Intermittent Oil Pressure

Slant 6 Engines

  1. grantwt

    grantwt Member

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    Hi all,

    New to the forum. Looking for help on my 68 Valiant. It has intermittent Oil Pressure.

    The motor I have in it is a ~76 bottom end, with a 68 head. It also has the 68 camshaft in it. Before I put it in, I put new bearings and rings in it.

    After I put it in, I had good oil pressure, 20psi at idle, 60psi at high revs. After a little running, the oil pressure dropped out, showing nothing at idle,, and only about 30psi at high rpm. It will even knock sometimes at idle. However, sometimes the oil pressure would suddenly fix itself, and be 20/60 again.

    I pulled the motor and replaced the pickup, and the oil pump wondering if something was wrong with oil pickup. I couldn't see anything wrong in the bottom end, so I put it back together. However, I still have virtually no oil pressure.

    Anybody have any ideas? It acts like there is a leak in the system, but I can't figure out how it sometimes stops leaking and has pressure again. Thanks,

    Grant
     
  2. my68barracuda

    my68barracuda Well-Known Member

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    If you are not reading off a known good mechanical gauge do that first.
    If you still have a low oil pressure issue after verifying that with a second gauge I would suspect that the oil by pass in the oil pump is sticking open. The bypass is intended to bleed off oil directly back into the oil pan at high oil pressures. If the piston sticks open it will bleed off oil at low oil pressure.
    You will need to remove the oil pump to service it, but that is not too bad with the slant’s external pump.
     
    Last edited: Mar 1, 2021
  3. Murray

    Murray FABO Gold Member FABO Gold Member

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    Does it have new main and cam bearings? And it has a new oil pump?
     
  4. dirty white boy

    dirty white boy 50 yr old Juvenal delinquent

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    oil pump drive stripped and slipping
     
  5. RustyRatRod

    RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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    He said he put a new oil pump in it.

    Things I would do. Like John said in post 2. Put a GOOD mechanical gauge on it. If you see the pressure loss THERE, get her back out again.

    I would from there, inspect the camshaft gear VERY closely. They have been known to fail. Look at the oil pump gear too. There was a rash of oil pump gear failures with the slant six for a time. If both those look good, "MY" thought is, there is a blockage in the system somewhere....probably some sludgy crap hung up in an oil galley. Unscrew the oil pickup. Inspect it very closely. Sometimes they can get sludge buildup under the shield and you cannot see it. Make sure the tube is unobstructed. Also make sure the passage in the block to the pump is clean and clear.

    I would remove the oil galley plug directly behind the cam gear and apply shop air pressure to see if something gets dislodged. This if course with the oil pan off and the oil pump removed. I say go in at the oil galley behind the cam gear because this will be applying pressure backwards and is more likely to dislodge "something" if anything is there. You'll also not want to be standing in the area where the oil pump mounts, rather, I would hold a gloved hand holding a shop rag over that area to see if you catch something on the way out.

    Next, I would remove the rocker shaft and rockers. Find the oil feed hole in the head. On the slant six, the hole for the oil feed is in the very last rocker shaft pedestal, that would be closest to the firewall. Apply shop pressure there. As mentioned, this will be applying pressure backwards through the system and possibly dislodge any obstructions.

    Beyond that, You can also run engine cleaning brushes through the oil galleys. "I" would much rather do that with the engine torn completely down at that point though, so I could rewash the block as if it has just come from the machine shop.

    The father down you go and more you do, you'll find "whatever" it is eventually.

    Now, before you go runnin back out to yank it back out.......you HAVE tried another filter, right?
     
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    • Charrlie_S

      Charrlie_S Well-Known Member

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      If you haven't done it, already, Change the oil filter.
       
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      • grantwt

        grantwt Member

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        Mechanical gauge, pressure switch, and engine knocking all agree!

        Yes, yes and yes! This is where it gets a bit weird. Cam is from a 68, and has the wider oil lands and block has narrower 75(?) and up bearings. I couldn't see that this would create a problem. Maybe someone else has some thoughts on this.

        Just finished removing motor to replaced pump. Camshaft gear looked fine.

        Man, I was really hoping for some advice that doesn't require yanking the motor again! :D I'm planning on moving, and I was trying to put a project back together, not take it all apart again.

        I did check when I had the pan off, and the pump inlet was clear. Also replaced the pickup tube, which I was shocked I could get new. So pump inlet is clear. I never checked the output for plugs. My thought was having a plugged oil gallery would cause high oil pressure as the oil would just go over the relief all the time.

        I never took off the cam gear cover. Makes me wonder if the plug has come loose or something. Either way, if I can pressurize with shop air, maybe I can find the leak. I should also check that I didn't put together something wrong in the rocker assembly.
         
      • grantwt

        grantwt Member

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        Also, I have tried multiple oil filters.
         
      • my68barracuda

        my68barracuda Well-Known Member

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        the oil by pass in the oil pump is sticking open.

        have you checked this?
        even if its a new oil pump, does not guarantee that it is good.
         
      • RustyRatRod

        RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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        You think it would be doing the same as the old one? That's a pretty huge coincidence......but possible, I guess.
         
      • dano

        dano Evil Handy Man

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        Are the bearings the correct size? Did you measure the journals?
         
      • grantwt

        grantwt Member

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        I had checked the bypass in the old pump before replacement. It seemed to be free to move and spring was fine. I have not checked the new one.
         
      • my68barracuda

        my68barracuda Well-Known Member

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        I did not see that you had checked the bypass at all, that is why I re-asked.
        The issue you are reporting: intermittent low oil pressure goes hand in hand with an intermittent oil by pass sticking.
        If it was excessive bearing clearances the oil pressure would be consistently low.
        Anything is possible, but it seems that debris blocking a passage would also be consistent.
        The oil pressure issue is intermittent, is that right? Sometimes it is fine, other times it is near zero at idle and maxes out at 30 or so at high rpm.
        Or is it the oil pressure is fine when the engine is cold and goes to near zero at idle and maxes out at 30 or so when the engine is hot? Please clarify.
         
        Last edited: Mar 1, 2021
      • volaredon

        volaredon Well-Known Member

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        Cam bearing issue? He did say that he put the cam from the older engine into this one then says he never took off the cam (timing) cover which has to happen in order to swap cams. Something ain't making sense
         
      • RustyRatRod

        RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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        Before I pulled the 170 out of Vixen late last year, she would so similar. Sometimes she would knock on startup both cold or hot. The oil light would stay on extended periods....like ten full seconds, then go out and the engine got quiet. Sometimes it didn't do it. Never once though, did it lose oil pressure once running.

        I suspected sludge in the oil pickup and I was sorta right. After I pulled it and got the pan off, there was a layer of sludge about a full 1/2" deep, with the exception of a round disc directly under the pickup. There was actually little sludge in the pickup itself.

        Although the bearings were still in fine shape, since I had new standard rod and main bearings, I replaced them, along with a new oil pump, oil pickup and timing set. Now it turns the oil light with the starter before the engine ever fires, even after sitting long periods.

        You got something going on affecting the supply of oil "somewhere", "somehow". When you had it all down, did you think to look at the block where the pickup screws in? I hate to say it, but I have seen a big block crack where the pickup screws in and they are very similar. When it got hot, the crack would open up and the oil pressure would drop. The guy took it to several shops and nobody could find it. He was a friend of mine so I told him to let me have a swing at it and I found it. He actually had another engine and we opted to go that way.
         
      • grantwt

        grantwt Member

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        I plastiguaged mains and rods when I was building. They were on the loose end of tolerance, bit still in tolerance.

        A sticky relief would be consistent with what I see. A big part of the reason I threw a new pump at it was just in case it was sticking.

        It has low pressure hot or cold. I might get 5-10psi cold at idle, but my gauge only really starts at 5psi so it's hard to tell. Right after I rebuilt it, it was fine but a few drives later the oil pressure suddenly dropped to nothing. Shut it off, wait a few minutes, and it would be normal. One time with it just tickling the gauge at 5psi, I put the clutch in, revved it to 3000rpm and it just switched back to 60psi. The last time I drove it, it ran normal for a whole weekend, and then on the last day of my permit it just fell to nothing.

        Since putting the new pump in, I haven't seen it go to "good". It has been reading low the whole time. It may just be to the point the bearings are toast, but I didn't see evidence of damage when I drained the oil last time. I've tried not to run it with the gauge at zero, but when oil pressure suddenly cuts out all of a sudden intermittently, I'm sure the damage is building up.
         
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        • grantwt

          grantwt Member

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          I swapped my ground cam from my 68 block in when I re-ringed and bearinged. Have not had the cover off since.

          A crack would make sense. I can't make sense of problems with clogging in the gallery or bearings causing intermittent problems. Vortexing some air into the inlet through a crack, or a clog would make sense. I halfways expected to find a shop rag rapped around the pickup when I pulled the pan.

          I'm planning on checking under the timing cover and valve cover, but after that I think it might be time to grind and bore my 68 block and crank and do it proper.
           
        • inertia

          inertia Well-Known Member

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          Run 20/50 dino oil..
          No synthetics .
           
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