what can cause my bad gas mileage?
NEWS
 

+ Reply to Thread
Results 1 to 12 of 12
  1. #1
    1st Gear
    Join Date
    Aug 2007
    Location
    Fredericksburg, VA
    Age
    26
    Posts
    483

    what can cause my bad gas mileage?

    so my 00 V6 is getting pretty bad gas mileage compared to what it used to get. in fact, about 4 mpg less on the highway than usual, and about 3-4 mpg city less. i've recently fixed my exhaust leak (replaced flex pipes prior to cats) and my tire pressure is good. i know i have an oil leak which i need to fix, but oil leaks haven't affected my mpg before. any ideas on what i should check?

    also, is it possible for the gas to be burning rich still even after fixing the exhaust leak? maybe i need to reset some settings or what-not?

  2. Remove Advertisements
    VW Passat Forum
    Advertisements
     

  3. #2
    6th Gear
    Join Date
    Feb 2001
    Location
    Hoover, AL
    Age
    36
    Posts
    9,102
    Do you have vag-com (or access to it?)

    Upstream O2 sensors are a common source of problems with mixture/economy, as well as the MAF.

  4. #3
    1st Gear
    Join Date
    Aug 2007
    Location
    Fredericksburg, VA
    Age
    26
    Posts
    483
    no, i don't have access to vag-com, unfortunately.

  5. #4
    I'm Urlik's Hero
    Join Date
    Jul 2004
    Location
    UK
    Age
    13
    Posts
    9,784
    When did you last service the car? When did you last change the plugs?

  6. #5
    1st Gear
    Join Date
    Aug 2007
    Location
    Fredericksburg, VA
    Age
    26
    Posts
    483
    not real sure, maybe two years max.

  7. #6
    6th Gear
    Join Date
    Feb 2001
    Location
    Hoover, AL
    Age
    36
    Posts
    9,102
    Since the cost of two upstream O2 sensors is about the cost of a VAG, now might be the time to invest. I put it off for over 8 years, but finally bit the bullet 6 months ago.

    If I were attacking it like an older car, I'd clean the MAF and change the upstream O2 sensors. A lot of sources recommend replacing cars' upstream sensors around 100k even if they're not acting up. To me, this is just needlessly throwing money at the car.

    I'd also at least inspect the plugs. The stock ones are supposed to be good for 40k (or more).

  8. #7
    I'm Urlik's Hero
    Join Date
    Jul 2004
    Location
    UK
    Age
    13
    Posts
    9,784
    Hopefully that was when you last changed the plugs and not serviced the car!

    New OEM plugs are cheap. I'd be going there first.

    Schedule suggests every 40k but a lot of people do them annually

  9. #8
    1st Gear
    Join Date
    Aug 2008
    Location
    Rochester, NY
    Posts
    151
    I have a MAC Tools OBD2 tool that reads voltages, what should the voltages be?

  10. #9
    1st Gear
    Join Date
    Jun 2007
    Location
    Canada
    Age
    52
    Posts
    403
    I would start by changing your plugs, air filter and oil/filter and then add a bottle of "good" fuel injector treatment to your next tank of gas. At least then you can rule out the obvious.

  11. #10
    2nd Gear
    Join Date
    May 2008
    Location
    New Hampshire
    Posts
    737
    One issue with a leaking exhaust manifold is that it fools the o2 sensors into thinking that the engine is running lean, so it will dump in more fuel. Depending on the location and severity of the leak, this could mess up the fuel economy for sure.

    The other advice is also spot on -- do a basic tune up with plugs, air and fuel filters, fuel injector cleaner (Lucas stuff works well) and maybe a MAF cleaning to get the basics out of the way. Also you may want to average a few tanks of fuel before deciding what gas mileage you're actually getting. Higher ethanol content will cause your fuel economy to drop, too.

  12. #11
    4th Gear
    Join Date
    Jun 2004
    Location
    mary land
    Posts
    3,844
    How's your driving habit? Controlling your right foot is the best thing you can do for gas mileage.

  13. #12
    4th Gear
    Join Date
    Nov 2009
    Location
    Upstate NY
    Posts
    3,294
    A stuck open thermostat will hurt MPGs, too. The ECU will dump extra fuel to get the coolant up to temp, so you will see 190* on the dial. This, of course, makes you think everything's fine. I believe a bad CTS can cause a similar problem, too.

  14. Remove Advertisements
    VW Passat Forum
    Advertisements
     

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
Powered by vBadvanced CMPS v4.1.2