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low oil pressure

Discussion in 'The Garage' started by big83chevy4x4, Oct 10, 2004.

  1. big83chevy4x4

    big83chevy4x4 3/4 ton status

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    on my buddies truck. i built his motor using a stock volume oil pump, i don't really like a high volume pump with a stock pan. installed it in his truck, has great pressure, 60 all the time with the stock ('77) mechanical guage (30 at idle when warm). he ran dinosour oil. we ran that till the tranny blew, always had good pressure. then he bought a 81 k20 and put in the motor on this truck. we took the stock mechanical to keep a eye on it when we were working on the other stuff.

    he changed the oil putting synthetic in it and we got a 3 guage pod with mech temp, volts, and mech oil. now when we start it when its cold it shows 60 then it drops down to about 4 psi at idle when warm and will not go above 20 psi when revved. it is a sunpro guage set.

    we pulled the line off at the guage and it came out slow at idle. the line doesn't look to be pinched. could it be that the line is too far in the guage or the block and not letting flow by? could it be the synthetic oil? or could it be a faulty guage?

    hes not very knolagable when it comes to mechanical things, like working on a truck. and he wont take my advise alot of the time, like when i said to get autometer guages. hes a pesimest also, thinks everythings gonna cost him $3000 to fix, like when i said the ball joints won't come out and said he would have to buy a new housing and it'll cost $2000 /forums/images/graemlins/rotfl.gif /forums/images/graemlins/rotfl.gif /forums/images/graemlins/rotfl.gif /forums/images/graemlins/rotfl.gif, he went nuts. /forums/images/graemlins/screwy.gif /forums/images/graemlins/screwy.gif

    anyway, any ideas on what to check, i don't wanna take the whole motor apart to find it was a bad guage or something.
     
  2. jon v

    jon v Registered Member

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    What viscosity oil are you using now? And, what was the viscosity you used before?
     
  3. big83chevy4x4

    big83chevy4x4 3/4 ton status

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    10w30 before
    10w30 after
     
  4. dyeager535

    dyeager535 1 ton status Premium Member

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    So what do the '81's gauges tell you?
     
  5. diesel4me

    diesel4me 1 ton status Premium Member

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    I'd try another mechanical gauge first-it might be the little ferrule on the plastic tubing crimped it almost shut,giving a false reading-I hate that plastic tubing,I use copper,it wont pop out of the gauge and flood your legs and interior with hot oil!-if it still has low pressure when it warms up it might have loose rod or main bearings--sounds like when your oil gets up to temparature it loses some viscosity and lets the bearing wear show up as a pressure loss.
    We had a customer rebuild a 302 ford for his daughter's maverick "grabber"years ago that had a similar oil pressure problem--fine when cold,but after 15 minites or so it would drop drastically,to 10psi or less at idle(more than once it was under 5 lbs!)and they tried 20w50 oil,it just took a few minites longer,but still did the same thing--they finally took the oilpan off,and checked the bearings(it had a reground crank kit in it)--they discovered the crank was turned .010 undersize,but the bearings that came with the crank were standard size--ten thousandths too big!--nobody thought to check them,figuring they were matched to the crank!---I found it hard to beleive the engine would run that long before the pressure died off-(it would knock ever so faintly when you revved it up a little when warm)-it must have been the oil that took up the "play"until it got up to temparature.They put in the correct .010 under bearings and it was ok after that,they got lucky and the crank suffered no damage. /forums/images/graemlins/crazy.gif
     
  6. ZooMad75

    ZooMad75 1/2 ton status Premium Member GMOTM Winner

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    I'd check it with another gauge. Small Blocks can run pretty low at idle. Mine sits around 10psi at idle warm. IIRC the spec is pretty low in the book to (7-10 psi at idle) but if someone has a book they can confirm or correct me. Mine will pick right up when revved and pulls 40-50 psi when rolling down the highway at 70.

    Look at it this way, if the engine was not touched (internally) from pulling from one truck and installing to the other one, nothing should have changed inside. All that has changed is using a new gauge. Taking the old gauge out of the 77 and bringing it to the newer truck might have got the line kinked, or tweaked the gauge itself.
     
  7. big83chevy4x4

    big83chevy4x4 3/4 ton status

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    [ QUOTE ]
    All that has changed is using a new gauge.

    [/ QUOTE ]
    a new guage and synthetic oil, this is the first time I have delt with synthetic stuff in a truck

    i was talking to him today and he said he put 6 quarts of oil in /forums/images/graemlins/shame.gif could this be the problem?

    i was also talking to another friend that had a similar problem, he pulled the pan and the screen was in the bottom of the pan, i don;t see how this could effect pressure. /forums/images/graemlins/dunno.gif

    ill drain the 1 quart out, then try a new guage.
     
  8. ZooMad75

    ZooMad75 1/2 ton status Premium Member GMOTM Winner

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    synthetic oil should not have changed oil pressure any if at all. it's still the same viscosity. Synthetics don't get thinner as they get hot, they actually take heat better to maintain viscosity when oil/engine temps rise and conventional oil breaks down. (sounds like a Castrol Syntec commercial! /forums/images/graemlins/grin.gif)

    6 Quarts in the pan may be the problem though. If he don't have the longer oil filter (truck filter) that has some extra capacity to it, the oil level may be too high in the pan. It could be high enough that the crank is whipping through it as it swings, areating the oil. Kinda like making a froth out of the oil with tiny bubbles. Air can't be pumped by the oil pump. The oil ends up like a foamy cappacino and oil pressure falls. If thats the case its happening not from heat, but the longer the engine runs the more the oil gets whipped up.

    I've personally never heard of this in a Small block, but I have very good experience with 8.1 big blocks, 1 quart over full will do the same thing. It didn't dawn on me until you said he had 6 quarts in it. Its a long shot, but dropping a quart out and firing it up again is easier than pulling the engine back out or dropping the pan.

    Easy test is to let it run till the pressure drops and check the oil again. if it comes up with a bunch of bubbles you know what you found.
     
  9. big83chevy4x4

    big83chevy4x4 3/4 ton status

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    makes sense, i was thinking the same thing after he told me.

    ill do that tomorrow
     
  10. big83chevy4x4

    big83chevy4x4 3/4 ton status

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    drained it till it was the correct level, and no change in pressure.
    could the oil pump be bad? i don;t think its the bearings, the motor has little time on it.
     
  11. big83chevy4x4

    big83chevy4x4 3/4 ton status

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  12. hawkeye649

    hawkeye649 1/2 ton status

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    Have you tried a new oil pressure gauge? How fast does it come out of the tube if you take the gauge off?
     
  13. mudjunkie 82

    mudjunkie 82 1 ton status GMOTM Winner

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    my buddys heep had the same problem. Turns out there was an big air bubble in the line.
     
  14. big83chevy4x4

    big83chevy4x4 3/4 ton status

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    when we unhooked the line from the guage and started it, when its cold it squirts out and when cold it comes out very slowly. /forums/images/graemlins/dunno.gif

    very considering changeing the pump with a z28 pump or something.
     
  15. hawkeye649

    hawkeye649 1/2 ton status

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    Possible....
    Screen is clear, right? Pick up tube clogged after the screen maybe (a reach...)
    What kind of tube do you have [for the gauge], plastic or copper? Have you checked for kinks? Maybe the tube warms up and then collapses?
    Pulled off the valve covers to see if they are getting oil? Any knocking? /forums/images/graemlins/confused.gif

    Doing work like this & having something go wrong always makes me feel like a retard, JUST for thinking I could do it. /forums/images/graemlins/frown.gif /forums/images/graemlins/screwy.gif
     
  16. big83chevy4x4

    big83chevy4x4 3/4 ton status

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    i don't know if the screen is clear or not, haven't pulled pan yet.

    it is a plastic line, no kinks, its not collapseing.

    didn't pull the valve covers yet, no knocking.

    there is oil pressure just very low, low enough to make you kinda leary.

    next time i go up there, im gonna try a different spot on the block, we have it at the port near the distributor, im gonna try near the oil filter. then pull the pan and see what is up.
     
  17. hawkeye649

    hawkeye649 1/2 ton status

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    Good luck /forums/images/graemlins/crazy.gif
     
  18. bigblock44k5

    bigblock44k5 1/2 ton status

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    The port by your distributer should be fine. I aggree with the ferule being crimped to tight on the line. They are only supposed to be 1/4 turn past hand tight. That cheap ass line crimps easily. Go get some 1/4" flexible copper, wont kind like that other crap and since its bigger it will give you accurate read(the gauge will go up and down relative to rpm, instead of hesitating with the small plastic hose). To check easily but messily, disconnect the line at the block, since you already disconnected it at the guage. This will omit the line altogether and tell you for sure if it is the problem or not.
     
  19. James670X

    James670X 1/2 ton status

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    A couple of things you could check would be 1. like alot of people on here have already said put another gauge on, or like you have already done make sure the one that's on it is 100%, 2. The pickup tube for the oil pump might be too close to the bottom of the oil pan, 3. The screen on the oil pump could be clogged, 4. Oil filter could have debris in it, and lastly 5. The oil pump may need replacing (tell tale sign is wild fluctuation in oil pressure when throttle is applied....goes from very low at idle knocking can occur to very high when revved up).
     
  20. 84_Chevy_K10

    84_Chevy_K10 Banned

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    [ QUOTE ]
    on my buddies truck. i built his motor using a stock volume oil pump, i don't really like a high volume pump with a stock pan.

    [/ QUOTE ]

    This statement represents a myth so deeply rooted it's pretty similar to the exhaust back pressure myth. An oil pump is not going to suck your oil pan dry, that is ridicoulous. If it was, it'd do it just as well with a stock volume as with a high volume pump. If you're really worried, clean up the casting flash so the oil can return to the pan better.

    I would verify the numbers with another gauge, but a mechanical oil pressure gauge is seldom wrong. If the new gauge fails I'd check the oil pump, followed by the bearings, and then the plugs in the end of the oil galleries in the block next.
     

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