Horrendous gas mileage
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HT4100 4.1, 4.5, 4.9 Discussion, Horrendous gas mileage in Cadillac Engine Technical Discussion; 1993 Sedan Deville, 65K car runs superb yet the darn thing is sucking gas down at what averages out to ...
  1. #1
    zentrade is online now Cadillac Owners Fanatic
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    Horrendous gas mileage

    1993 Sedan Deville, 65K car runs superb yet the darn thing is sucking gas down at what averages out to be 6-7 MPG! Not even a Hummer H1 or 2 is that bad!!

    How can that be when the car runs so good?, Just cleaned out throttle body not so long ago . Air filter looks ok..no codes being put up, No SES or SVS lights either.

    I figure even with city driving, stop and go, a bad MPG example would be 10 to 13 perhaps

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    awadecki is offline Cadillac Owners Enthusiast
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    Re: Horrendous gas mileage

    Yeah, that's pretty awful. You likely have a leaking injector or bad FPR, which has now caused your oxygen sensor to go bad too. Check the fuel pressure at the rail (just buy a gauge at the local store). The nipple is located under the black cover just above the valve cover on the radiator side.

    Connect the gauge, turn the key on, and read the pressure. See if it drops after a couple of minutes, if it does you have a bad injector. If the pressure exceeds about 40 psi with the car running, your FPR should be replaced.

  4. #3
    BlackTTC6 is offline Cadillac Owners Member
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    Re: Horrendous gas mileage

    How new is your fuel filter?

  5. #4
    zentrade is online now Cadillac Owners Fanatic
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    Re: Horrendous gas mileage

    Quote Originally Posted by awadecki View Post
    Yeah, that's pretty awful. You likely have a leaking injector or bad FPR, which has now caused your oxygen sensor to go bad too. Check the fuel pressure at the rail (just buy a gauge at the local store). The nipple is located under the black cover just above the valve cover on the radiator side.

    Connect the gauge, turn the key on, and read the pressure. See if it drops after a couple of minutes, if it does you have a bad injector. If the pressure exceeds about 40 psi with the car running, your FPR should be replaced.
    Ok im going to check that out.Thanks! If it turns out to be an injector..any particular way to narrow down which one(s)?

  6. #5
    zentrade is online now Cadillac Owners Fanatic
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    Re: Horrendous gas mileage

    Quote Originally Posted by BlackTTC6 View Post
    How new is your fuel filter?
    Not sure on the fuel filter, Never replaced it since i owned car (Dec 2006) and dont know the history....Cheap fix so i guess while im at the parts store i can pick one up.Thanks!

  7. #6
    HUF's Avatar
    HUF
    HUF is offline Cadillac Owners Connoisseur
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    Re: Horrendous gas mileage

    Quote Originally Posted by zentrade View Post
    Ok im going to check that out.Thanks! If it turns out to be an injector..any particular way to narrow down which one(s)?
    You can lift the fuel rail and see which one is leaking when ignition is ON. Usually, all eight should be replaced when one of them goes bad. You can buy a set for some $200-$250.

  8. #7
    zentrade is online now Cadillac Owners Fanatic
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    Re: Horrendous gas mileage

    Quote Originally Posted by HUF View Post
    You can lift the fuel rail and see which one is leaking when ignition is ON. Usually, all eight should be replaced when one of them goes bad. You can buy a set for some $200-$250.
    Ok thanks! I am going to get on it as soon as possible and i will report back. Got some nasty wethaer coming so might take a bit.

  9. #8
    gdwriter's Avatar
    gdwriter is offline Cadillac Owners 10000+ Posts
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    Re: Horrendous gas mileage

    Several members here have installed the Bosch fuel injectors from Five-0 Motorsport, myself included. Significantly less than OEM, and they work great. I noticed a significant improvement in throttle response after putting them in. And I average anywhere from 18-20 MPG in a 30/70 mix of city and highway driving.

  10. #9
    BlackTTC6 is offline Cadillac Owners Member
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    Re: Horrendous gas mileage

    Try the fuel filter first and see what happens.

  11. #10
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    I~LUV~Caddys8792 is offline Moderator
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    Re: Horrendous gas mileage

    I get an average of 13.5-15.2 MPG in my type of driving.... 70 city/30 highway. Maybe I should look into the fuel filter, as I've never done one in the two and a half years I've owned it. Where's it located on the car?
    "They call Alabama the Crimson Tide, call me Deacon Blues."

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  12. #11
    zentrade is online now Cadillac Owners Fanatic
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    Re: Horrendous gas mileage

    Quote Originally Posted by gdwriter View Post
    Several members here have installed the Bosch fuel injectors from Five-0 Motorsport, myself included. Significantly less than OEM, and they work great. I noticed a significant improvement in throttle response after putting them in. And I average anywhere from 18-20 MPG in a 30/70 mix of city and highway driving.
    Thanks! If i find the problem to be the injectors, i`ll look into those you suggested.

  13. #12
    behind-bars is offline Cadillac Owners Connoisseur
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    Re: Horrendous gas mileage

    Quote Originally Posted by I~LUV~Caddys8792 View Post
    I get an average of 13.5-15.2 MPG in my type of driving.... 70 city/30 highway. Maybe I should look into the fuel filter, as I've never done one in the two and a half years I've owned it. Where's it located on the car?
    Should be a little silver tube located toward the rear of the car on the drivers side. Think its around the back door just inside the frame. I have a fuel filter rolling around in my trunk that i never got installed because i couldnt see it. To be honest I didnt have any jacks to get under it so I couldnt really get a good look.

    To the original poster, You can replace the o2 sensor for about 20$. Not sure if it would be wise to do it now or wait until any problems found. I replaced mine and my mileage went up, dont think i have injector problems, my o2 just got old.

  14. #13
    zentrade is online now Cadillac Owners Fanatic
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    Re: Horrendous gas mileage

    Ok thanks! I think thats something im definitely going to replace as an earlier reply had suggested that if there are any other issues such as injectors or FPR..Those issue could have fouled the 02 sensor. So im still waiting to check everything out firts and then will replace it either way.

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