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Thread: Horrible (IMO) gas mileage

  1. #1
    Join Date
    Feb 2011
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    Horrible (IMO) gas mileage

    I've put a couple of tanks of gas through my SC now. I only drive it to work, and it's all back roads with a couple of stop signs. No real stop n' go city driving.

    I'm averaging about 16mpg. This seems horrible to me for a bone stock car.

    Any suggestions on problems to check for? I've noticed it only makes about 9lbs of boost, so perhaps a vacuum leak? The ignition system seems to be in decent shape - good idle and no misses. No bogging or signs of a fuel problem either.

  2. #2
    Join Date
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    Stay out of boost if you want gas mileage.

    Don't worry about gas mileage if you're boosting.

    Check your exhaust and free that up first - see the threads for what's worked best for others here.

    RwP

  3. #3
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    Feb 2011
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    I don't get above 45 mph on my drive to work and I keep off the boost. Only reason I know about the 9psi issue is from playing with it when I first got the car. This think absolutely chugs gas.

  4. #4
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    Quote Originally Posted by HelluvaEngineer View Post
    I don't get above 45 mph on my drive to work and I keep off the boost. Only reason I know about the 9psi issue is from playing with it when I first got the car. This think absolutely chugs gas.
    The 45MPH thing is probably the issue. I would classify that as city driving. Low speeds, stops on the way, 4Klbs car. That seems about right on the gas mileage.

  5. #5
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    Seems about right. Plus the gas nowadays with the ethanol is less potent so the engine burns more of it to burn Stoich. And our engines weren't designed to run the e10 efficiently.
    ~Chris
    SCCoA Member #1826
    Riding jack stands.

  6. #6
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    Portland, OR 97219
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    Run the codes, check the air filter, air up the tires. But yeah, 16 around town (not open highway same speed) seems pretty close to expected.

  7. #7
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    Feb 2011
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    Thanks for all the input. I'll check the codes, but I suspect everything is functioning within specs. I run high tire pressures anyway, so I guess I'll have to live with it.

    Sucks that my pickup gets better mileage, and runs 87 octane :-(

  8. #8
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    Apr 2002
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    St. Charles, MO
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    Quote Originally Posted by HelluvaEngineer View Post
    I don't get above 45 mph on my drive to work and I keep off the boost. Only reason I know about the 9psi issue is from playing with it when I first got the car. This think absolutely chugs gas.
    Not really chugging that much gas if your getting 16 mpg. Is this an automatic or manual trans. With an auto at 45 mph you are really lugging the motor in OD.

    David
    Last edited by David Neibert; 11-08-2011 at 06:24 PM.
    91 SC AOD 4.2..2.3 Whipple........10.92 @ 126.70
    93 SC AOD 347 Turbo.................10.04 @ 138.99
    My Garage

  9. #9
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    16MPG is not normal, 1st thing to do is hunt for vac. leaks. seal off/block the throttle body, use compressed air & nozzle to pressurize the intake system via any of the hoses going to the intake. listen for leaks, the big ones will be easy to hear & find, next spray soapy water on suspect areas & seal/fix one at a time. See how long it takes for your Vac/boost gage to drop to 0 when you turn off the engine. mine takes 6 seconds. If your good on vac leaks then check the EGR valve, I found mine was commanded to far open when driving, it ran fine but vac was around 10 @ 60 & I know that is not good. I disabled the EGR when I unplugged the vac solenoid & plugged the vac line to it. What is your Vac/boost read at 60-70 MPH?
    "Airplanes are interesting toys but of no military value."

    Major General Ferdinand Foch, 1911

  10. #10
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    Quote Originally Posted by David Neibert View Post
    Not really chugging that much gas if your getting 16 mpg. Is this an automatic or manual trans. With an auto at 45 mph you are really lugging the motor in OD.
    It's an AOD. Supposedly engines are most efficient at high load, WOT (lugging), from what I've read anyway.

  11. #11
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    Quote Originally Posted by 35thauto View Post
    What is your Vac/boost read at 60-70 MPH?
    Thanks. These are really useful suggestions.
    I'll have to check the 60mph reading soon. My tach is dead so it's hard for me to give much useful info, but the highest vac reading I've noticed during cruising is 22 (in hg?) and the highest boost I've seen is 9 psi. This seemed low to me.

    I'm sure the EGR has never been replaced.

  12. #12
    Join Date
    Oct 2004
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    How far is your drive to work.....

    Short trips are bad for MPG.....
    Flying is the 2nd greatest thrill known to man. Landing is the first !!!!

    Every day, I set a personal record for consecutive days I have remained alive!

    If you cut a crumb in half, you don't get two half crumbs, you get two crumbs
    Doesn't this violate some law of physics?

  13. #13
    Join Date
    Feb 2004
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    287
    If it makes you feel better i only get like 12mpg on my sc with just driving it around here on nice days. When i drove it back from dave dalke ohio to chicago area, i was tailgating my dad on the express way and got 21mpg. So there is a big range possible from city driving to highway driving, for me 16 mpg would be what i get for mixed city/hwy.

  14. #14
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    I don't know how much modifications you have all done to your cars, but my 1990 is mildly modified from stock and gets about 21 mpg in my daily commute. That's about 60% highway, 40% city. I check the mileage at each fillup with a quick mental calculation. (Just out to 1 decimal place, to keep my math skills sharp.)

    I think some of these mileage numbers sound awful. Might be worth looking into.

    I should also say that my mileage seemed to drop 1 or 2 mpg when the 10% ethanol gas came out. Yech.
    God bless!!!

    1990 SC auto ... red outside, gray cloth inside ... ~180,000 miles (and climbing) ... driven daily!

  15. #15
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    Aug 2004
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    al.
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    90/95

    on a trip about 2000 miles round trip in the 90 i got 26 on the hwy no cats or resonator kn panel filter only
    on a recent trip of 1000 miles round trip in the 95 20-21 on hwy all stock except cold air.. the 95 avgs 20 no matter so i guess that good for these cars.

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