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  #1 (permalink)  
Old 04-07-2017, 11:12 AM
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Default Oil pressure question

I just bought my car right before winter so I don't have many miles on it yet. Not new, it is about 15yrs old and about 15k miles on it. I am still working my way through the car checking everything out.

One thing that irks me is the oil pressure gauge always reads around 52-55psi since I got it (At cold startup, warmed up, idle, driving around, etc....it doesn't seem to move much more than 5psi any time I look at it) When you shut the car off, it sinks to zero again so its not stuck.

Its a mechanical gauge and this is the first car I have had with a mechanical gauge so I am trying to understand whats going on. If the sender was bad, don't they usually read zero, act erratic or peg the gauge? Seems weird that it always reads about the same value. Doesn't seem to me like it would be a gauge issue. I don't see any damage in the line from sender to gauge...No oil pump noise, etc. Its a 1969 351w...Thoughts?
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Old 04-07-2017, 12:21 PM
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If I understand you correctly, a mechanical gauge should have a small oil tube going to the back of the gauge directly from the oil port on the lower left front of the block, so you should be seeing a true reading of fluid pressure. Typically about 10 lbs higher at cold startup, and after everything warms up you'll see maybe a 10psi drop. Standard SB oil pumps are typically 35-40 psi. A high volume pump can see 60 psi, cold.
If this is not the case, and there is a sending unit with a wire, then it is an electric gauge, and the readings don't surprise me. It is basically an idiot light that goes to a gauge instead of a light.
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Old 04-07-2017, 02:02 PM
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Agree with the above. A lot of electrical gages have a lower limit (very low) and as long as pressure is above that it shows a steady reading. Oil pumps produce varying pressure depending on rpms, oil temp, etc - and a true mechanical gage will reflect that.
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Old 04-07-2017, 03:30 PM
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Mine has done the same thing, two completely different engines...I think its due to a high volume oil pump that is slightly relieving all the time? At least thats what i've talked myself into...My stock Duramax diesel's oil pressure guage (as with most other non hotrods ive owned) responds like its hooked to the tach...

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Old 04-07-2017, 04:48 PM
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I have a mechanical gauge with the clearish oil filled tube going to the port in the front of the block. I have always heard rules of thumb like 10psi pressure for every 1000rpm, etc. but this thing just sits steady all the time. There is a fair amount of air in the oil line to the gauge but from what I have read, that shouldn't make any difference because the pressure is such a low range it doesn't really compress the air much and it doesnt take much pressure to move the needle, etc. So don't need to bleed that air out from what I can tell.

So, I'm just stumped how the gauge can be steady like that all the time. A mechanical gauge like this should be moving. Doesn't give me a good feeling when oil pressure is the life of the engine. Any other thoughts?
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Old 04-07-2017, 05:17 PM
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If you have a high volume regular pressure oil pump and you put a heavy weight oil in the engine, the pressure relief could be lifted all the time. Then the PRV spring will act like a pressure regulator. So this could be happening. Or the gauge could be defective. Should be pretty easy to connect a test gauge to the engine block and verify your gauge is correct or prove it is full of it.
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Old 04-07-2017, 05:59 PM
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AND I've had three different guages; one original full sweep smiths, one electric full sweep smiths, and one half sweep smiths with diff senders, and they ALL behaved the same.

And because I was concerned about the same issue I have a tee with a mechanical guage at the filter adapter-sender....reads 50ish all the time.

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Old 04-07-2017, 06:04 PM
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Old 04-07-2017, 06:05 PM
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I think olddog and tortuga hit the nail on the head! That makes perfect sense. Thank you very much.
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Old 04-08-2017, 09:32 AM
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Depending on what weight oil you are running, I might would recommend going to a lighter oil.

Why? If at idle, warm engine, your oil pressure is not dropping enough to close the PRV, then who knows how much oil is flowing through the engine. Slow oil flow through the engine allows the oil that is in the bearings, doing the work, to be shear thinned longer, which puts more heat into the oil. So you end up with really hot, thin oil in the bearings, where it counts. By putting too thick of oil in the engine (slow flow), you can actually end up with thinner oil in the bearings, when it counts, than you would have had, if you had used a thinner (faster flowing) oil to start with.

Seems counter intuitive, but the oil pressure never moving seems wrong too, no?

Last edited by olddog; 04-08-2017 at 09:37 AM..
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