Odd oil pressure drop...

Discussion in 'Oil and Lube Systems' started by Michel Resdi, Jun 22, 2018.

  1. Michel Resdi

    Michel Resdi New Member

    I drive a 67 396/325 Impala sports coupe,that i brought from the USA to France ,in 1990.
    135000 miles on the odometer.

    The car has not been driven,for ten years,so i decided to give it some fresh stuff before going back to the road.
    New timing chain and gears,and a pair of head gaskets,among other treats.

    After initial start up everything seemed to be ok.

    It ran fine,temp and oil pressure ok. (i have added some extra instruments)

    I let it warm up and iddle ,drove aroud the block a few times: no issues .


    Then ,after start up,i noticed a drop on the oil pressure meter,and the oil pressure idiot light went on.

    I killed the engine,and checked the engine bay.

    No leak whatsoever,the oil level was at the low mark,as i was planning to change the oil after a short time.


    I replaced the oil filter: same thing.
    Oil pressure builds up on starting ,then falls down slowly to almost none...
    I put a gage with an oil feed tube ,a new filter again ,and saw that pressure built up as the filter was filled then slowly falled down to zero.

    This not temperature related,as the engine was cold.

    I took the oil sump out,and checked the pump : it seems ok to me.

    What could cause this condition?

    Thanks for reading so far. Any suggestion welcome.
     
  2. Grumpy

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member

    pressure is a measure of resistance to flow volume,
    if the pressure drops is indicating EITHER the pump flow volume has been reduced ,
    or the level of oil flow volume resistance DECREASED ,example a oil gallery plug got loose and fell out.
    Id change the oil filter and oil, and pour in 5- quarts of fresh oil and see if that makes a difference as a first test,
    if the pressure is near zero you should confirm that with a second gauge and if its not a defective gauge,
    I know you don,t want too think about this, but I may be time to pull and inspect and rebuild that engine,
    you will have extensive damage result if you run an engine with near zero oil pressure,
    thus its time to find the cause before you run the engine to the point,
    it starts throwing broken parts through the block or heads,

    its sounds like a big job but having done that over a hundred times on bbc engine ,
    its not that difficult, and the early 1967 396 is one of the easier engines too work on.
    in my opinion , and experience and from lots of G.M. engine, race testing.
    theres no need for oil pressure to exceed about 65 psi,
    it takes power to spin the oil pump against that extra resistance, it induces extra wear on the distributor and cam gears,
    and it does nothing to reduce bearing wear or increase cooling on the bearing surfaces, he should open some bearing clearances marginally
    (maybe an extra half thousandth on the mains) to increase oil flow volume reaching the main bearings, and use a lower resistance oil pump bye-pass spring.
    extra oil flow volume cooling the bearings and valve train will do more for durability than oil pressure exceeding 65-70 psi
    [​IMG]
    swaping in a new oil pump, would certainly not hurt and as a test its not that hard or expensive,
    you should pull the engine apart and locate and correct the failed bearing or oil passage plug ETC.
    personally with 140K or so miles Id take the time to install new cam,
    crank and rod bearings and give the bore walls a good inspection,
    if they look ok, a mild hone and new rings after cleaning the pistons and if it was my engine a new cam and lifters
    , any flat tappet hydraulic cam approaching 140K miles is suspected of wear issues
    . Id check the valve guides and seals and get a cylinder head rebuild while I was at it.


    BE AWARE NOT ALL BBC OIL PUMPS FIT ALL BBC ENGINES, the LATER GEN V, and GEN VI HAVE A DIFFERENT PUMP due to main cap clearance issues (YEAH IM AWARE they can be retro fitted)
    [​IMG]
    [​IMG]

    spending the time too read through the links and sub-links will prevent a lot of wasted money and time and mistakes
    http://garage.grumpysperformance.com/index.php?threads/another-496bbc.5123/

    http://garage.grumpysperformance.co...alves-and-polishing-combustion-chambers.2630/

    http://garage.grumpysperformance.co...tion-of-crank-durring-short-blk-assembly.852/

    http://garage.grumpysperformance.com/index.php?threads/installing-an-oil-pump-pick-up-tube.1800/

    http://garage.grumpysperformance.com/index.php?threads/basic-info-on-your-v8-lube-system.52/

    http://garage.grumpysperformance.co...ing-parts-and-a-logical-plan.7722/#post-72126

    http://garage.grumpysperformance.com/index.php?threads/precision-measuring-tools.1390/#post-68850

    http://garage.grumpysperformance.com/index.php?threads/big-block-chevy-info.710/

    http://garage.grumpysperformance.co...e-springs-and-setting-up-the-valve-train.181/

    http://garage.grumpysperformance.co...g-and-installing-connecting-rods-pistons.247/


    http://garage.grumpysperformance.co...402-bbc-with-a-4-454-crank-or-even-4-25.2165/

    http://www.superchevy.com/how-to/148-0309-396-ci-chevrolet-chevelle-engine-rebuild/

    http://garage.grumpysperformance.com/index.php?threads/block-prep.125/

    https://www.summitracing.com/search...-5l-396/engine-family/chevy-big-block-mark-iv

    https://www.themotorbookstore.com/how-to-rebuild-big-block-chevy-engines.html

    http://garage.grumpysperformance.com/index.php?threads/bits-of-396-bbc-build-info.3968/
     
    Last edited: Aug 7, 2018
  3. Grumpy

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member

    its always a good idea to verify machine work, theres always some machine shops that either do low quality or shoddy work, and you can,t expect an engine to operate correctly if the clearances or consistency of the bearing and lubrication surfaces are not correctly set, you would not bet the first or last guy to be told a crank was cut to a size that was not the true size or sold the wrong bearings for an application, you should always verify machine work, with both a precision measuring tool and plasti-gauge
    [​IMG]
    [​IMG]
    [​IMG]

    MEASURE CAREFULLY
    [​IMG]
    [​IMG]
    [​IMG]

    http://garage.grumpysperformance.com/index.php?threads/can-i-get-it-polished.9214/

    http://garage.grumpysperformance.co...guess-on-clearances-and-journal-surface.9955/

    http://garage.grumpysperformance.com/index.php?threads/bearing-clearances.2726/

    http://garage.grumpysperformance.co...tion-of-crank-durring-short-blk-assembly.852/

    http://garage.grumpysperformance.com/index.php?threads/rotating-assembly-bearings.9527/
     
  4. Michel Resdi

    Michel Resdi New Member

    Thank you very much for these explanations.

    I already proceeded the way you suggest for the first steps: oil filter change ,oil filling to the high mark,use of another oil gage(mechanical),with the same results.
    The pressure rises to 60psi,then falls down enough to turn the idiot light on...

    I will take the mill out and search for the things you mentioned.
    (I had the heads rebuilt with hardened seats,and new guides ,a few years ago.)

    (By any chance,could the bypass valve on the oil filter housing cause this symptom?)
     
  5. Grumpy

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member

    thats a very common question but the answers no!
    [​IMG]
    [​IMG]

    [​IMG]
    [​IMG]
    [​IMG]
    [​IMG]

    [​IMG]
    [​IMG]
    ALL THAT OIL FILTER BYE-PASS VALVE DOES IS ROUTE OIL FLOW PAST THE OIL FILTER
    IF IT BECOMES SO CLOGGED WITH TRASH THAT THERES
    A 10 PSI DIFFERENCE IN THE RESISTANCE TO OIL FLOW THROUGH THE FILTER
    VS AROUND IT INTO THE BLOCKS OIL PASSAGES, oil enters the area over the oil filter in the block and is forced into the outer holes in the oil filter perimeter down through the case and filter element and up through the central hollow screw retention stud into the blocks oil passages, if the resistance too flow is too great the oil filter bye-pass valve routes oil around the filter directly from oil pump to the blocks oil passages.
    [​IMG]

    MORE USEFUL INFO
    [​IMG]
    [​IMG]
    yes the oil flows around the mounting stud,from oil pump to main cap to reach the engine oil passages, thru the oil filter
    [​IMG]
    [​IMG]
    [​IMG]
    failure to use the correct oil pump,mounting stud, bolt or nut or carefully check clearances when mounting an oil pump can cause problems
    [​IMG]
    [​IMG]
    ONE RATHER COMMON MISTAKE IS USING THE WRONG OIL PUMP STUD OR BOLT TO MOUNT THE OIL PUMP AS IF EITHER EXTENDS THRU THE REAR MAIN CAP IT CAN AND WILL BIND ON THE BEARING AND LOCK OR RESTRICT, SMOOTH ROTATION
    [​IMG]

    [​IMG]




    [​IMG]
    http://garage.grumpysperformance.co...-friction-and-pumping-losses.8966/#post-31978
    obviously the first two steps in diagnosing any oil pressure problem you suspect is to check your dipstick for the proper oil level in the engine and then test the pressure with an independent test gauge to verify your cars gauge is correct, and you do have a problem , not a defective sensor or gauge
    the oil pressure sensor location, is next to the distributor base,on most sbc engines on the rear of the block, but if your trying to track down an oil pressure issue it could be either a bad sensor, bad connection on the sensor or gauge, or a defective gauge, causing the high pressure reading OR it may be a defective oil pump, or bearing clearances or an oil passage plug, or the wrong oil viscosity, or a busted oil pump pick-up or incorrect installation of the oil pump or pick-up,defective oil filter or several other potential sources if its a low pressure issue as previously mentioned.

    [​IMG]
    broken , or improperly installed oil pump pick-ups can cause several major oil pressure issues
    [​IMG]

    [​IMG]
     
    Last edited: Jun 22, 2018
  6. Maniacmechanic1

    Maniacmechanic1 solid fixture here in the forum

    It sounds like the 396 BBC Engine Main Bearings are Severly worn out.
    Running oil clearances at .004.-.005".
    Too much.
    .002-.0025 desired.
    Ditto same on the Rod bearings new.
     
  7. Loves302Chevy

    Loves302Chevy "One test is worth a thousand expert opinions."

    2 other things to check:
    the oil pump pickup might not be set to the proper clearance off the bottom of the pan.
    the oil is not returning to the sump fast enough because the returns are somehow blocked (sludge, old silicone).
     
  8. Grumpy

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member

    yes Ive seen several cases where guys failed to install the oil pump pick-up at the proper minimum 3/8"-to-1/2" off the oil pan floor clearance,
    the result is the pump is starved for oil intake flow.
    [​IMG]


    [​IMG]



    [​IMG]
    [​IMG]

    [​IMG]


    almost every mechanics tool box needs a few basic measuring tools and supplies
    [​IMG]
    [​IMG]https://www.amazon.com/Claytoon-Set...d=1466872286&sr=8-17&keywords=plastilina+clay

    http://www.utrechtart.com/Plastalin...currency=USD&gclid=CN3G75zOw80CFQgaaQodKbgFjA

    http://garage.grumpysperformance.co...m-sure-your-convinced-its-the-oil-pump.11085/

    http://garage.grumpysperformance.co...m-oil-pump-installed-now-no-oil-pressure.525/

    http://garage.grumpysperformance.com/index.php?threads/bbc-oil-pump-in-a-sbc.2598/
     
    Last edited: Jun 22, 2018
  9. Michel Resdi

    Michel Resdi New Member


    Thank you.
     
  10. Michel Resdi

    Michel Resdi New Member

    Thank you
     
  11. Michel Resdi

    Michel Resdi New Member

    I took the cover off on the oil pump pick up screen.
    There was a lot of silicone sealant,and sludge and metalic debris...
    Looking like copper...
     
  12. Maniacmechanic1

    Maniacmechanic1 solid fixture here in the forum

    The engine is Hurt.
    Time to pull it.
    Tear it down and rebuild.
     
  13. Grumpy

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member

    excess silicone trash clogging the oil pump pick-up and internal oil passages in the block,
    from guys putting too much or improperly located silicone sealant on gaskets,
    is a common source of problems when the excess crap gets into the oil passages

    http://garage.grumpysperformance.co...oil-passages-and-improved-oil-flow-mods.3834/

    http://garage.grumpysperformance.com/index.php?threads/metallic-debris-in-filter.12364/

    http://garage.grumpysperformance.co...k-after-a-cam-lobe-rod-or-bearings-fail.2919/

    http://garage.grumpysperformance.co...oil-passages-and-improved-oil-flow-mods.3834/
     
    Last edited: Oct 9, 2018
  14. Michel Resdi

    Michel Resdi New Member

    yes,i think i have no choice.
    Thank you.
     
  15. Michel Resdi

    Michel Resdi New Member

    Thank you.
    Great stuff you gave me links to.
     
    Last edited by a moderator: Oct 9, 2018
  16. Grumpy

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member

    any progress?
    I had hoped you would take a bunch of clear pictures of the engine , dis-assembly, cleaning, inspection, and rebuild
     
    Last edited: Oct 9, 2018

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