Low oil pressure?

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    This is sort of OT!  My 71 pickup’s 390 has recently started leaking badly, rear main seal.  I have also recently started seeing low oil pressure.  Will a leaking rear main cause low oil pressure?


    It would depend on the reason for the leaking rear main.  If the seal is worn out, of course it will leak, but that may not affect oil pressure.  However, if the rear main is leaking because the rear main bearing is worn then you could also have low oil pressure.  There are other reasons for low oil pressure including worn cam bearings, a worn out oil pump, or oil that has too low of viscosity or just low oil levels.  You could also have an oil pressure gauge or sending unit that is malfunctioning.  Hopefully it isn’t too serious or expensive to fix.  By the way, does a ’71 have a PCV system?  If so, you might want to be sure that everything is functioning properly.  A plugged PCV valve or a constricted hose can create tremendous pressure in an oil pan that will push oil out of any weak spot.



    Virgil, no PVC before, but I have installed one.  This truck was a recent purchase.  Low pressure turned out to be a rear main seal, and a plugged pickup screen, which now mandates new rod bearings.  Was surprised the bearings were only lightly oil starved.  This engine was recently rebuilt.  The builder used a rubber rear main seal (should have used rope) and it was pinch, hence the rear main leak.  The FOD in the screen looked like the original plastic timing gears got chewed up (too hard to be gasket material) and collected in the pan.  The engine does have steel timing gears now, builder just didn’t clean the block, not sure how you screw this up.  I am fortunate to have a old school FE mechanic that is doing all the work with the engine in the truck.  He works at his own pace, but I can’t argue with the results or the money!


    Steve & Liz Jackson

    James,  I had one of my 390’s loose oil pressure to the point of it knocking, but still running.  I was racing it at the time, so I assumed I spun a bearing.  When I pulled the motor, I found all bearings very good shape.  Then I found the rear camshaft freeze plug had come out.  This allows the pressure drop.  I had put “press in” plugs when rebuilding it many years ago, but now I know better and use a “screw” in style, with locktite!  Just another thought.


    Steve Jacskon



    Steve, didn’t know about the plug!  Here’s a shot of the rear main.  Only had my cell phone camera, not real clear but you can see the rubber seal split and the oil starved bearing surface.




    Steve & Liz Jackson


    OUCH, that looks, not too good.  I hope that never happens to me(again).  Too many stories involving racing.  If you do rebuild or redo, make sure you get someone to do the machine work that is familiar with the Ford engine, especially the FE.  My latest 428 was done that way.  I searched out the one I wanted and spoke with him several times before using his services.




    Steve, it actually looks better than I would have imagined!  My guy is diffinately an old school FE mechanic.  He takes his time and double checks his work, wish the previous owner of this engine would have done the same!

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