Rebuilt engine, low(er) pressure.

Discussion in 'Engine Performance + Maintenance' started by NacIK, Jan 9, 2017.

  1. NacIK

    NacIK aka Dirty Hoe

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    So, I hit the 250k mark and had to rebuild the engine because I cracked a head and melted a piston. I finally had the funds to put it together and the first time I was getting low pressure, around 40 cold and 10 when warm. The rockers were also not getting oil. I took the engine apart again and realized the main bearings were too small. I replaced it a few years ago after I spun a bearing and forgot that the crank was probably machined. I replaced the standard bearings and put the .020's it called for.

    I put everything back together, primed it up, and was seeing oil on the rockers finally. However the pressure was still at 20 psi and there quite a bit of oil seeping around the distributor shaft and into the galley. I finished putting it together and it gets 20 cold and 40 when warm. The pressure does not fluctuate with rpm at all. Before it would get 60 on start and 40 when warm and reach 60 with higher rpm.

    I replaced the oil pressure sensor because I snapped the original one when I pulled the engine to swap the bearings so I don't think that is the problem. My gauge seams to work fine. Does this seam normal or should I be concerned? I know the older engines can run as low as 8 psi and be fine, but I don't know about the hydraulic lifters.
     
  2. NacIK

    NacIK aka Dirty Hoe

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    Just because I know someone will ask:

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    And now:

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  3. L29Sub

    L29Sub Newbie

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    Be concerned. Shouldn'the go up with heat? With correct çam and mains it should maintain 30-40 lbs hot--60 cold and 20% + - drop hot.
    Some worn out Detroit diesel 2 strokes operated with low oil pressure at idle, such as you describe. Gas engines won't survive that.
    You could have loose cam bearings, loose mains, loose lifter bores or bad lifters, wrong/bad oil pump/mount/pickup, bearings incorrectly installed, bad oil pressure guage, oil pump drive gears/shaft, wrong oil filter adaptor or check valves...ad infinitum. If you lost a main in the past, the block and cap bearing surface may be enlarged. Replace the block. Don't interchange the main bearing caps.
    Did you plastigage the mains? Mains can be fitted relatively easily. Cam bearings should be replaced and will require an engine machine shop.
    A cheap mechanical oil pressure guage will verify your electric guage.
    Good luck!
     
  4. L29Sub

    L29Sub Newbie

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    Add a bad distributor cam gear drive pin to the list of possibilities....
     
  5. someotherguy

    someotherguy I'm Awesome

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    Were clearances actually checked (plastigage) after installing bearings or you just put 'em in? :(

    Richard
     
  6. NacIK

    NacIK aka Dirty Hoe

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    Almost everything was replaced. Treed 2 different dizzys, shaped between old and new oil pump with same thing. Rod bearing spun before, not main. New rod, heads, bearings, cam bearings..... almost everything new. Mic'ed and use plastiguage. Checked out good. The only thing I can think of is there was a lot of oil seeping up from the distributor home into the galley when I was priming it with the intake off.

    Machine shop said it will be fine as long as the rockers are getting oil and the oil psi is at least 15. They are also the ones that cleaned the block and replaced the cam bearings.
     
  7. L29Sub

    L29Sub Newbie

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    True enough--to a point. You'll know the pressure is too low when the lifters rattle at idle. IMHO, you have the oil pressure of a well worn 200K engine, yet paid for a fresh engine. It may work great for local use, but something isn't quite right.
    What did the mains and rods plastigage to?
     
  8. someotherguy

    someotherguy I'm Awesome

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    TBI small block oil pressure absolute minimum standard, straight from the 1994 factory service manual; a little more specific than the "10 psi per 1,000 rpm" rule of thumb you often hear:

    6 psi @ 1,000 RPM
    18 psi @ 2,000 RPM
    24 psi @ 4,000 RPM

    Mind you those are rock-bottom numbers; I'm used to seeing better than that on a healthy engine even with plenty of miles. I've also noticed that Vortec small blocks generally seemed to have higher overall oil pressure. Don't have the factory guidelines on those.

    Richard
     

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