Low oil pressure

Discussion in 'Troubleshooting & Diagnosis' started by valleyguy, Dec 11, 2020.

  1. sandlapper

    sandlapper Veteran Member

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    upload_2021-1-15_20-55-29.png

    from GM Heritage archive resto kit for 2000 chevy express van rpo L31 5.7L 350 ci
    iron head Vortec last of the gen 1 sbc

    <*>the unit in GM's above chart (psiGage) is not same as psi; and can be misunderstood. No, I'm neither chemist or physicist; am open for corrections & willing to learn.
    to convert psig to psi, seems one must add ambient air pressure to psiGage.
    we all know ambient air pressure at 32*F at sea level is 14.7 psi.
    fyi altitude increase causes ambient air pressure decrease; somewhat noticeably.
    ambient air temp increase causes ambient air pressure increase; but with relatively little effect.
    e.g. ambient air pressure at a mile high garage on a 70*F is about 12.15 psi.
    JMO, seems an easy way to interpret above GM pressure chart is to add 15 psi to either of the three specs. e.g 6 psiG @ 1000 rpm + 15 is about 21 psi.

    * here's chart for 77 Camaro ... from same source as above chart... note oil pressure unit is PSI (not PSIG)
    upload_2021-1-16_15-41-22.png
     
    Last edited: Jan 16, 2021
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  2. Rosster

    Rosster Veteran Member Gold Member

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    probably too low but most idled around 20psi when warm if clearances were good.
     
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  3. badazz81z28

    badazz81z28 Veteran Member

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    There probably simply a difference between "good" and "acceptable" versus "will survive". In my 81 when I had a leaky rear seal, I ran that engine with the oil pressure light on all the time. It managed to live. If GM says 6 PSI at 1000 is the min, should be ok. I think the LS engines are rated the same...I think its 1 psi per 100rpms. I wouldn't be a fan however.
     
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  4. sandlapper

    sandlapper Veteran Member

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    but ... again ... the chart says 6 psiG @ 1000 ... not 6 PSI @ 1000 ... 6 psiG = about 20-21 psi

    your older car probably has a PSI instrument ... Not a more modern psiG instrument like the LS vehicles have.

    I struggled with it for some time before I lifted the needle off my later truck's psiG instrument and put it about where PSI would be. Not so unlike sticking a piece of electrical tape over an "always on" CEL aka MIL.

    PSI and psiG are Not the same ... kinda like horsepower measurement ratings; SAE Gross v. SAE Net

    ALSO
    since 1981, GM OE sbc oil pumps have changed. IIRC, beginning in early 1990's GM changed from an M55 sbc pump to an M155 sbc pump. They are Both STD volume and pressure, but the difference is in the pickup tube diameter. M55 has 5/8" OD tube while M155 tube OD is 3/4". M155 is reverse-compatible and should be considered during rebuild. The change was made because GM found the larger OD (& thus ID as well) helped pump suck up oil at low to mid rpm. Both sbc & BBC pumps are efficient at Pushing oil but rather inefficient at Pulling oil. Honestly, I don't know when GM began moving away from PSI and to psiG instruments.
     
    Last edited: Jan 20, 2021
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  5. Rosster

    Rosster Veteran Member Gold Member

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    It’s common to see OEM gauges read PSI (which is really psig) but I have never seen an actual PSI gauge factory installed in any car. There would be no value in doing so as it would read 14.7 psi at sea level while the engine isn’t running.
     
    Last edited: Jan 20, 2021
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  6. Spellbound

    Spellbound Veteran Member

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    Does it matter where the oil pressure sensor is located on the engine? The factory idiot light is on the head I believe while my aftermarket gauge is on the top for example.
     
  7. biker

    biker Veteran Member

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    The factory sensor is on the back of the block, behind the intake, drivers side. There is also a port above the oil filter. There are no pressure sensor locations in the heads, only coolant temp sensors.
     
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  8. Steemo

    Steemo New Member

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    I'd like to have both a gauge AND the idiot light. So, run a "switched" sender for my stock idiot light, about 5psi folks here are saying. I'll put that one up top just behind the Dist. I'd also like to use The Gauge Marks Oil pressure gauge to replace the clock in the gauge cluster. So a Borg Warner S768 is the sender to use for that apparently. Biker, can you confirm in my pic here the port to use above the oil filter? Is that it? Looks like it's still open on mine here. Or is it the plugged hole closer to the filter base? Good thing you've mentioned this, or I might have had oil puking out on first start-up. That is still to come. OilPressureByFilter.jpg
     
  9. COPO

    COPO Veteran Member Lifetime Gold Member

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    I use a T fitting and use both idiot light and gauge behind the distributor which is the factory location.
     
  10. biker

    biker Veteran Member

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    Hmmm... the open hole you see is for the clutch "z-bar" linkage. The blank, orange plug just above the filter pad is where the port should be. Maybe your block isn't drilled for it. You can use a tee for both sensors in the port up by the distributor as Mark mentioned above.
     

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