Oil pressure drop after change

ZeeOSix

$100 site donor 2022
Joined
Jul 22, 2010
Messages
34,363
Location
PNW
Actually, the oil pressure is driven by the high-shear viscosity, not the low-shear kinematic viscosity (KV) reported on the data sheet. The base-oil viscosity is a better approximation to the high-shear viscosity than the KV. Since you have a synthetic blend versus a full synthetic, the base-oil viscosity index (VI) will be something like 110 vs. 125 or 115 vs. 130, respectively, and you will see even more than a 20% difference in the cold oil viscosity and oil pressure, explaining the cold-oil pressure drop by switching to a better oil, and yes, there is also the less restrictive fresh and larger-capacity oil filter, further reducing the overall, across-the-full-lubrication-circuit oil pressure.
As mentioned by WobblyElvis, when the oil pressure sensor is located after the oil filter then the oil filter has no effect on the oil pressure if the PD pump is still putting the same volume of oil through the oiling system.

The only time an oil filter would effect the oil pressure in that situation is if the filter was so clogged up that it made the PD oil pump go into pressure relief, which I highly doubt is happening at an idle.

The only two things that effect the oil pressure at the pressure sensor located after the filter are: 1) oil viscosity and 2) oil flow volume.
 

ZeeOSix

$100 site donor 2022
Joined
Jul 22, 2010
Messages
34,363
Location
PNW
2010 Silverado 1500 with 140,000. I switched from Valvoline High Mileage synthetic blend to Valvoline High Mileage full synthetic both 5w-30 and from a Valvoline VO88 oil filter to a Wix Xp. Why did my oil pressure go from 41-42 psi at cold idle to around 28 at cold idle after switching?
Variable volume oil pump, or old fashioned oil pump on this vehicle?
 
Joined
Dec 29, 2010
Messages
5,872
Location
Paramount, California
As mentioned by WobblyElvis, when the oil pressure sensor is located after the oil filter then the oil filter has no effect on the oil pressure if the PD pump is still putting the same volume of oil through the oiling system.

The only time an oil filter would effect the oil pressure in that situation is if the filter was so clogged up that it made the PD oil pump go into pressure relief, which I highly doubt is happening at an idle.

The only two things that effect the oil pressure at the pressure sensor located after the filter are: 1) oil viscosity and 2) oil flow volume.
Yes, with the idealized assumption that the oil flow generated by the oil pump only depends on the rpm and not on the pressure drop across the oil pump caused by the total flow resistance in the circuit, including the oil filter. In reality, there is some dependence on the pressure in addition to the rpm. See the following study done at 2,000 rpm.

 
Last edited:
Joined
Nov 22, 2004
Messages
9,054
Location
Texas
I just changed my '04 w/ 5.3 using RGT 5w20, oil pressure is 40 psi hot but up to 50 psi at cold startup, also using Fram Ultra filter. I was leery of running this but read that lots of others are using 5w20 with no noticable difference in wear when having it tested.

Everything seems fine but right now I kinda wished I would've made one change instead of a different oil and filter at the same time, I mostly run Delco filters and have worked out well for me, I figured if it doesn't work out I can just dump it and go back to 5w30.
Why are you using 5w/20 in a 5w/30 spec'd vehicle?
 

ZeeOSix

$100 site donor 2022
Joined
Jul 22, 2010
Messages
34,363
Location
PNW
Yes, with the idealized assumption that the oil flow generated by the oil pump only depends on the rpm and not on the pressure drop across the oil pump caused by the total flow resistance in the circuit, including the oil filter. In reality, there is some dependence on the pressure in addition to the rpm. See the following study done at 2,000 rpm.

Yes, there will always be some PD pump "slip". Thing is, it will not suddenly change between oil changes since the pump slip is a function of how tight the clearances are between the moving parts of the pump. In that example figure in your link (which may not represent every PD pump), the total pump slip is only 5 l/min out of 40 at an output pressure of ~50 PSI, or 5/40 = 12.5% when the pump hit's pressure relief.

Also, the OP is seeing a drastic oil pressure change at idle, so that's the lowest pump volume output possible with the engine running. If the pump was really worn, he would see even worse difference (between a healthy pump and a worn pump) in oil pressure at higher RPM and volume output.

What's the oil pressure go up to if the engine is revved to say 3000~3500 RPM while the oil is still pretty cold? Will it make the pump hit pressure relief?
 
Last edited:

BlueOvalFitter

Site Donor 2022
Joined
Feb 28, 2003
Messages
11,721
Location
Cajun Country, La.
Oil pickup tube oring is a known problem. Low pressure can occur as the pick-up tube o-ring can suck air where it meets the block.
I watched a few videos on YT about this. The O-ring can be replaced with the engine still in the truck. It's a tedious job, but taking your time, it CAN be done.
 
Joined
Jul 14, 2020
Messages
1,139
Yes, there will always be some PD pump "slip". Thing is, it will not suddenly change between oil changes since the pump slip is a function of how tight the clearances are between the moving parts of the pump. In that example figure in your link (which may not represent every PD pump), the total pump slip is only 5 l/min out of 40 at an output pressure of ~50 PSI, or 5/40 = 12.5% when the pump hit's pressure relief.

Also, the OP is seeing a drastic oil pressure change at idle, so that's the lowest pump volume output possible with the engine running. If the pump was really worn, he would see even worse difference (between a healthy pump and a worn pump) in oil pressure at higher RPM and volume output.

What's the oil pressure go up to if the engine is revved to say 3000~3500 RPM while the oil is still pretty cold? Will it make the pump hit pressure relief?
I'm not sure about the OP but I've seen mine hit 70psi under those conditions. Hot idle is 30-34 depending on grade of oil and how long I've been running it.
 
Joined
Jul 14, 2020
Messages
1,139
I watched a few videos on YT about this. The O-ring can be replaced with the engine still in the truck. It's a tedious job, but taking your time, it CAN be done.
From what I read this usually results in low oil pressure on cold starts that progressively gets worse until it's always affecting it. Not sure if this is always the case.
 
Joined
Apr 7, 2019
Messages
429
Location
VA
Why are you using 5w/20 in a 5w/30 spec'd vehicle?
Because I can and winter is around the corner where I mostly do short trips and the thinner oil helps on those cold mornings and get up to operating temp. sooner. 5w20 in theory has less VII should help make it more shear resistant. I have read several reports from Blackstone and others that show there is no noticeable increase in wear using 5w20 vs 5w30 in this generation.

Also I don't live in a warm place like TX as you do, I live up in the mountains and it can get fairly cold here.
 

Jamie86

Thread starter
Joined
Nov 11, 2021
Messages
8
That guy had a torn/stuck-bypass-valve/clogged oil filter.

Is your oil pressure normal at the normal operating temperature? If that's the case, there is probably nothing to I'll pay attention to it later on today. I THIN

That guy had a torn/stuck-bypass-valve/clogged oil filter.

Is your oil pressure normal at the normal operating temperature? If that's the case, there is probably nothing to worry about.
I'll check later on today. My memory is really bad but I think it's normal at about 42-43. I know its operating all within specs on both ends so I'm not necessarily worried. I was just wondering why there would be such a big change in pressure with the only change being in that of the oil and filter.
 

Jamie86

Thread starter
Joined
Nov 11, 2021
Messages
8
The plot thickens. Coincidence?
Oil pressure was what it's always been, 42-43, after I changed the sending unit and screen. Oil pressure didn't change until I switched to full synthetic and a better filter.
 
Joined
Jan 20, 2019
Messages
110
Location
Tampa, FL
My engine didn't seem to like some filters I've used in the past, the gauge would read lower psi and jitter around alot at hot idle, I've since used fram ultras and the gauge reads higher psi and zero jitter GM 5.7 engine also.
 
Top