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Priming the oil pressure gauge

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    Priming the oil pressure gauge

    Having removed my instrument panel and disconnected the mechanical oil pressure gauge, can anyone tell me whether I need to spin the engine over to get some oil back in the pipe before I reconnect the gauge or does it self prime in some way.

    TIA

    #2
    No, just connect the pipe up. Doesn't matter if there is air in the pipe - 50 psi of air gives the same reading as 50 psi of oil !
    Now Stagless but have numerous car projects
    So many cars, so little time!

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      #3
      Great, thanks

      Comment


        #4
        Chris.
        It just takes longer for the gauge to read, particularly in the winter when the oil is thick, if there is air in the pipe as the oil has to go some way up it to get the air compressed. For a satisfyingly quick reading on the gauge I have always bled the air out of mine.
        Mike.

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          #5
          easy enough to bleed the air but potentially a messy job.

          Personally I would run the engine, crack off the union to the back of the gauge and as soon as I see oil nip it back up again. A huge pile of tummy wipes will keep the mess to a minimum.
          Stags and Range Rover Classics - I must be a loony

          Comment


            #6
            Originally posted by richardthestag View Post
            easy enough to bleed the air but potentially a messy job.

            Personally I would run the engine, crack off the union to the back of the gauge and as soon as I see oil nip it back up again. A huge pile of tummy wipes will keep the mess to a minimum.
            But why do you need to bleed the air??? pressure is pressure no matter what medium transmits it.
            Now Stagless but have numerous car projects
            So many cars, so little time!

            Comment


              #7
              Thanks all for your opinions. Doesn’t seem to matter either way but I’ll probably try bleeding mine as the pressure gauge had been slow to react when first starting the engine. Hopefully that’s not a sign of anything else being wrong!

              Comment


                #8
                Originally posted by marshman View Post

                But why do you need to bleed the air??? pressure is pressure no matter what medium transmits it.
                no idea, just seems sensible. Not going to bother googling but does air compress more than oil? if so might that have an impact?


                I don't loose sleep if there is air in the pipe.
                Stags and Range Rover Classics - I must be a loony

                Comment


                  #9
                  Originally posted by marshman View Post

                  But why do you need to bleed the air??? pressure is pressure no matter what medium transmits it.
                  By that logic I’ll drain my brake fluid.

                  Comment


                    #10
                    With my very basic science knowledge I might be able to help. As I understand it, the atoms in a quantity of gas are further apart and therefore less dense and easier to compress whereas in a liquid the atoms are packed closer together and therefore harder to compress. This is why it’s imperative for safety reasons that the brake system is bled thoroughly, as air can lead to inefficient compression and brake performance. So, I’d say it’s probably best to bleed the oil gauge too.

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                      #11
                      I have never bled oil presssure gauge lines, and they have always worked just fine, as does my current one. IMHO a line full of oil would be very viscous at low temperatures which could lead to poor gauge response?
                      Header tanks - you can't beat a bit of bling.

                      Comment


                        #12
                        Originally posted by KOY 23 View Post

                        By that logic I’ll drain my brake fluid.
                        Your car, your choice. But I wouldn't
                        Last edited by marshman; 14 April 2024, 22:19.
                        Now Stagless but have numerous car projects
                        So many cars, so little time!

                        Comment


                          #13
                          Originally posted by PhillyJH View Post
                          With my very basic science knowledge I might be able to help. As I understand it, the atoms in a quantity of gas are further apart and therefore less dense and easier to compress whereas in a liquid the atoms are packed closer together and therefore harder to compress. This is why it’s imperative for safety reasons that the brake system is bled thoroughly, as air can lead to inefficient compression and brake performance. So, I’d say it’s probably best to bleed the oil gauge too.
                          Yes all gases are more compressible than fluids but, as Marshman says Pressure is Pressure.
                          The only difference is if air is in the gauge pipe more oil will enter the pipe to compress the air, does this matter in the oil gauge, no because there I are pints of the stuff. Does it matter in your brakes yes because you only have a very small amount in the master cylinder.
                          If the pressure were changing very rapidly then the air would act like a damper but this isn’t the case in our engines.
                          So do I bleed oil pressure pipes? no but if you want to it do it, it makes no difference.
                          - Alan

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