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Bad Fuel Economy

Discussion in 'VT - VX Holden Commodore (1997 - 2002)' started by DeathSammich, Nov 29, 2017.

  1. DeathSammich

    DeathSammich Member

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    I have a V6 VT commodore and it seems to be using 14L per 100km. I've checked this myself by totaling how much I drove by how much I've put in and by checking the digital readout. I've changed the O2 sensors, spark plugs, leads, coil packs, fuel pump and have a cat back exhaust (probably does nothing) a mace CAI (again probably done nothing) and have also tried 95 and 98 octane with no change. The car seems to drive fine and idle fine. Other threads ive read people manage to pull decent economy (as low as 8L long distance and a max of 12.5 mixed) but I can't seem to get these figures. I do a bit of mixed driving but I never drive in the stop start city. Thanks.

    I have also checked for fault codes and I got no faults
     
  2. VT&VX

    VT&VX Active Member

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    That is what we get
     
  3. mpower

    mpower Well-Known Member

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    you're using as much fuel as my V8's - is that normal!?
     
  4. DeathSammich

    DeathSammich Member

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    Yeah see my dad's 6 litre uses the same as me that's why I thought it was wrong.
     
  5. ephect

    ephect Donating Member

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    Only thing not listed is MAF sensor. Try a clean or replace.

    another thought is a dirty air filter

    hows the trans, does it shift ok, no slippage?
     
  6. DeathSammich

    DeathSammich Member

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    I'm using the same as my dad's 6 litre V
    Oh yeah forgot to mention. It has a brand new pod filter because of the mace CAI and I have cleaned the MAF sensor. Trans is fine
     
  7. uniacidz

    uniacidz Harden the Frak Up

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    Long shot but some oils didnt like my old Falcon and had not great fuel economy and ended up using a Mitsubishi Mineral Oil 15w40

    Seemed to work, got better economy

    Maybe i drove it better, but did work

    My current car is temperamental and only Penrite, only taken years or so to work that out
     
    Last edited: Nov 30, 2017
  8. FNC

    FNC New Member

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    Hi
    I drive gentle and get about10.5ltrs/100 around suburbia and about 7.8ltrs/100 on hwy (I only use 91)
    I have a VX & WH (both 3.8) which I do most of the work on myself.
    It sound like you have covered a considerable amount of possibles but I notice (especially this time of year with a lot of flowers out) that the air cleaner and maff need cleaning a lot more regular (maybe the pollins from flowers) If I dont clean more regular I start using more fuel. I also throw in the odd bottle of injector cleaner. The only other things I can think of would be injectors or a possible small vacuum leak. Cheers
     
  9. MYVESSV8

    MYVESSV8 Well-Known Member

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    how heavy is your foot, tyre pressures, what load do you carry all the time, oil viscosity,
     
  10. DeathSammich

    DeathSammich Member

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    Light foot as I try to get good economy. I check the tyre pressure every fortnight to make sure it's good, weight is just me a
    And the car I even removed the spare wheel and Jack, and the oil is penrite 20w 60
     
  11. greenacc

    greenacc Searching for the billion

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    What tyre pressure are you running? The placard pressure is the bare minimum if you want good economy.
     
  12. rossybob

    rossybob New Member

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  13. rossybob

    rossybob New Member

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    Try nulon 5w30 your oil seems to heavy to me
     
  14. Sean880

    Sean880 Well-Known Member

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    14 lit per 100 in mixed running on an old 3.8 engine in a big car with lots of kms is not all that bad. That's 20 miles per imp. gallon.

    1. Take it on a highway 150 km run without lead footing it and check the consumption. If you can get between 8 and 9 Lit per 100 km that's not an indication that the engine is gulping too much fuel.

    2. The engine oil you are using is wrong. If you want to make the engine work harder,the oil pump to work harder and a sub optimum lubrication rate keep using that incorrect heavy viscosity oil. You can very safely use Castrol Edge 5w-30 fully synthetic in that engine.

    3. Check for dragging brakes. Place the car on a very slight incline and let it roll. If the brakes are free it should roll immediately.

    4. Engine Compression. It's an old engine and may well be significantly down on compression with normal wear. If so the engine will run less efficiently and you will use more fuel. Have a compression test done and see where it stands.

    5. Exhaust restrictions - cat converter or muffler breaking down internally.
    You can check for exhaust restrictions by connecting a vacuum gauge to the intake manifold. At idle, the engine should show a high and steady vacuum reading . If the reading is low or it gradually drops, this is indicative of an exhaust restriction.

    6. Faulty Coolant Sensor
    If the sensor is defective and reads lower than normal, or always reads cold, the engine computer will keep operating in open loop - so the fuel mixture remains rich. A richer fuel mixture is required while a cold engine is warming up. If the mixture remains rich once the engine is warm you will of course get poor fuel economy.

    7. Injectors
    You are apparently not getting any engine misfires on any cylinder or a check engine light so I would not suspect an injector issue.

    Does the increase in fuel consumption you experienced coincide with any modifications you have made that affect engine operation?
     
  15. losh1971

    losh1971 Well-Known Member

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    The only real way to gauge economy is to fill the tank until it reaches the top of the filler neck. Drive it 200km and then refill to the same point and check the litres. Do that 3 or 4 times then check your economy. Trip meters often give a false reading. If your not getting better economy with 95 and 98 then I'd say your method of testing is out. Every car I have run on premium fuel gets better economy after 2-3 tanks of continual use. Also comes down to the brand of fuel. I have always got the best economy by using Caltex 95 and 98.
     
    Last edited: Dec 12, 2017

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