Oil pressure guage fluctuates a bit at idle

Discussion in 'OT Driven' started by P07r0457, May 26, 2008.

  1. P07r0457

    P07r0457 New Member

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    GF has a chevy blazer with the 4.3L V6. It has 170k miles but it's still in darn good shape, considering. Drives around town great, and has lots of pep. She wasn't real good with maintenance so I took it to the shop and had them replace all the fluids (oil, t-case, trans, front/rear diffs).

    We didn't notice this prob before (but it may have been there, not sure):

    Oil pressure guage reads approx 60psi on a cold morning start, and stays around 40psi when warm and driving around. If you idle for a while at a red light or drive through the oil press guage will sit around 30psi and move back and forth a few millimeters. Is that normal? I know oil pressure at idle will be less than when driving, but shouldn't it settle at a constant pressure? It's fluctuating and that bothers me.

    Am I just being anal, or is this a precursor to a bigger problem?
     
  2. deusexaethera

    deusexaethera OT Supporter

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    If it's an electronic gauge (and it almost certainly is), then the fluctuation is more likely due to an old sensor or failing voltage regulator than actual changes in oil pressure. The engine is spinning fast enough even at idle that anything really affecting oil pressure would show up as a vibrating gauge, not a wavering gauge.

    If you feel up to it, take the voltage regulator off the car and clean it inside and out with a wire brush. My dad just did this on his old van and all of the gauges started reading properly again, except for the tach, but we know that sensor is shot anyway.
     
  3. P07r0457

    P07r0457 New Member

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    interesting that you mention that. There is a voltage gauge, as well. it reads a steady 14V when running. However, if the blinker is on, the voltage gauge shows a similar fluctuation down-up-down.

    It's due for a tire rotation, anyway, so when I take it in i'll have them test the batt/alt. I hope its something that easy.

    but then I wonder why the voltage gauge would read a steady 14V when the oil pressure guage would fluctuate.
     
  4. deusexaethera

    deusexaethera OT Supporter

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    That means the alternator is good, because 14V is what it should be putting out -- just enough over 12V to push current back into the battery to keep it charged. If the readout is steady normally, then that suggests the voltage regulator is also good. If it's dipping when the lights come on, that sounds more like a loose/rusted ground.

    It's possible the alternator is managing to push 14V, but can't muster the amperage it needs to power everything, though. It might be worth getting the alternator tested to see if it's putting out the rated amperage, but my first guess is still that the grounds aren't conducting as well as they should, because few electronic components in a car get more abuse and neglect than the ground cables. When a ground is loose, anything running on higher voltage will try to push current into things running on lower voltage, which obviously will affect their operation.
     
  5. P07r0457

    P07r0457 New Member

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    When the lights are on it's fine. It's just when the blinkers are on and the vehicle is at idle.

    I'm not so sure about that. It's got a subwoofer and amps and the system hits hard. If there was a ground issue, I imagine the stereo wouldn't be working properly. It is also the highest-demand item for the electrical system.

    And our "higer voltage will try to push current into things on lower voltage" is just plain retarded. If you don't know what the fuck you're doing, get out of my thread.
     
  6. deusexaethera

    deusexaethera OT Supporter

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    Well, if you've got a better explanation for why loose grounds can cause systems on separate circuits to malfunction in ways that make them seem connected, let's hear it.
     
  7. guumer

    guumer New Member

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    do you ever get off ot?
     
  8. IslanderOffRoad

    IslanderOffRoad Do you even lift kit? OT Supporter

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    If its a "real" gauge, rather than an idiot light that looks like a gauge, thats normal.

    alot of people freaked because of sensitive fluctuating gauges, so they went to ones that stay steady within a range. i'd say its fine unless its making huge swoops to either side.
     
  9. critter783

    critter783 OT Supporter

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    My Chevy does the same on both gauges. My voltage gauge is a little more pronounced, but I think I have one bad or weak battery. The oil pressure gauge does the exact same thing though, and the mechanic I have look at it says its fine. He's been a neighbor for over 20 years, and he runs a pretty large Mac-authorized service center, so I trust him to be honest with me.

    On the other hand, I've always heard that the electrical systems in late 80's to mid-90's GMs were a clusterfuck.
     
  10. P07r0457

    P07r0457 New Member

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    it's a 1998 blazer, if that helps.

    I'm hoping its just a bad batt. Gonna get it tested this weekend.
     
  11. alltracman78

    alltracman78 New Member

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    If it's minor fluctuations and within the range of normal pressure I wouldn't worry about it, unless your super picky about everything working perfect on the vehicle.

    However, if you want to get rid of it regardless, check the battery [like you stated] and the alt, because you can get them tested for free some places.
    If both are good, or the problem persists after replacing one, you need to decide how far you want to go. Or you can do these first.
    It will be one of two things
    1- The actual pressure is pulsating. Get a mechanical gauge on there to verify. If so, it's something internal to the engine. Slightly rough running engine, worn pump, dirty oil pickup, whatever.
    2- An actual electrical problem. This can be the fun one, especially in corrosion prone areas. Take a good look at the sender wire near the connector. Make sure it isn't corroded where it's crimped to the contact pin or up on the wire. Make sure the sender to connector contact is good and clean. Check your alt and fusebox connections for corrosion. Check various engine/dash grounds for good contact, ect.

    Electricity takes the path of least resistance.
    Once the resistance in a loose/bad ground [or any part of the circuit for that matter] becomes more than the resistance of the load in a separate connected circuit the current will want to flow through that circuit, activating any extra loads it passes through, like a bulb. And yes they are connected, your turn signal circuit isn't going to cause something weird with your O2 sensor, but it can cause the taillights to act funny.
     
  12. P07r0457

    P07r0457 New Member

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    That Raises the question of what is the acceptable pressure range? It seems to be going 60Psi cold. 20-30psi hot and 40psi under load when hot. Is that safe. I was woried that 20psi was way too low. I thought. 30 was more in line with safe operating range.
     
  13. CastorTroy

    CastorTroy New Member

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    It's normal. Every car I've ever had fluctuated a bit at idle. You have to figure most mass produced cars have about a whole dollar invested in a sender and gauge, it's not the most accurate gauge you will ever use. There can also be slight fluctuations in RPM's and other factors that will result in a slight changes in pressure.

    The rule of thumb for oil pressure is about 10 PSI for every 1000 RPM's. 20 PSI warm at idle and 40 during normal driving is fine. I had a old Chevy truck with a 305 motor that had (and verified) ~5 PSI idle and 10 PSI maximum under load, and it ran like that for years until I sold it.
     
  14. P07r0457

    P07r0457 New Member

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    hmm, odd. it idles at ~550rpm and idles very smooth. I guess I was making something out of nothing. I'll still have the batt/alt tested, but if those come back okay then I guess it's in the clear. Thanks everyone.
     
  15. deusexaethera

    deusexaethera OT Supporter

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    10psi/1000rpm is about right for hot oil on a warm day.
     
  16. P07r0457

    P07r0457 New Member

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    that doesn't make sense... because 550rpm would be 5.5psi, yet it idles above 20psi, and 2000rpm would be 20psi, yet it runs 40psi under load. So that scale is useless. Likewise, I can take it up to 5500rpm and it's still 40psi.

    you need to stop repeating shit you heard someone else say.
     
  17. aim2kill

    aim2kill New Member

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    deployed.


    yes and no, while most will take the path of least resistance, there will still be lingering current in the old path (privided its not open) and sometimes having current from the same source will make things go freaking crazy. my grandfather was a salty old 'backyard mechanic' even though he really was an operating eng. mechanic, he used to say "if its on your car, electrical, and you can say 'thats some freaky shit', then its a ground." might check to see if your leaking some current somewhere possibly from i :dunno: maybe a rubbed cable?

    otherwise, the fluctuation isnt something i would be concerned with, i have seen a couple older GM products whose gauges would tent to rock back and forth, (in one case the motor would vary a couple hundred rpm and i guess it crossed a volume threshold in the pump, raising it several psi) but neither were reasons to panic or spend large quantities of money on.

    wow, :sigh: i hate when i ramble :hs:
     
  18. Justintiime

    Justintiime OT Supporter

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    you don't do your own work? :hsugh:
     
  19. Justintiime

    Justintiime OT Supporter

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    What sensor? :ugh:
     
  20. Justintiime

    Justintiime OT Supporter

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    Typical in blazers. Mine did it.
     
  21. Justintiime

    Justintiime OT Supporter

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    This is normal too.
     
  22. P07r0457

    P07r0457 New Member

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    we've had that discussion before :hsugh:
     
  23. Justintiime

    Justintiime OT Supporter

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    refresh my memory then because I must not have believed the bullshit I was reading :big grin:
     
  24. P07r0457

    P07r0457 New Member

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    some bickering that took place a while back. you criticized me for not "doing my own work". I retorted that I used to (went on to explain the engine work and tranny work that I had done) but that I now would rather pay someone else to hassle with it. You made a few more comments and ended with something to the effect that you'd never pay someone else to work on your vehicle. I finished with "that's nice, but I'm in a financial position where I can pay someone else to work on my vehicles and not have it impact my lifestyle"

    that's all i remember from it.
     
  25. Justintiime

    Justintiime OT Supporter

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    getting lazy in your old age? :p
     

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