Is my oil pressure somewhat okay?

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Hello good people of bobistheoilguy.

I have a 1999 Sierra 1500 4x4 that had a 4.3 vortec. I put in a 4.3 from a 2003 Silverado 1500 and the engine has been a dream in my gutted Sierra that I use to hit a few dirt and mud trails. It use to have what I think was okay oil pressure. Cold start 40psi. Warm idle exactly 20. And 45 when cruising. Then suddenly I saw the gauge bobbing around 10 at idle. I thought the mechanical gauge might be messed up since I dropped it so I put another. And the same thing. I changed the oil feeling dumb I didn’t realize it’s actually been a while and after cold start was 45 warm idle 25 and cruising 50 to 45. But as soon as I got off the freeway. It was bobbing 12-16psi. But as soon I touch the gas shoots to 35-40. No engine noise and is running great. Could it be the oil pump or pick up tube? Someone told me that might be pointless to try and do and that I should just get the engine overhauled. Any help or opinions would be greatly appreciated. Should I just Drive it until it’s done done??
 
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While that's sufficient pressure, it's abnormal for that engine is known for 40 psi warm idle. First thing would check is the sending unit and screen. If still showing low, it's likely a bad pickup screen o-ring. The o-ring is notorious for failing in those engines. One way to test that would be, right before changing the oil, pour in 2 quarts of your cheapest API locally and jack the rear of the truck up as high as you can to fully submerge the pickup inlet to the oil pump. If pressure suddenly jumps up at warm idle, that's your issue.

If it's a bad o-ring, the oil pressure is low due to the o-ring allowing air into the pump. Aeration isn't a good thing regardless of the cause so it's something you'll want to address.
 
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I would look further online what the actual spec is which is usually at 2000 RPM (HOT). Your pressures could be normal. During the summer, a heavier weight oil can't hurt. If you have faithfully done oil changes when you were supposed to, it would be very unlikely the pump's pickup screen is plugged with modern oil.
 
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Hello good people of bobistheoilguy.

I have a 1999 Sierra 1500 4x4 that had a 4.3 vortec. I put in a 4.3 from a 2003 Silverado 1500 and the engine has been a dream in my gutted Sierra that I use to hit a few dirt and mud trails. It use to have what I think was okay oil pressure. Cold start 40psi. Warm idle exactly 20. And 45 when cruising. Then suddenly I saw the gauge bobbing around 10 at idle. I thought the mechanical gauge might be messed up since I dropped it so I put another. And the same thing. I changed the oil feeling dumb I didn’t realize it’s actually been a while and after cold start was 45 warm idle 25 and cruising 50 to 45. But as soon as I got off the freeway. It was bobbing 12-16psi. But as soon I touch the gas shoots to 35-40. No engine noise and is running great. Could it be the oil pump or pick up tube? Someone told me that might be pointless to try and do and that I should just get the engine overhauled. Any help or opinions would be greatly appreciated. Should I just Drive it until it’s done done??

What oil do you use? I had a 1995 Chevrolet Tahoe with the 5.7L and on 5w30 oil pressure always got super low after a highway run. No abnormal engine noises, but it dropped enough for low oil pressure light to come on a couple times. I switched to 5w40/0w40 oils and never had that issue again.
 

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Your concern, justifiably, stems from the significant change in hot pressure and I believe @RDY4WAR has touched on a very appropriate course of action as well as a possible diagnosis. We've talked about GM pick-up o-rings on here before, it's a common issue.

Whether the new reading is still within the acceptable range or not really isn't relevant, you had a rapid, and significant, change from what it was reading prior and that clearly warrants investigation as it means something happened/changed.
 
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What oil do you use? I had a 1995 Chevrolet Tahoe with the 5.7L and on 5w30 oil pressure always got super low after a highway run. No abnormal engine noises, but it dropped enough for low oil pressure light to come on a couple times. I switched to 5w40/0w40 oils and never had that issue again.

The '95 5.7L is a Gen II Votec/LT based engine which has a very different oiling system. It's normal for them to have just 10-15 psi at hot idle from the factory with a 30 grade oil. The later LS based engines are whole different animal as they consistent 38-42 psi at hot idle. The poor design of the stock windage tray in the Gen II, coupled with a (relative) low oil capacity, made it susceptible to aeration and high oil temps after sustained rpm so the low oil pressure after a long drive isn't uncommon. The later LS engines are typically rock steady though.

Regardless of the engine, any sudden drop in oil pressure is an indication of something gone wrong. It may not be catastrophic, but it's not good.
 

Jesseyourcalipers

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Seems fine to me. An extremely basic rule of thumb is 10psi per 1000 rpm so if it's idling at 750-1200 rpm 12psi is completely normal.
That is what my dad said. He said be worried when it’s 0 and won’t move lol. But what is concerning is that there was a change. It use to be very very strictly 20psi when at red lights.
 

Jesseyourcalipers

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While that's sufficient pressure, it's abnormal for that engine is known for 40 psi warm idle. First thing would check is the sending unit and screen. If still showing low, it's likely a bad pickup screen o-ring. The o-ring is notorious for failing in those engines. One way to test that would be, right before changing the oil, pour in 2 quarts of your cheapest API locally and jack the rear of the truck up as high as you can to fully submerge the pickup inlet to the oil pump. If pressure suddenly jumps up at warm idle, that's your issue.

If it's a bad o-ring, the oil pressure is low due to the o-ring allowing air into the pump. Aeration isn't a good thing regardless of the cause so it's something you'll want to address.
My dad mentioned the pick up tube but could it be as simple as the o ring?? And I have just a brass fitting and a mechanical gauge hooked up. I’m probably going to end up checking the o ring and pick up tube or replacing everything. But could it be a bandaid on the problem like someone told me and it being a worn engine? It runs great
 

Jesseyourcalipers

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I would look further online what the actual spec is which is usually at 2000 RPM (HOT). Your pressures could be normal. During the summer, a heavier weight oil can't hurt. If you have faithfully done oil changes when you were supposed to, it would be very unlikely the pump's pickup screen is plugged with modern oil.
Yeah that’s what my dad said. I’ll probably give that a go and run thicker oil since I’m probably going to check the pick up tube and it’s summer.
 

Jesseyourcalipers

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What oil do you use? I had a 1995 Chevrolet Tahoe with the 5.7L and on 5w30 oil pressure always got super low after a highway run. No abnormal engine noises, but it dropped enough for low oil pressure light to come on a couple times. I switched to 5w40/0w40 oils and never had that issue again.
I use 5w-30. But probably 5w-40 a way better idea it being summer
 

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Yeah that’s what my dad said. I’ll probably give that a go and run thicker oil since I’m probably going to check the pick up tube and it’s summer.
If it is sucking air (not good) because of a failed o-ring, I would not continue to operate it. As @RDY4WAR noted, aeration is not good and there is the potential for you to cause damage. He outlined a process to determine if this is the case, I'd give that a whirl and then look up how difficult it is to change the part if it ends up being supported by the results of the test.
 
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My dad mentioned the pick up tube but could it be as simple as the o ring?? And I have just a brass fitting and a mechanical gauge hooked up. I’m probably going to end up checking the o ring and pick up tube or replacing everything. But could it be a bandaid on the problem like someone told me and it being a worn engine? It runs great

Oil pressure doesn't just suddenly drop from engine wear. In fact, you'd have to wear a bearing very deep before you'd clearance it enough to cause that. If that is happening, you have much bigger problems to worry about than oil pressure. Also, it would be obvious if that was the case. You'd hear a rod knock or something audibly alerting you to the issue. I am 99.99% sure it's the o-ring on the pickup tube. You have all the classic symptoms of a bad o-ring.


Yeah that’s what my dad said. I’ll probably give that a go and run thicker oil since I’m probably going to check the pick up tube and it’s summer.

That's going the opposite direction. Higher viscosity oil will be more difficult to pump which would draw even more air by a bad o-ring. Higher viscosity also traps air easier and doesn't dissipate it as well when dwelling in the pan. All you'd do with a 40 grade oil is take that risk of bearing cavitation from aeration and increase it.

Pressure is a measure of resistance to flow. The factors in that equation are the oil's viscosity, the oil pump (volume), and the restriction (oil galley/orifice). The oil pump and restriction are both constant so going to a higher viscosity will decrease flow, hence an increase in pressure. People put way too much importance on pressure. Flow is more important than pressure.

Do what I stated above with 2 additional quarts of cheap oil (same grade to keep viscosity constant) and lifting the rear end of the truck up as high as you can get it to submerge the oil pump pickup which located on the very front crank snout of the LS based engines. If pressure jumps up, you've found your issue. This is a very common issue that can only be fixed by replacing the o-ring. High Mileage oils and seal conditioners will not fix it. Higher viscosity will not fix it. No oil supplements or different oil brands will fix it. It can only be fixed by replacing it. It's not too difficult to replace, just time consuming.
 

Jesseyourcalipers

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Oil pressure doesn't just suddenly drop from engine wear. In fact, you'd have to wear a bearing very deep before you'd clearance it enough to cause that. If that is happening, you have much bigger problems to worry about than oil pressure. Also, it would be obvious if that was the case. You'd hear a rod knock or something audibly alerting you to the issue. I am 99.99% sure it's the o-ring on the pickup tube. You have all the classic symptoms of a bad o-ring.




That's going the opposite direction. Higher viscosity oil will be more difficult to pump which would draw even more air by a bad o-ring. Higher viscosity also traps air easier and doesn't dissipate it as well when dwelling in the pan. All you'd do with a 40 grade oil is take that risk of bearing cavitation from aeration and increase it.

Pressure is a measure of resistance to flow. The factors in that equation are the oil's viscosity, the oil pump (volume), and the restriction (oil galley/orifice). The oil pump and restriction are both constant so going to a higher viscosity will decrease flow, hence an increase in pressure. People put way too much importance on pressure. Flow is more important than pressure.

Do what I stated above with 2 additional quarts of cheap oil (same grade to keep viscosity constant) and lifting the rear end of the truck up as high as you can get it to submerge the oil pump pickup which located on the very front crank snout of the LS based engines. If pressure jumps up, you've found your issue. This is a very common issue that can only be fixed by replacing the o-ring. High Mileage oils and seal conditioners will not fix it. Higher viscosity will not fix it. No oil supplements or different oil brands will fix it. It can only be fixed by replacing it. It's not too difficult to replace, just time consuming.
Thank you so much for your time. I will be doing as you said. And yes very time consuming. Lol has a lift kit so I’m going to need to take out the motor mount bolts to crank the engine up a bit to clear the oil pan sliding out. It is what it is lol but thank you very much for your time and words.
 

Jesseyourcalipers

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Oil pressure doesn't just suddenly drop from engine wear. In fact, you'd have to wear a bearing very deep before you'd clearance it enough to cause that. If that is happening, you have much bigger problems to worry about than oil pressure. Also, it would be obvious if that was the case. You'd hear a rod knock or something audibly alerting you to the issue. I am 99.99% sure it's the o-ring on the pickup tube. You have all the classic symptoms of a bad o-ring.




That's going the opposite direction. Higher viscosity oil will be more difficult to pump which would draw even more air by a bad o-ring. Higher viscosity also traps air easier and doesn't dissipate it as well when dwelling in the pan. All you'd do with a 40 grade oil is take that risk of bearing cavitation from aeration and increase it.

Pressure is a measure of resistance to flow. The factors in that equation are the oil's viscosity, the oil pump (volume), and the restriction (oil galley/orifice). The oil pump and restriction are both constant so going to a higher viscosity will decrease flow, hence an increase in pressure. People put way too much importance on pressure. Flow is more important than pressure.

Do what I stated above with 2 additional quarts of cheap oil (same grade to keep viscosity constant) and lifting the rear end of the truck up as high as you can get it to submerge the oil pump pickup which located on the very front crank snout of the LS based engines. If pressure jumps up, you've found your issue. This is a very common issue that can only be fixed by replacing the o-ring. High Mileage oils and seal conditioners will not fix it. Higher viscosity will not fix it. No oil supplements or different oil brands will fix it. It can only be fixed by replacing it. It's not too difficult to replace, just time consuming
And it doesn’t matter it’s not an ls engine? Process and test still works? May be wrong but I thought lu3 was not an ls being called a 90degree engine. I am a beginner so please excuse if they are dumb questions
 
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Oil pressure doesn't just suddenly drop from engine wear. In fact, you'd have to wear a bearing very deep before you'd clearance it enough to cause that. If that is happening, you have much bigger problems to worry about than oil pressure. Also, it would be obvious if that was the case. You'd hear a rod knock or something audibly alerting you to the issue. I am 99.99% sure it's the o-ring on the pickup tube. You have all the classic symptoms of a bad o-ring.




That's going the opposite direction. Higher viscosity oil will be more difficult to pump which would draw even more air by a bad o-ring. Higher viscosity also traps air easier and doesn't dissipate it as well when dwelling in the pan. All you'd do with a 40 grade oil is take that risk of bearing cavitation from aeration and increase it.

Pressure is a measure of resistance to flow. The factors in that equation are the oil's viscosity, the oil pump (volume), and the restriction (oil galley/orifice). The oil pump and restriction are both constant so going to a higher viscosity will decrease flow, hence an increase in pressure. People put way too much importance on pressure. Flow is more important than pressure.

Do what I stated above with 2 additional quarts of cheap oil (same grade to keep viscosity constant) and lifting the rear end of the truck up as high as you can get it to submerge the oil pump pickup which located on the very front crank snout of the LS based engines. If pressure jumps up, you've found your issue. This is a very common issue that can only be fixed by replacing the o-ring. High Mileage oils and seal conditioners will not fix it. Higher viscosity will not fix it. No oil supplements or different oil brands will fix it. It can only be fixed by replacing it. It's not too difficult to replace, just time consuming.
Pretty sure OP has a 4.3 which is SBC based and not LS based. Not sure that the LS pick up o-ring test will accomplish anything in his case.
 
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My mistake. I saw 2003 Silverado and read 4.8L. The test I stated above would not apply. I need to stop posting before having my coffee.
 

Jesseyourcalipers

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My mistake. I saw 2003 Silverado and read 4.8L. The test I stated above would not apply. I need to stop posting before having my coffee.
Okay that’s why I asked lol and it happens I’m not very functional without my coffee so I can understand lol. Would it be worth checking out the pick up anyways?
 
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