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EXTREME OIL PRESSURE FLUXUATION!! HELP!! '98 K1500 5.7L Vortec

Discussion in 'Chevy C/K Truck Forum' started by MROY16, Apr 14, 2014.

  1. MROY16

    MROY16 New Member

    I have a 1998 GMC K1500 Z71 offroad with the Vortec 5.7L 350ci V8. Purchased in Kelowna, BC in October 2013 at 157,000kms (98,000mi). Currently has 164,000km (102,500mi).

    Cold air intake
    True dual exhaust (no cats, dual in/single out Magnaflow muffler)

    Nothing else done to the engine this far.

    Idle is 600RPM, my oil pressure fluxuates from nearly 0 at idle and back to normal (275kPa/40PSI) at WOT. Oil Pressure is perfectly fine when engine is cold, but once it gets to operating temp the oil pressure starts fluxuating insanely.

    Not sure why this is happening, started happening more after the exhaust was done (cats removed). Never burned oil until today (April 12, 2014). Started burning oil for about 5mins, then stopped and hasn't since. NEED HELP!!! This is my only vehicle and I am going to university right now so don't have enough money or time to fix major repairs. I'm hoping it is something cheap and simple! CEL/MIL is on for O2 Sensor since cats were removed but getting an O2 Simulator to eliminate that issue.

    At this point my thoughts are:

    Oil pressure sending unit
    Oil Pump
    Oil pump pickup screen/tube

    PLEASE POST ANY SUGGESTIONS AND/OR EXPERIENCES OF THIS PROBLEM AND HOW IT WAS REPAIRED!
  2. JTWard

    JTWard Member

    Well the first thing I'd do is check for any logged codes, but avoiding that, and since you have 164.000 miles, calm down and just go out and get a oil pressure sending unit, make sure your getting one meant for a gauge. And then start it up, I'm quite sure all you have is a worn out oil Pressure sending unit. After 164.000 miles, it's normal to fail, no big deal. If you had a shot oil pump you'd hear the valves tapping away. :rules:
  3. MROY16

    MROY16 New Member

    Truck only has 102,500 miles, has 164,000kilometers (i live in canada so metric measurements).

    and no codes besides the O2 sensor codes.
  4. JTWard

    JTWard Member

    OK, then just go get a oil pressure sending unit, I have to assume you have a oil pressure gauge, so you would need a special sending unit, their different from just a 'Check engine' light. But I am very confident your oil pressure gauge sending unit is bad and all you need is to replace it. I believe there are two on your engine, (But I'm not Positive) if you went to an older parts house with some experienced counter guys, they could tell you in a minute or just write done your VIN and they can get you the exact switch or sending sensor you need. I have a 1990 Silverado and mine is back by the distributor. I'm not sure of your exact unit, but you have a LS series engine, and if so. . it's under the intake manifold. I have a 2003 Corvette with the LS1 engine, your engine is either an LS4 or LS5 which are just like their kin, but are aluminum block The LS1,2,3,6,7,8,and 9 are Aluminum block and heads. The truck engine are exactly but made from cast iron. I'm not sure how difficult it would be, but I'm not there, so I can only wish "Good Luck". I don't think you have lost any oil pressure as the truck has a 105.000 miles on it and oil pumps rarely go bad in an instant.
  5. poncho62

    poncho62 Member

    My truck has 240,000 km on it and the oil gauge fluctuates quite a bit too, not down near zero at idle. It is fairly normal with higher mileage. Yours does seem a bit low. I would replace the sender and see if that helps. I have run older SB Chevy engines for years with not a lot of oil pressure.
  6. kbonanny

    kbonanny New Member

    These Oil Pressure sending units are notorious for being inaccurate as they age. I would remove the oil sending unit and install an oil pressure gauge. Have someone start the truck and watch the pressure. Go from stone cold all the way through when it is fully warmed up. Have someone inside the truck with the E-Brake set, have them put it in gear hot. That will tell you all you need to know. If the manual gauge shows no problems, replace the sending unit and drive on. If the mechanical gauge shows low oil pressure, you will have to dig into it. Usually a new oil pump & screen will solve this type of problem so long as it has not been overheated badly and it has been well maintained. The oil burning has nothing to do with oil pressure with the exception that if you burn enough, you will lose pressure.
    Any questions? feel free to ask. I have the same exact truck and my sender would read roughly 10 lbs hot @ idle, and never went above 40. These trucks also have instrument panel cluster issues which is a common issue to these. If your gas gauge is "shaky" there is a good possibility you have a bad board.
    Regardless of everything else, the quickest and most accurate way to diagnose this type of problem is to tap in a mechanical oil pressure gauge. Don't use a cheap POS either....they have a nasty habit of leading people the wrong direction.
    Kirk Bonanny
  7. vncj96

    vncj96 New Member 1000 Posts

    The instrument cluster in your truck is another weak link, they go out. Get a mechanical gauge and check the oil pressure first, you can rent one at most auto part stores. Most likely the pressure is just fine and I wouldn't even spend the money on the sending unit.
  8. JTWard

    JTWard Member

    OK, then just go get a oil pressure sending unit, I have to assume you have a oil pressure gauge, so you would need a special sending unit, their different from just a 'Check engine' light. But I am very confident your oil pressure gauge sending unit is bad and all you need is to replace it. I believe there are two on your engine, (But I'm not Positive) if you went to an older parts house with some experienced counter guys, they could tell you in a minute or just write done your VIN and they can get you the exact switch or sending sensor you need. I have a 1990 Silverado and mine is back by the distributor. I'm not sure of your exact unit, but you have a LS series engine, and if so. . it's under the intake manifold. I have a 2003 Corvette with the LS1 engine, your engine is either an LS4 or LS5 which are just like their kin, but are cast iron blocks, The LS1,2,3,6,7,8,and 9 are Aluminum block and heads. The truck engines are exactly but made from cast iron. I'm not sure how difficult it would be, but I'm not there, so I can only wish "Good Luck". I don't think you have lost any oil pressure as the truck has a 105.000 miles on it and oil pumps rarely go bad in an instant.
    Last edited: Apr 27, 2014
  9. vncj96

    vncj96 New Member 1000 Posts

    It is NOT an LS engine its an L31 Vortec 5.7

    - - - Updated - - -

    It is NOT an LS motor its an L31 Vortec 5.7
  10. JTWard

    JTWard Member

    I stand corrected.

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