration disappeared and the stern came up out of the “hole” the prop was trying to dig.
    Result :- one happy owner, one happy Perkins.

    Please post some pics and provide some measurements but you are unlikely to get a "reasonable" oil pressure at what it tantamount to a fast-ish idle of 1,200 rpm.

    Cheers from Martin B.
    retired yachtsman and design engineer
    Mandurah, Western Australia.

  21. #21
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    Default Re: What is normal oil pressure (hot) for a newly rebuilt engine?

    Not a diesel...

  22. #22

    Default Re: What is normal oil pressure (hot) for a newly rebuilt engine?

    I did leave a lot of the story out and yes, it is not a diesel. It is also not the engine specified for the boat. The boat is a 36 foot wood sloop (Lapworth-36) of 12,000 pound displacement. The drive is 1:1. The specified engine had 25 HP and was 90 ci. Someone put in a used engine a long time ago and it is a 4-112 or 112ci and rated at 31HP. The prop I had on the engine was a two blade folding prop of 7 pitch and the combination would leave me unable to get back to the marina in strong winds. I consulted with Martec and they recommended the 14 x 10 pitch two blade folding prop. There is some tradition to get higher pitch with the folding props as I guess they have more slip. The old prop was 7 pitch because that is all Martec made initially. We had a race last year with gusts of 55 knots and I was one of the few boats that was able to get back into the marina so I am not complaining about the new setup. That said, I am beginning to think the low oil pressure is in fact due to the low RPM and not a fault of the builder. I have purchased a mechanical SW pressure gauge and intend to take the boat out and see if I can sail under at some higher RPM and note the oil pressure. Just kidding about sailing under but I will get more speed than I typically ask for and see what happens.

    I plotted the pressure readings vs RPM and they do not look right so before I say anything else I really do want to check things with the new gauge.

    The reason I made the post is that I thought someone here would have a similar engine and be able to tell me what they see oil pressure wise.


    Allen

  23. #23
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    Default Re: What is normal oil pressure (hot) for a newly rebuilt engine?

    Can you just put the gear in neutral and run the engine up to 2200?
    (That was manfacturers suggested full speed, from the book above)

  24. #24

    Default Re: What is normal oil pressure (hot) for a newly rebuilt engine?

    Quote Originally Posted by Canoeyawl View Post
    Can you just put the gear in neutral and run the engine up to 2200?
    (That was manfacturers suggested full speed, from the book above)
    I am going to take the boat out and plot RPM vs boat speed and oil pressure using the Stewart warner mechanical gauge. I will plot the results. I will increase the RPM until the boat sails under :-)

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