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Thread: Bad gas mileage

  1. #1

    Join Date
    08 Nov 2007
    Location
    San Francisco, California

    Bad gas mileage

    I know this has been covered a lot, but when I searched I saw everyone suggesting to replace plugs/wires/fuel filter/02/etc...

    Basically, I'm only getting around 21mpg no matter how I drive. I believe the most I've gotten out of a tank of gas is 280 miles, and that was 90% freeway.

    I've replaced the plugs, wires, fuel filter, 02 sensor, air filter (CAI), oil, tranny fluid (though i dont see what difference this would make) and they have not helped my gas mileage at all. I have a test pipe on this car but I know that wouldn't affect anything since the 02 is in the engine bay. Sometimes it gets so bad that when I come to a stop I can smell the gas. I replaced the stock cat with a test pipe hoping it would help my gas mileage.

    Anyone have any ideas? When I got my car smogged they saw the timing was off so they adjusted it for me, and I passed smog with almost perfectly neutral numbers (when I had the cat on, of course)

    Car has 105k on it.

  2. #2
    G2IC FOUNDER
    Join Date
    29 Oct 1997
    Location
    Maryland USA
    You can actually smell it the fuel? Did you check to see if you have a fuel leak maybe? Where's the smell coming from -- engine bay or out the exhaust? Just a thought.
    NEIL B. - 1991 DA945. Owner since 1997.
    MAY 2014 APPROX ODOMETER: 510K km / 317K miles.
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  3. #3
    G2IC Moderator
    Join Date
    11 Nov 1998
    Location
    Milpitas, CA
    Quote Originally Posted by juanpac0 View Post
    Sometimes it gets so bad that when I come to a stop I can smell the gas. I replaced the stock cat with a test pipe hoping it would help my gas mileage.
    Catalytic converter helps get rid of unburnt fuel. Replacing a cat with test pipe isn't going to help with the fuel smell.

    http://auto.howstuffworks.com/catalytic-converter1.htm
    Catalytic converters are designed to reduce all three:

    * Carbon monoxide (CO) is a poisonous gas that is colorless and odorless.
    * Hydrocarbons or volatile organic compounds (VOCs) are a major component of smog produced mostly from evaporated, unburned .fuel.
    * Nitrogen oxides (NO and NO2, together called NOx) are a contributor to smog and acid rain, which also causes irritation to human mucus membranes.

    Put a new catalytic converter on, not a worn out 100K one.

    There are many many factors that affect mpg.

    - tire pressure
    - alignment
    - weather
    - proper viscosity oil
    - how much extra weight in car
    - valve adjustment

  4. #4

    Join Date
    08 Nov 2007
    Location
    San Francisco, California
    Quote Originally Posted by N FUL FX View Post
    You can actually smell it the fuel? Did you check to see if you have a fuel leak maybe? Where's the smell coming from -- engine bay or out the exhaust? Just a thought.
    Out of the exhaust, it's due to my test pipe.

  5. #5

    Join Date
    08 Nov 2007
    Location
    San Francisco, California
    Quote Originally Posted by spikeymike View Post
    Catalytic converter helps get rid of unburnt fuel. Replacing a cat with test pipe isn't going to help with the fuel smell.

    http://auto.howstuffworks.com/catalytic-converter1.htm
    Catalytic converters are designed to reduce all three:

    * Carbon monoxide (CO) is a poisonous gas that is colorless and odorless.
    * Hydrocarbons or volatile organic compounds (VOCs) are a major component of smog produced mostly from evaporated, unburned .fuel.
    * Nitrogen oxides (NO and NO2, together called NOx) are a contributor to smog and acid rain, which also causes irritation to human mucus membranes.

    Put a new catalytic converter on, not a worn out 100K one.

    There are many many factors that affect mpg.

    - tire pressure
    - alignment
    - weather
    - proper viscosity oil
    - how much extra weight in car
    - valve adjustment
    Tire pressure is ~ 34psi daily driven, alignment is fine, weather is sunny, 10w-30 (I think) high mileage oil is being used, although it got bad gas mileage before the oil change too. All synthetic oils (GM syncromesh in tranny, castrol in engine), extra weight, probably 50lbs for my subs+amp and then me, valve adjustment - is this something i could do myself? How would I go about doing it?

    I think the biggest issue that people don't really pay attention to is that I get bad gas mileage and same amount of miles out of a tank no matter how i drive. I can be shifting ~2k regularly,, 2.5k regularly ~3k regularly, etc.. and not see a difference in gas mileage. All the added weight shouldn't matter as much on the freeway either, as I already have a rolling momentum and the engine doesn't have to work as hard.

  6. #6

    Join Date
    05 Jan 2006
    Location
    Marysville, Wa. 98270
    injector o-rings? Only about $25 for a set of 4.

  7. #7

    Join Date
    25 Jan 2007
    Location
    Bakersfield, California
    +1 on the O-Rings. Same thing happened to me. Smelled gas real bad so I sniffed around the Engine and whamo found it. O-rings were fried. Replaced them and all is well.

  8. #8
    G2IC Moderator
    Join Date
    11 Nov 1998
    Location
    Milpitas, CA
    But he said...

    Quote Originally Posted by juanpac0 View Post
    Out of the exhaust, it's due to my test pipe.

  9. #9

    Join Date
    23 Jan 2004
    Location
    Thahood, California
    i smell gas when i first start the car, i've been guessing it was the fuel injector o ring, but i never have time to do this stuff....

  10. #10

    Join Date
    05 Jan 2006
    Location
    Marysville, Wa. 98270
    Quote Originally Posted by juanpac0 View Post
    Sometimes it gets so bad that when I come to a stop I can smell the gas.
    The smell of fuel when opening the door and getting out leads me to believe what I said about the injector o-rings.

  11. #11

    Join Date
    08 Nov 2007
    Location
    San Francisco, California
    Well, the thing is, the smell wafts from the rear up to the front, when I'm standing at the engine bay when the car on I can't smell any gas or anything unless the wind blows the exhaust smell toward me. I think this is all leading to me needing a valve adjustment, but I only have 105k on the car, how often are we supposed to do it? I know on my E30 it was every 20k or so miles.

  12. #12

    Join Date
    05 Jan 2006
    Location
    Marysville, Wa. 98270
    Does your gas cap hiss when you open up the filler tube? have you checked the tank drain stopper bolt on the bottom rear of the tank, this could be leaking/seeping. Between the filler neck attached to the car body and the tank inlet there's a rubber hose to absorb road vibrations, this could be unclamped or pulled nearly off either filler or inlet tubes.

  13. #13

    Join Date
    08 Nov 2007
    Location
    San Francisco, California
    Yes, it does hiss, I always thought that was weird. So you're saying I'm literally leaking gas? I never see any stains from it on the ground or anything. How do I check the hose?

  14. #14

    Join Date
    05 Jan 2006
    Location
    Marysville, Wa. 98270
    The hissing is a good thing. Means your pump is working ok and the fuel rail return is doing its job along with the charcoal canister vapor collection and return. IIRC if you pull the drivers rear wheel and the fender splash shield the filler neck and gas tank inlet tube s/b visible. The tank drain is where I described. So, again, the drain or fill hose don't have to be letting gas pour out, they only have to be seeping fuel while the car is running and the tank is pressurized. Of course, this is all a moot point if you do have excess unburned fuel from your exhaust.

  15. #15

    Join Date
    08 Nov 2007
    Location
    San Francisco, California
    Lol, looks like I'm back to square one. Okay, how about a simple question. How many miles do you guys usually get out of a full tank? Most I've gotten was about 280.

  16. #16

    Join Date
    05 Jan 2006
    Location
    Marysville, Wa. 98270
    Measure the O2 and see if it meets specs. Of course how you accelerate and shift points have everything to do with that side of mpg. My daughters auto 92 ls got near 30mpg mixed driving with me behind the wheel, she only gets 23mpg because all the in-town, stop and go, jack rabbit starts and such. So for me 300+ per tank, her 250-275.

  17. #17

    Join Date
    08 Nov 2007
    Location
    San Francisco, California
    How do I go about measuring it? It was a replacement Denso unit I bought btw. It's just that I hear that some people get 350 miles out of a tank and I wonder what the hell I'm doing wrong when I'm going 65mph on the freeway for most of the tank and still only getting 280 out of a tank at best.

  18. #18

    Join Date
    28 May 2005
    Location
    San Antonio, Texas
    i wish i was getting 300+ a tank. my best ever was 230 and i had to fill up 11 gallons then. i do tend to drive like an ass so im sure that has a lot to do with it.

    i second the injector o rings. i use to get like 17mpg tilll i changed those. it leaked real bad when it was cold. i could see the fuel leak out around the injector.
    '93 Milano Red GS-R #0363

  19. #19

    Join Date
    08 Nov 2007
    Location
    San Francisco, California
    Quote Originally Posted by RedHeadPyr0 View Post
    i wish i was getting 300+ a tank. my best ever was 230 and i had to fill up 11 gallons then. i do tend to drive like an ass so im sure that has a lot to do with it.

    i second the injector o rings. i use to get like 17mpg tilll i changed those. it leaked real bad when it was cold. i could see the fuel leak out around the injector.
    If you only get 230 out of a tank then you do still get like 17mpg. But yeah, where are the injectors themselves so I can check this?

  20. #20

    Join Date
    09 Jan 2008
    Location
    Texas, USA
    Have you checked for vacuum leaks? I just found a leaky hose on my Teg that was going from my MAP sensor to my manifold, and gained about 12 mpg to my fuel economy when I fixed it.

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