No codes, very bad gas mileage
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Cadillac Seville / Cadillac Eldorado Forum Discussion, No codes, very bad gas mileage in Past Cadillac Vehicle Discussion; Alright I've fixed a lot so far but still trying to solve an issue on my 91 seville. It has ...
  1. #1
    carden2 is offline Cadillac Owners Member
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    No codes, very bad gas mileage

    Alright I've fixed a lot so far but still trying to solve an issue on my 91 seville. It has no current codes. I've cleared and ran it several times to work and back and can't get any codes. The car has poor acceleration especially up hills and at 60 mph. I've got a new catalytic convertor, new plugs and wires, distributor, air filter, fresh oil change. Replaced a bunch of vacuum line connections today. I cleaned the throttle body and checked out the egr valve. The egr vale was really black on the end not real sure how to clean it. Anyone have any idea where to start on this? I notice if I give it gas at all it's instant mpg is like 8-12 nothin over 30 even if your coasting. So any ideas, I appreciate the help. I've fixed my a/c and driver seat with help from here so far.

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    ronluh is offline Cadillac Owners Member
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    Check the fuel pressure regulator.......should be dry when u pull vacuum hose ......if not replace. FPR

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    carnut is offline Cadillac Owners Master
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    Re: No codes, very bad gas mileage

    If its running too lean, it wont always set a code. Whats the fuel pressure as driven?

  5. #4
    carden2 is offline Cadillac Owners Member
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    Re: No codes, very bad gas mileage

    Alright to check the fpr, do I start the car then shut it off and pull the vacuum hose?

    The fuel pressure can I check this from the CPU as I'm driving?

  6. #5
    Submariner409's Avatar
    Submariner409 is offline If it won't run, chrome it
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    Re: No codes, very bad gas mileage

    Quote Originally Posted by carden2 View Post
    Alright to check the fpr, do I start the car then shut it off and pull the vacuum hose?
    The fuel pressure can I check this from the CPU as I'm driving?
    The FPR vacuum nipple and hose should be absolutely dry - NO gas.

    Fuel pressure itself is not monitored - you need to attach a fuel pressure gauge to the test port on the fuel rail/throttlebody test port and monitor it mechanically.

    Instant fuel mileage is useless for anything but watching engine load changes: Your actual consumption over time/mileage/gallons used is the only true mileage figure.

    What sort of mileage do other 4.9 engine owners get ? ----- city, suburban, open highway.

    Please update your profile to show car owned, miles on the speedo, maybe approximate location - it sure helps with diagnosis.

  7. #6
    Ranger's Avatar
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    Re: No codes, very bad gas mileage

    FPR is checked with the engine at idle.

    Fuel pressure must be checked with a fuel pressure tester.
    http://shop.advanceautoparts.com/p/a...7818/9030328-P

    Remember, EGR stands for "Exhaust Gas Recirculation". Exhaust gas = burnt fuel. It's going to be black and look like the inside of your tail pipe.

    Don't put much into that instant MPG. That's more for your amusement than anything else, although it should go to about 70 on coast down.

    EDIT:
    Oops, Sub beat me to it.

  8. #7
    carden2 is offline Cadillac Owners Member
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    Re: No codes, very bad gas mileage

    Alright I updated my information.

    I pulled the vacuum while the car idled it doesn't show any gas or feel like it's wet at all.

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    Ranger's Avatar
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    Re: No codes, very bad gas mileage

    Then you can rule out the FPR.

  10. #9
    carden2 is offline Cadillac Owners Member
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    Re: No codes, very bad gas mileage

    So I need to test fuel pressure now and that will diagnosis what fuel injectors?

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    Submariner409's Avatar
    Submariner409 is offline If it won't run, chrome it
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    Re: No codes, very bad gas mileage

    Huh ??

  12. #11
    MoistCabbage's Avatar
    MoistCabbage is online now Cadillac Owners 10000+ Posts
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    I think he's asking if checking the fuel pressure will diagnose the fuel injectors.

    If so, no. That would diagnose the fuel pump.

    ----------

    Let's eat, grandma.

    Let's eat grandma.


    .....Punctuation is important.
    carnut likes this.

  13. #12
    carden2 is offline Cadillac Owners Member
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    Re: No codes, very bad gas mileage

    Alright I'll check that and the car has never stalled on me or not started first try if that helps point to what else to check. I'll get a gauge and check the fuel pressure

  14. #13
    Ranger's Avatar
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    Re: No codes, very bad gas mileage

    Weak fuel pressure will not affect your fuel mileage.

  15. #14
    carden2 is offline Cadillac Owners Member
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    Re: No codes, very bad gas mileage

    so should i chevk something else. it starts and runs good overall. it just gets horrible gas mileage, has a hesitation to accelerate where you can put the pedal to the floor and wont do much, going up hills it wont pickup speed. i drive in heavy traffic, the car goes to about 220 then temp will drop so that seems fine. anything outside checking fuel pressure?

  16. #15
    Ranger's Avatar
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    Re: No codes, very bad gas mileage

    Check the fuel pressure and if that's right, check for a leaky fuel injector. MAP sensor OK? (it should set a DTC if not)

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