Oil Pressure Below Mercruiser Limits Per Manual

[FL370Pilot]

Hi Braintrust,
Just overhauled my engine again after it ran out of oil which was covered in another thread. Now with a fresh overhaul and new oil pump I am getting low(er) oil pressure than expected. Mercruiser says anything below 30psi at 2,000 rpm is too low. I am getting around 15-18psi at 2,000 rpm and 20-25 at 3000 rpm. At idle it is around 5psi. It does not set off the alarm at any point once the engine is started. I have heard 10psi per 1,000 rpm is a good rule and I am still a little under that. I am running 10W40 for summer running in Texas.

 

[Kola16]

How old is the gauge? Not all oil pressure gauges are the same. Is the reading accurate is what I am getting at.

You mentioned you ran it out of oil. What was the cause of that? Did you fix that problem? Are you still losing oil?

What engine are you running?

 

[Alumarine]

Is this a mechanical gauge?
Without starting an oil debate why not try it with a recommended weight which is 25W40.

 

[Kola16]

I found your other thread. I would test it with a temporary gauge. You can get them for $20.
https://forums.iboats.com/threads/1993-mercruiser-4-3l-v6-ran-out-of-oil.744386/#post-5673491

 

[FL370Pilot]

I found your other thread. I would test it with a temporary gauge. You can get them for $20.
https://forums.iboats.com/threads/1993-mercruiser-4-3l-v6-ran-out-of-oil.744386/#post-5673491

Sorry forgot to say that Yes I did verify it with a mechanical guage.

 

[FL370Pilot]

Is this a mechanical gauge?
Without starting an oil debate why not try it with a recommended weight which is 25W40.

I can try that. I didn't see anything but 40W in the manual but now I see it clear as day.

 

[FL370Pilot]

How old is the gauge? Not all oil pressure gauges are the same. Is the reading accurate is what I am getting at.

You mentioned you ran it out of oil. What was the cause of that? Did you fix that problem? Are you still losing oil?

What engine are you running?

Yes the problem is fixed. New Block and all new bearings. No oil loss that I can see. Only about 20 minutes on the engine so far.

 

[Scott06]

To me i think this means you dont have the correct bearing clearances on the reman or possibly a oil gallery plug was left out. Maybe try 15 or 20 w 50 and see if the pressure comes up. What were the bearing clearances when the engine was assembled ? Hate to say it but to me i would be questioning the engine rebuilder
i think the merc oil pressure alarm switch goes off at 4 psi

i just looked at the video i had of when i did my repower in 2016 on a new 5.0 running on a pallet prior to install had 45 psi oil pressure at idle ish 700 rpm with 15-40 rotella oil at 150 degree engine temp

 

[Rick Stephens]

If it wasn't Plastigaug'd, it needs to be. Too much money there not to pull the motor, pull the pan and check clearance.

 

[Scott Danforth]

how did you overhaul your motor?

be very very very specific

did you plastigage the bearings?
did you replace the cam bearings?
did you get a new oil pump?
did you go to thread-in oil galley plugs or did you use welsh plugs?
did you braze the oil pickup to the oil pump body?
why are you running 10W anything vs the oil recommended?
did you re-size the bearings (both mains and rods)?
did you re-use the stock rods or buy new?
did you recondition the crank or buy new? if you reconditioned it, did you get the proper undersized bearings?

the 10 psi per 1000 rule of thumb is a minimum.

you are below that.

stock motor with stock sloppy bearing clearances will run 40-50 psi at idle and be hitting the pump relief pressure at 1500 psi and above
rebuilt motor with properly clearanced bearings and a good melling pump with a 80 psi relief will be running higher than that and be at relief pressure by 1000-1200 psi

 

[Searay205]

I chased a low oil pressure issue in a Roush 427R for months. Bearing clearances were good, before and after I replace them, lol, Super long story short the oil put relief valve spring was to short from factory. My symptoms were good oil pressure up to 4K rpm at 40 psi then it would drop off to 40psig at 6000 rpm. Correct spring 65psig oil pressure at 6K rpms.... Sounds like you have a bad gauge (maybe new gauge is bad). Did your crank have scoring?????? Low oil pressure across all rpm usually main bearing clearance, low oil pressure at high rpm usually rod bearings. Remember rod bearings rotate and centrifugal force throws oil out at high rpm, main bearings just sit in same position. Plastigauge is fine as rough check but Mics are what you really want, do you know how to read? If not take to machine shop they will check for free probably

 

[wshekar]

the 10 psi per 1000 rule of thumb is a minimum.

Does this mean the manual is not accurate? I find it odd that the manual only lists oil pressure at 2 RPMs (Idle & 2K). Idle = 4 psi min, 2K = 30 psi min. That is under and over the "rule of thumb". Seems like a bad rule of thumb. Would be nice if they gave an oil pressure at WOT.

I sit at ~45psi at idle (at temp).

 

[Scott Danforth]

Does this mean the manual is not accurate? I find it odd that the manual only lists oil pressure at 2 RPMs (Idle & 2K). Idle = 4 psi min, 2K = 30 psi min. That is under and over the "rule of thumb". Seems like a bad rule of thumb. Would be nice if they gave an oil pressure at WOT.

I sit at ~45psi at idle (at temp).

oil pressure relief valve is 55 psi on the stock pump (most aftermarket pumps and high performance pumps are available with springs from 70-100 psi). other than maybe too thin of oil at idle you should be about 25-40 psi, at about 1000 RPM and above you should be at relief pressure. anything less than that means the oil is flowing out of some sloppy location in the motor.

on a fresh rebuild, you should be at relief pressure, unless someone botched the rebuild.

 

[wshekar]

oil pressure relief valve is 55 psi on the stock pump. you should be about 25-40 psi, at about 1000 RPM and above you should be at relief pressure.

That clarifies the expected oil pressures on a well running motor.

The engine in question is clearly low - even by the manual mins. I'd expect the issue to be with the engine/pump rather than using the wrong oil even in the Texas heat (barely hitting 90s here now).

 

[Searay205]

I had 5 psig oil pressure at idle in a 3.0 liter once. Changed the oil and had 20 pig at idle. Oh that right the oil was 6 years old. That was 25 years ago and engine still running strong. I like the 20W40 suggesting may get you comfortable with your oil pressure.

 

[FL370Pilot]

So I put in 25W40 and it works much better. The engine was overhauled by Blain's engines in Dallas. They have a great reputation fwiw.