Mercruiser GM V6 and V8 oil pressure

[QBhoy]

Hi all

On my own MPi V8 and any other GM V6 and V8 I’ve ever had or been on, the oil pressure at idle and good oil has always been around 40 psi at idle and at worst about 35psi at idle with used oil.
I was literally just watching a water test from a local dealer there. He was showing the instruments one by one and showing the oil pressure at 20 psi, whilst commenting that this was normal.
I can’t say I agree and would think this was a concern.
I then checked the manual for the V6 and V8 mercs and to my surprise, it said that the minimum acceptable oil pressure at idle is 4 psi !!! Surely not ? Am I being daft and missing something ?

 

[Bondo]

Ayuh,..... Acceptable oil pressure is 10 psi per 1,000 rpms,.....

 

[QBhoy]

Ayuh,..... Acceptable oil pressure is 10 psi per 1,000 rpms,.....

Man that seems way low. I’m certain I’ve never seen any anything like as low as that. Would you not be worried if your engine was indicating that figure Bondo ?

 

[Bondo]

Ayuh,..... I'd probably change the oil,......

My motors have always been tighter than that,..... 'n I run 15W-50 oil,.....

I worry more about to much oil pressure, til the oil warms up,.....

 

[QBhoy]

Ayuh,..... I'd probably change the oil,......

My motors have always been tighter than that,..... 'n I run 15W-50 oil,.....

I worry more about to much oil pressure, til the oil warms up,.....

Fair point.
Used to always run 15w40 in my old carb engines. Currently run the crappy quicksilver synthetic blend in the mpi. For nothing other than to keep the service file OEM. Change it twice a year though. I’m not a fan of it. Better than the normal 25w40...but not much.

 

[Scott Danforth]

the "rule of thumb" is 10 psi for every 1000 RPM

so at idle, while the 4psi is OK with mercruiser, I agree it is low.

many worn out SBC's are out there running 10 psi at idle with 5W-20 oil

However as Bondo said, change your oil. long carbon chains are sheared causing viscosity degradation. also contaminants could be holding the PRV in the oil pump open.

on motors I build, I use an 65-80# spring, coated bearings (slightly tighter tolerance) and a Hi Volume oil pump. That way I have 65-80 psi at idle running 15W40. the PRV also limits the oil pressure on the high end. I also have a few drilled out oil galley plugs to spray oil on the back side of the timing set, and I have the dizzy hole notched to spray the dizzy gear with oil


keep in mind, pressure is the result of the restriction of flow. So assuming a pump that is not compromised, low oil pressure is usually the result of oil viscosity/engine clearances/lifter leakage/rocker-arm leakage, etc allowing the oil to flow back to the sump faster than it may have when new.

Stock SBC PRV is about 50psi that means that when the pump sees 50psi, the spring opens and the pump dumps some of the outgoing oil to the incoming side. low oil pressure after that is the engine not being able to keep up with leakage due to viscosity breakdown either due to oil shear or temperature.

the Z28 pump (3849807) is about 20% more pressure and 20% more flow (simply due to a 65psi spring) that requires more pressure.

first thing to do..... change the oil



Oil pump bypass springs.

----pump -----------wire dia.-----coils---------free length------oil pressure-----spring color
stock M55 SBC-----.037”-------23.0-----------2.235”-------------47psi-----------plain
stock Z-28 SBC-----.041”-------24-¾----------2.210”-------------NA-------------white large end
misc SBC-------------.037”-------24.0-----------2.220”-------------NA-------------plain
misc HP SBC--------.042”--------26-¾---------2.390”-------------NA-------------blue

stock 454 BBC------.040”-------26.0-----------2.580”-------------47psi-----------plain
stock 454 BBC------.041”-------26-¼------2.400”to 2.450“----52 to 65psi-------green
HP BBC--------------.040”-------21-¾----------2.460”--------------NA------------yellow
HP BBC--------------.042”-------26-¾----------2.650”--------------NA------------pink
HP BBC--------------.0415”------26-½----------2.550“-------------NA-------------blue
misc BBC-------------.039”-------26-¾----------2.530”--------------NA------------plain

 

[QBhoy]

the "rule of thumb" is 10 psi for every 1000 RPM

so at idle, while the 4psi is OK with mercruiser, I agree it is low.

many worn out SBC's are out there running 10 psi at idle with 5W-20 oil

However as Bondo said, change your oil. long carbon chains are sheared causing viscosity degradation. also contaminants could be holding the PRV in the oil pump open.

on motors I build, I use an 65-80# spring, coated bearings (slightly tighter tolerance) and a Hi Volume oil pump. That way I have 65-80 psi at idle running 15W40. the PRV also limits the oil pressure on the high end. I also have a few drilled out oil galley plugs to spray oil on the back side of the timing set, and I have the dizzy hole notched to spray the dizzy gear with oil


keep in mind, pressure is the result of the restriction of flow. So assuming a pump that is not compromised, low oil pressure is usually the result of oil viscosity/engine clearances/lifter leakage/rocker-arm leakage, etc allowing the oil to flow back to the sump faster than it may have when new.

Stock SBC PRV is about 50psi that means that when the pump sees 50psi, the spring opens and the pump dumps some of the outgoing oil to the incoming side. low oil pressure after that is the engine not being able to keep up with leakage due to viscosity breakdown either due to oil shear or temperature.

the Z28 pump (3849807) is about 20% more pressure and 20% more flow (simply due to a 65psi spring) that requires more pressure.

first thing to do..... change the oil



Oil pump bypass springs.

----pump -----------wire dia.-----coils---------free length------oil pressure-----spring color
stock M55 SBC-----.037”-------23.0-----------2.235”-------------47psi-----------plain
stock Z-28 SBC-----.041”-------24-¾----------2.210”-------------NA-------------white large end
misc SBC-------------.037”-------24.0-----------2.220”-------------NA-------------plain
misc HP SBC--------.042”--------26-¾---------2.390”-------------NA-------------blue

stock 454 BBC------.040”-------26.0-----------2.580”-------------47psi-----------plain
stock 454 BBC------.041”-------26-¼------2.400”to 2.450“----52 to 65psi-------green
HP BBC--------------.040”-------21-¾----------2.460”--------------NA------------yellow
HP BBC--------------.042”-------26-¾----------2.650”--------------NA------------pink
HP BBC--------------.0415”------26-½----------2.550“-------------NA-------------blue
misc BBC-------------.039”-------26-¾----------2.530”--------------NA------------plain

great reading there Scott.
This isn’t my boat Scott. It’s in reference to a dealer advertising a boat for sale with a video of a sea trial. When watching it, I saw the oil pressure was showing 20 psi and got to thinking about my own boats past and present and more recently a boat a friend just bought with the same engine. All the ones I’ve had and know of all sit around 40 psi at idle with good oil and even with well used oil...not much below. That’s why I checked the specs from merc. I was amazed to see their figure of minimum 4 psi at idle and minimum 18 psi at 2000 rpm. I would have a panic if I ever saw 20 psi at idle. The oil would have to be a hell of a thin and lost its viscosity to do that. Never mind show 4 psi. Seems crazy low to me.
Begs the question...if 4 psi is deemed acceptable at idle, what the hell does the low oil pressure alarm at ?

 

[Lou C]

Well on my old engine (both before and after the top end rebuild I did back in '17) I have always had different oil pressure readings at idle depending on operational conditions. For example, after a cold start oil pressure has always been at or slightly above 40. And as long as the engine has not been up on plane, 40. But right after coming off plane, it has always dropped to approx 20-30 psi, and if allowed to idle a bit (oil is cooling off I assume) it comes back up to 40. I've never seen it lower than 20 psi. Running it is usually between 50-60 psi. This is with Merc 25/40 oil, which I have found after a Blackstone analysis still holds its rated viscosity just fine. So what you saw on that engine, may be common depending on if the engine was just run on plane for a while and just came off plane. In this pic I was running on plane and slowed down to idle, and the pressure came up to 40 again in a few minutes of idling....on plane, 50-60.

 

[QBhoy]

Well on my old engine (both before and after the top end rebuild I did back in '17) I have always had different oil pressure readings at idle depending on operational conditions. For example, after a cold start oil pressure has always been at or slightly above 40. And as long as the engine has not been up on plane, 40. But right after coming off plane, it has always dropped to approx 20-30 psi, and if allowed to idle a bit (oil is cooling off I assume) it comes back up to 40. I've never seen it lower than 20 psi. Running it is usually between 50-60 psi. This is with Merc 25/40 oil, which I have found after a Blackstone analysis still holds its rated viscosity just fine. So what you saw on that engine, may be common depending on if the engine was just run on plane for a while and just came off plane. In this pic I was running on plane and slowed down to idle, and the pressure came up to 40 again in a few minutes of idling....on plane, 50-60.

I’d say what you have said is spot on what most of them do.
the one in the review was ticking along at about 800-1000 rpm showing 20 psi. But my main point is about the manual suggesting that 4 psi is acceptable. Utter madness to me !

 

[Lou C]

I recall reading that the small block Chevrolet oiling system was very well designed and I guess they could run with lower than typical oil pressure readings without damage. Keep in mind that from 1955 to well into the 90s in most used in autos these had nothing more that a warning light; most other than high performance applications did not have an oil pressure gauge. So the whole time you owned it you really had no idea what your oil pressure was as long as it was above a certain minimum level. Might be that the light went on at or just under 4 psi!

If you didn't have a high performance GM car, they used warning lights for overheating, low oil pressure and battery voltage. The only US maker who consistently provided gauges even on base model cars was Chrysler, most had a temp gauge, ammeter and some had oil pressure gauges, most had warning lights for oil pressure and brakes. Some Fords had a temp gauge with warning lights for oil pressure and charging.

My '98 Jeep has gauges for temp, oil pressure, volts and fuel level. The 4.0 inline six idles at 30 psi and ranges from 30 to 55. Current mileage is 179,000.
My '07 has only temp and fuel level, lights for oil pressure and battery voltage.