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oil pressure

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by MudRebel350, Jun 10, 2000.

  1. MudRebel350

    MudRebel350 1/2 ton status

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    My oil gauge usually sits around 30. Today while driving for about 45 minutes it would drop a little bit when I was at lites. When I'd get back on the gas it would go up to 30 again and not a hair past. Later in the day I went out again and the gauge was sitting at 30 and when I was on the gas it was a hair over 30. Is this normal?

    J.D.
    88 Gmc Jimmy
     
  2. Whii338

    Whii338 Registered Member

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    Mud, my 350(88) always ran between the first hash mark(im guessing thats 15psi) and 30 when idling... when i overhauled the engine.....also new cam and bearings and so on...also a new oil pump and it runs exactly the same pressure at idle(about 10 psi more at speed now) and mine had 170,000 before the overhaul....also i wonder about the accuracy of the stock gauges(if thats what you have)...mine ran fine on that pressure for a long time only reason i overhauled it was it need a valve job and new guides....i hope this helps...maybe try some aftermarket gauges for a little more accuracy.
     
  3. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    Re: oil pressure Class is in!

    There are a lot of things that effect oil presure (hehe you see who it is this may be a long one). Hmm where to start...
    Ok lets start with the obvious and we will work out way down to the less obvious. Oil. Different grade oils will run different pressures. Really for most of us a 10w40 as a good selection. in a milder climate then a 10 w 30 and a place thats stays cool and in the winter a 5w30. On a high mile engine you may want to go up a grade like run a 20w50 in hot areas and durring summer in moderate climates. there can be as much as a 10 psi difference depending on the engine and grade oil.
    Next would be the temerature of the engine. With a multi grade oil this is not as obvious because the oil does tend to hold a farly even viscocity. if you have a real accurte gage and really pay attention you will see when stone cold high presure then it will come down as the engine heats and then come back up just a hair when full operating temp is achived and if you have an engine the really gets the oil hot it will come back down. My Supra has a fairly sensative gage. When the engine is cold it will idle with about 25 psi once warm it comes down to 17. Now cold at 3400 rpm (80mph) the oil is thick enough that it starts puging through the pressure valve and will hold a steady 40psi but once it's fully warm it will have 45 psi. Yeah you read that right it got higher when hot now here is why
    The oil filter...yeah the oil filter is the reason.....Poder it and then read on :)
    The oil filter is a restriction. If the oil is thick it takes more pressure to force the oil through the filter. The pressure gage is after the filter so the pressure is lower at mid temps. On the inlet side of the filter it's probably 15psi higher and the bypass is purging so it doesn't rip the filter media and pump dirt into the engine. THIS is why it is a VERY bad idea to run high RPM when cold. Always let you motor war a little before really getting on it. on all my cars I am pretty easy on them till the temp is up to normal.
    Ok now one I bet nobody thought of.........the voltage in the electrical system. I can see the confused looks now!
    Ok I'll explain. It's simple once you think about it. If your vehicle has an electric gage movement the voltage being supplied to it changes depending on conditions that include, power demand, outside temperature, battery temperature, time of day, and engine speed. Ok I saw a few lightbulbs come on with our brighter students.
    Right after your car starts the voltage will be low till the alternator can get the battery fully charged. Don't belive me? go get a meter and test the voltage of a car that has sat overnight. Start the car and shut the engine right off and recheck the voltage. It should be lower. Now you just took a meter and saw a change, well the oil pressure gage is a meter that mesures resistance. Since the voltage being suplied can change it will change what the gage reads. It would be cost prohibative to have a conditioned power suply for a $10 gage. Idle voltage over driving voltage is obvious. The alternator is running slower at idle and depending on the electrical load it may drop as low as 12v at idle but be running 14volts while driving. Ahhh saw the rest of the lights come on with that.
    Well I hope that explains why your oil pressure can change.
    Basic rule of thumb is 10psi for every 1000 rpm. As long as your in those guide lines you should be fine.
    Class dissmissed!

    Diging it in the dirt with my K5's
    Grim-Reaper
    http://grimsk5s.coloradok5.com/
     
  4. MudRebel350

    MudRebel350 1/2 ton status

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    Re: oil pressure Class is in!

    Thanx alot guys. I thought I had a bum oil pump. I wasnt in the mood to get under in the 95 degree weather and have oil drip on my head to change the pump. I also checked my oil last nite and was a quart low. Although today I drove it home from my dads and the oil pressure was doing the same thing which is normal, so anyway I feel better now that I dont have to do an oil pump.

    J.D.
    88 Gmc Jimmy
     
  5. DMK

    DMK 1/2 ton status

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    Sounds good to me.
     
  6. blazer72

    blazer72 1/2 ton status

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    I had the bad pump and when I got in there for a close look bad bearings too, So one reground crank and new bearings even the cam's plus a new cam too!!!! just got it back it tonight I will start it up after work monday!! One thing on what grim said some new gages have voltage regulators built in to them My VDOs do.
     
  7. Blazer79

    Blazer79 1/2 ton status

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    Re: oil pressure Class is in!

    Nice "article" Grim. Have you ever considered writing one for the main page of CK5? I'm sure Steve will be glad to post it.

    By the way, your writing reminded me of our friend Sled Dog, anyone heard of him?

    <font color=blue>//////
    http://blazer79.coloradok5.com
    </font color=blue>[​IMG]
     
  8. Burt4x4

    Burt4x4 3/4 ton status

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    Re: oil pressure Class is in!

    The Emperor Grim-Reaper has spoken......All remain in silents with jaws open and a drool of spit running down your chin....[​IMG]
    Damit Grim when you gonna move to California and be my neighbor??? I need a good neighbor like you!![​IMG]
    Grim you ROCK! [​IMG]

    Rock ON![​IMG] 72 K5 [​IMG] Led Zeppelin
    http://burt4x4.coloradok5.com
     

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