low oil pressure
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Senior User
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low oil pressure
so I have a 94 f350 with a 351 I rebuilt the motor a few months ago new rings bearings cam and heads and oil pump finally got a real oil pressure gauge hooked up I have between 15 and twenty psi idling then it won't get past 25 psi when running the truck it is a standard volume and pressure pump and I'm running 10w40 oil what do y'all think
Cargo Master
Joined: Jan 2004
Posts: 3,118
Likes: 149
From: South Dakota
I would suspect, for some reason, your gauge is not capable of going past 25. It's strange that it won't read above 25 when the oil is cold. I would try to temporarily hook up a different gauge and/or sending unit.
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Lead Driver
Joined: Feb 2002
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From: Greater Austin, Texas
I am thinking the bypass valve spring in the oil pump is broken or otherwise bad and allowing the bypass to open too early.
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I always get lower psi when using a thicker oil.
I was also wondering if that psi is hot or cold because I get around 20 when hot and never worried about it. 40 psi when cold.
I was also wondering if that psi is hot or cold because I get around 20 when hot and never worried about it. 40 psi when cold.
Lead Driver
Joined: Jul 2017
Posts: 9,601
Likes: 1,174
From: San Jose, CA
Haven't done one myself in the newer trucks, but it was ALWAYS a pain in the butt doing in place on older trucks. Especially 4wd trucks with the rear-sump pans. Pump in the front, long pickup tube going all the way to the rear, usually equals big PITA.
If you're using a Fram filter, I would change it.
Because thicker oils can also raise the pressure, my first "test" would be to add some thickening "viscosity modifier" type additive. Something like STP Oil Treatment, or Motor Honey that are very thick.
This is not a cure, but at least it will tell you if viscosity is the primary reason for the low reading. I don't dispute killer's experience, and think it's totally valid. But in my experiences the thicker oils have also raised the pressures. I used 20w/50 in my 302 and it had 70psi cold, with 50-60psi warm at rpm and 40psi at hot idle all day long. Right up until I sold the engine still running with 175k miles on it. Always had good pressure.
The change of a defective filter could also be a simple and inexpensive test.
My thinking isn't necessarily that these are the only two possibilities. More that one can of additive and/or one filter change and losing maybe a quart or less of oil in the process, are both easy and inexpensive alternatives to an oils change and, ultimately a pump change.
And speaking of expensive and big hassle changes, when you first fired up the engine did you perform the standard cam break-in procedure? Seems like your engine should be right at the cusp of being either a flat-tappet or a roller cam, so should have been run at high rpm for at least 20 minutes immediately upon first startup if you still had a flat-tappet type.
If it's a roller cam, then no problem.
Not really sure what exactly a bad lifter can do to oil pressure, but figured I'd ask anyway.
With 10psi per 1000 rpm being the standard recommendation for the last 40 years or more now, as said you're fine at idle, but running out of head-room by 3,000 rpm.
Good luck.
Paul
If you're using a Fram filter, I would change it.
Because thicker oils can also raise the pressure, my first "test" would be to add some thickening "viscosity modifier" type additive. Something like STP Oil Treatment, or Motor Honey that are very thick.
This is not a cure, but at least it will tell you if viscosity is the primary reason for the low reading. I don't dispute killer's experience, and think it's totally valid. But in my experiences the thicker oils have also raised the pressures. I used 20w/50 in my 302 and it had 70psi cold, with 50-60psi warm at rpm and 40psi at hot idle all day long. Right up until I sold the engine still running with 175k miles on it. Always had good pressure.
The change of a defective filter could also be a simple and inexpensive test.
My thinking isn't necessarily that these are the only two possibilities. More that one can of additive and/or one filter change and losing maybe a quart or less of oil in the process, are both easy and inexpensive alternatives to an oils change and, ultimately a pump change.
And speaking of expensive and big hassle changes, when you first fired up the engine did you perform the standard cam break-in procedure? Seems like your engine should be right at the cusp of being either a flat-tappet or a roller cam, so should have been run at high rpm for at least 20 minutes immediately upon first startup if you still had a flat-tappet type.
If it's a roller cam, then no problem.
Not really sure what exactly a bad lifter can do to oil pressure, but figured I'd ask anyway.
With 10psi per 1000 rpm being the standard recommendation for the last 40 years or more now, as said you're fine at idle, but running out of head-room by 3,000 rpm.
Good luck.
Paul
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Senior User
Joined: Oct 2018
Posts: 166
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Haven't done one myself in the newer trucks, but it was ALWAYS a pain in the butt doing in place on older trucks. Especially 4wd trucks with the rear-sump pans. Pump in the front, long pickup tube going all the way to the rear, usually equals big PITA.
If you're using a Fram filter, I would change it.
Because thicker oils can also raise the pressure, my first "test" would be to add some thickening "viscosity modifier" type additive. Something like STP Oil Treatment, or Motor Honey that are very thick.
This is not a cure, but at least it will tell you if viscosity is the primary reason for the low reading. I don't dispute killer's experience, and think it's totally valid. But in my experiences the thicker oils have also raised the pressures. I used 20w/50 in my 302 and it had 70psi cold, with 50-60psi warm at rpm and 40psi at hot idle all day long. Right up until I sold the engine still running with 175k miles on it. Always had good pressure.
The change of a defective filter could also be a simple and inexpensive test.
My thinking isn't necessarily that these are the only two possibilities. More that one can of additive and/or one filter change and losing maybe a quart or less of oil in the process, are both easy and inexpensive alternatives to an oils change and, ultimately a pump change.
And speaking of expensive and big hassle changes, when you first fired up the engine did you perform the standard cam break-in procedure? Seems like your engine should be right at the cusp of being either a flat-tappet or a roller cam, so should have been run at high rpm for at least 20 minutes immediately upon first startup if you still had a flat-tappet type.
If it's a roller cam, then no problem.
Not really sure what exactly a bad lifter can do to oil pressure, but figured I'd ask anyway.
With 10psi per 1000 rpm being the standard recommendation for the last 40 years or more now, as said you're fine at idle, but running out of head-room by 3,000 rpm.
Good luck.
Paul
If you're using a Fram filter, I would change it.
Because thicker oils can also raise the pressure, my first "test" would be to add some thickening "viscosity modifier" type additive. Something like STP Oil Treatment, or Motor Honey that are very thick.
This is not a cure, but at least it will tell you if viscosity is the primary reason for the low reading. I don't dispute killer's experience, and think it's totally valid. But in my experiences the thicker oils have also raised the pressures. I used 20w/50 in my 302 and it had 70psi cold, with 50-60psi warm at rpm and 40psi at hot idle all day long. Right up until I sold the engine still running with 175k miles on it. Always had good pressure.
The change of a defective filter could also be a simple and inexpensive test.
My thinking isn't necessarily that these are the only two possibilities. More that one can of additive and/or one filter change and losing maybe a quart or less of oil in the process, are both easy and inexpensive alternatives to an oils change and, ultimately a pump change.
And speaking of expensive and big hassle changes, when you first fired up the engine did you perform the standard cam break-in procedure? Seems like your engine should be right at the cusp of being either a flat-tappet or a roller cam, so should have been run at high rpm for at least 20 minutes immediately upon first startup if you still had a flat-tappet type.
If it's a roller cam, then no problem.
Not really sure what exactly a bad lifter can do to oil pressure, but figured I'd ask anyway.
With 10psi per 1000 rpm being the standard recommendation for the last 40 years or more now, as said you're fine at idle, but running out of head-room by 3,000 rpm.
Good luck.
Paul
Cargo Master
Joined: Oct 2016
Posts: 2,738
Likes: 217
From: Northern VA
15w40 Rotella went in all my old beater trucks. However, I think lighter oil is actually better for these old engines. Most wear occurs when starting cold, lighter oil gets where It needs to be quicker. On my 32k mile 351w I am sticking with 5w-30 or 10w-30 at the highest.
Lead Driver
Joined: Jul 2017
Posts: 9,601
Likes: 1,174
From: San Jose, CA
Great result, congrats!
Too bad you didn't just change the filter first, and the oil next. Just to narrow down for sure which one it was.
Guessing the filter though, since the oil going from 10w to 15w all by itself should not be enough to make that much difference.
But the end result is what matters. Good one.
Paul
Too bad you didn't just change the filter first, and the oil next. Just to narrow down for sure which one it was.
Guessing the filter though, since the oil going from 10w to 15w all by itself should not be enough to make that much difference.
But the end result is what matters. Good one.
Paul