Why does my car get poor gas mileage?

Discussion in 'Community Discussion' started by aquajet, May 19, 2007.

  1. aquajet macrumors 68020

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    Feb 12, 2005
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    VA
    #1
    I've been driven mad lately trying to figure out why my '99 VW Beetle with 170k miles seems to be chugging gasoline. I'm obsessed with trying to get this thing to be more efficient considering gas is above $3 again. It gets 25mpg on the highway these days. It used to get over 30. Unacceptable for a small car IMO.

    Parts that have been changed within the last 1.5 years: catalytic converter, both O2 sensors, mass airflow sensor, coolant temp sensor, air filter (with a K&N), timing belt and related accessories. It also has new spark plugs.

    My mechanic told me it has low vacuum under heavy acceleration and that it could be due to the foreign matter inside the muffler (probably from the old cat converter). But a muffler shop told me that there's no obstruction in the system. It doesn't burn oil so I assume it still has good compression.

    So, before I bring it in again, I would like to try to figure out why the vacuum is low. I suppose this could be related to the decreased fuel efficiency?? What else should I be looking for?
     
  2. Abstract macrumors Penryn

    Abstract

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    #2
    Are your driving habits different? Did you move to a new city?


    Or maybe you're not a very efficient driver.
     
  3. MacNut macrumors Core

    MacNut

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    Jan 4, 2002
    Location:
    CT
    #3
    Check all the seals and gaskets. I don't know if I would complain about 25 mpg tho.

    An 8 year old car with over 170k miles is a lot of wear and tear, its not gonna run like new anymore.
     
  4. majiktt macrumors newbie

    majiktt

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    Oct 6, 2004
    Location:
    Tucson, AZ - USA
    #4
    I can't comment on the mechanical aspects, but have you checked things such as the tire pressure, oil grade, and that you're not carrying around unneccessary load you didn't have before? Also, maybe your air filter is dirty already?
     
  5. aquajet thread starter macrumors 68020

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    VA
    #5
    Now that you mention it, it spent its entire life in Maryland until ~1.5 years ago when I bought it from my father. I'm in Texas. But it would still go longer on a tank even after it was moved. I guess it's been slowly degrading over the last year or so.

    I drive it gingerly most of the time, but I run it up to redline every few weeks to blow the cobs out.

    Ugh. You try doing that under the hood of a Beetle! :rolleyes: :D

    Yeah, my tires are at 32 psi. I use synthetic 0W 40. Air filter is still clean.

    It's usually just me in the car. You'd think I get better mileage since I've lost 50 pounds over the last two years.
     
  6. pseudobrit macrumors 68040

    pseudobrit

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    #6
    Check all your vacuum lines. Then think about getting a new cat back exhaust. Remember if you're doing lot of highway driving the Beetle is an aerodynamic nightmare with a Cd of .38 or something.
     
  7. aquajet thread starter macrumors 68020

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    VA
    #7
    I miss my '97 Jetta with 5 speed and the same 2.0L engine and 36 mpg. :rolleyes:

    The only things I've done myself on this vehicle are changing the oil and air filter. What's the best way to go about detecting a leak in the vacuum lines?
     
  8. clownie macrumors member

    Joined:
    Sep 23, 2003
    #8
    Beetle Mileage

    Check for fuel leaks. My last VW developed obvious (as in dripping) leaks in three of four fuel injectors... the rubber cracked and the cloth jackets just made some of the leaks harder to find...
     
  9. IJ Reilly macrumors P6

    IJ Reilly

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    #9
    The car should run very poorly if it's got an actual leak in the vacuum lines, so I doubt that one.

    Your engine is pretty elderly. Even if you don't have any obvious compression issues, you're still talking about a lot of internal wear.

    That being said, it's not clear how you are comparing mileage. A 15% difference is not that much, when you consider that even an increase in highway driving speed of 10 MPH can produce that sort of mileage hit, especially in a car which isn't exactly an aerodynamic wonder.
     
  10. Zwhaler macrumors 603

    Zwhaler

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    Jun 10, 2006
    #10
    Before I opened this thread, I thought, "I bet he has a VW". I don't know why, but those things just naturally get bad gas mileage. Even the small ones.
     
  11. aquajet thread starter macrumors 68020

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    VA
    #11
    The fact remains that I haven't changed my driving habits, so I'm not really convinced my highway speeds have much to do with it.

    To be honest, I'm a bit suspect of the exhaust tech who examined the system by shoving a small rod down the length of the pipe. I'm thinking it might be time to disassemble the system and do a visual check instead.

    Like I said, my gas Jetta got 36 mpg cruising at 75 mph across the US. The turbo diesels can get 50+ if I'm not mistaken.
     
  12. 7254278 macrumors 68020

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    NYC
    #12
    With a 170,000 miles and all the maintenece your doing wouldnt it just be better to put up with the bad gas mileage(which is still decent) and just save up for a new car?
     
  13. aquajet thread starter macrumors 68020

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    #13
    A new car? I don't plan to ever buy a new car, used only and cash only. I don't plan to dump this car either until it breaks in half. Even if the engine exploded, I would just replace it. :D
     
  14. MACDRIVE macrumors 68000

    MACDRIVE

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    #14
    That's the way it's supposed to be; you have low vacuum under acceleration because of all the air rushing into the engine. You will have high vacuum at idle speeds because the air entering the engine is at a minimum.

    How does the engine perform other than a decrease in fuel mileage?

    Do you still have a smooth idle?

    Do you still have plenty of power?
     
  15. kajitox macrumors 6502a

    kajitox

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    May 2, 2007
    #15
    I guess I shouldn't mention the mystery that is 18 MPG with a 1999 Nissan Maxima (80K mi).

    Thus far I haven't found any problems with it, but still get really poor milage.
     
  16. Roger1 macrumors 65816

    Roger1

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    Jun 3, 2002
    Location:
    Michigan
    #16
    Have you thought about getting a tune up? With my car, a tune up involves checking sensors, cleaning the air intake thingie, etc. It might be worth looking into.
     
  17. killr_b macrumors 6502a

    killr_b

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    Suckerfornia
    #17
    You may have a blown head gasket.

    That'll eat right into your gas milage without adding new sounds or odd feelings to your car- it will still drive "normally".
     
  18. quagmire macrumors 603

    quagmire

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    Apr 19, 2004
    #18
    Why a used car? Why not new? I understand financially, but never planning to buy new? Frankly, I rather take the risk of a lemon then risk abused car.

    Anyway..... it is probably due to wear and tear. At that mileage, fuel mileage would be down, power will be down, etc. It is all due to normal wear and tear. But, I would check for anything wrong with the car if this was a sudden drop if nothing changed on your side.
     
  19. adk macrumors 68000

    adk

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    #19
    I used to have a 2000 jetta with the same engine as your beetle, and never even once managed over 25 mpg.
     
  20. pseudobrit macrumors 68040

    pseudobrit

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    #20
    The best thing you could do is find someone with VAG-COM and log some data while driving. That way you keep yourself from throwing money at it.


    Mine has gotten as high as 56 @ 75mph on a long distance trip.
     
  21. Porksword macrumors member

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    Mar 30, 2006
    Location:
    Grand Rapids, MI
    #21
    I'm surprised this wasn't mentioned already

    I had a 97 Corolla with 115000 on her and changing the fuel filter on it helped my milage among other things. This would have been done during a 30k, 60, etc. dealer tune-up but if you haven't had one done in a while, it's worth trying. If you do it yourself its only a $35-ish dollar job.
     
  22. aquajet thread starter macrumors 68020

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    VA
    #22
    Because I don't like the idea of absorbing the initial depreciation. I'd rather buy a well-maintained used vehicle with cash. No interest payments, minimized depreciation.

    VW's 2.0L motor is a solid one which will last a long time provided it's not abused and is well-maintained. My Jetta attained 36 mpg at nearly 200k miles. There's nothing to lead me to believe there's any significant wear inside the engine.

    Good thought. I'll look into that.

    I decided to try and poke around the inside of the rear muffler because while shaking it back and forth, I can hear what sounds to be nuts and bolts and pebbles. I was also able to remove some material by using a coat hanger. :rolleyes: I'm thinking it's time to cut it off and see how much crap is really hiding in there.
     
  23. daze macrumors 6502

    daze

    Joined:
    Mar 11, 2006
    Location:
    San Jose, California
    #23
    You've got the correct attitude. I am never buying another new car ever again!

    My 2002 Grand Am is running fine since I bought it 5 years ago... Oh, by fine, I mean that I've had to get the ignition module replaced twice, and the wheel hub broke, and the CD player's LCD died... Yes, and the last payment is due this month. It's got 73K on it, and guess what? I will probably never buy a GM car again. But I plan on keeping this one till it dies. I am hoping for another 3-5 years of usage out of this car.

    The best feature of this car is the after market Pioneer CD player I put in it, which plays AAC files. Love it!
     
  24. quagmire macrumors 603

    quagmire

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    Apr 19, 2004
    #24
    Was this a sudden drop in fuel economy or have you noticed a steady drop?
     
  25. climhazzard85 macrumors member

    climhazzard85

    Joined:
    Dec 28, 2005
    #25
    Um wow....


    Anyway, has your area recently mandated a 10% ethanol mix in the gasoline? It did here, and it hurt my gas mileage.
     

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