low oil pressure at start builds as engine warms

tigermoth
tigermoth Expert Adviser
edited November -1 in HUDSON
hello, besides my terraplane, i have another vintage car (4 cyl. XPAG in an MG TD) just out of overhaul that starts with 20psi oil pressure and as the engine gets warm it builds to 40 psi and holds that. anybody seen anything like this? the oil relief valve has been checked...new spring, new ball and nothing between the ball and seat. the engine also has an emergency by-pass valve. i know its not hudson, but the hudson guys are the best. any thoughts would be appreciated. regards, tom

Comments

  • dwardo99
    dwardo99 Expert Adviser
    Maybe the screen on the pickup in the oil pan is clogged and thick oil can't get through it. I've seen some nasty oil screens.
  • mars55
    mars55 Senior Contributor
    Well, another thing that could cause it is that oil pump has too much clearance and as the engine heats the clearance is taken up. I think you need to drop the oil pan and give the oil system a good check for blockages and bad clearances in oil pump and bearings.
  • Uncle Josh
    Uncle Josh Senior Contributor
    Keep in mind that pressure is a function of restriction given a constant volume.



    Something is warming up and causing more restriction. or the pump is somehow making more volume.



    Hmmm, if the ball check is set to 40 psi, then a reading of 20 means that the pump can't put out enough volume to overcome the leakage in the system or itself.





    There was a lot of wisdom in 'warming up' your old car before you ran it hard, and the old TD might need that.
  • junkcarfann
    junkcarfann Expert Adviser
    I would check the gauge before tearing apart the engine. It is possible that the gauge itself, or the sending apparatus, is not working correctly.



    If it is a mechanical gauge (with a pipe or tubing feeding direct pressure from the oil passage to the gauge), a restriction in that is very possible.



    Get another gauge (preferrably one with a wide sweep of the needle), and plumb it anew to the engine, the closer to the engine the better. Harley-Davidson fans often install a gauge that is threaded directly into the engine itself.



    I would do that before tearing apart the engine, which to me seems like ripping the wires out of the wall to find the cause of a faulty light, only to find that the light bulb has burned out.
  • Geoff
    Geoff Senior Contributor
    This could be the way of overcoming the fact that oil supplied for English cars has a built-in "leakability" factor! Seriously though, this goes against everything that usually happens, as normally the oil thins out and pressure drops. Is it perhaps that there is a restriction to the intake that is not allowing the oil to flow freely to the pump? As the oil thins out it is then flowing easier and gets up to full pressure? Very mysterious. Please let us know what you find, and good luck,

    Geoff.
  • rambos_ride
    rambos_ride Senior Contributor
    tigermoth wrote:
    hello, besides my terraplane, i have another vintage car (4 cyl. XPAG in an MG TD) just out of overhaul that starts with 20psi oil pressure and as the engine gets warm it builds to 40 psi and holds that. anybody seen anything like this? the oil relief valve has been checked...new spring, new ball and nothing between the ball and seat. the engine also has an emergency by-pass valve. i know its not hudson, but the hudson guys are the best. any thoughts would be appreciated. regards, tom

    Dumb question....If the gauge is electrical with a sender - Can the gauge just be wired backwards?
  • Unknown
    Unknown
    Find a reliable pressure gauge and mount it on the engine. That will tell you if gauge bad. I have seen air cause problems in mechanical gauges with a long line from engine to dash, much like air in brake lines causing a soft pedal, air doesnt compress as quick as oil.. Also go back and figure what you didnt do in your overhaul. maybe the cam bearings?
  • Hudsy Wudsy
    Hudsy Wudsy Senior Contributor
    My recollection is that these engines particularly sensitive spring and check ball issues. New or not, it might be a small investment to replace that check ball spring with another new one. I suppose that it would also be worthwhile to make sure that it comes from the most reliable of sources.
  • tigermoth
    tigermoth Expert Adviser
    hello guys, thanks for the replies. i will try to provide more info without coming across as shooting down all ideas. the engine assembly was completed this summer in an engine overhaul shop owned by a friend of mine. the oil screen and pan were boiled out at the same time as the block, the oil pressure gauge is pressure tube only and was verified with the shop's calibrated gauge. i also suspected the gauge supply line and douched it out. i will check that again and after reviewing the parts manual there is a small leather washer missing from where the copper tube connects to the back of the gauge. i will replace that. thank you all again..any other thoughts are most welcome and i will let all know what i find. regards, tom
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