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Cadillac Seville / Cadillac Eldorado Forum Discussion, Mpg in Past Cadillac Vehicle Discussion; Help, I had the plugs on my 01 seville changed and now my mpg went from 28 to 19. Is ...
  1. #1
    rjba5450 is offline Cadillac Owners Member
    Automobile(s): 2001 cadillac seville
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    Mpg

    Help, I had the plugs on my 01 seville changed and now my mpg went from 28 to 19. Is there something in the computer that needs to be reset or could they be the wrong plugs?

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  3. #2
    Submariner409's Avatar
    Submariner409 is offline If it won't run, slap in a big stereo
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    Re: Mpg

    Depends on who and how and what did the plugs. If they were done at a GM dealer you should be OK in that department; anywhere else, and without your direct input, anyone's guess. There's quite a list that will contribute to suddenly decreased gas mileage., but none of the possible problems would have anything to do with the PCM (computer - Powertrain Control Module, in the air cleaner box.)

    Read the entire "sticky thread" at the top of the main Seville page titled "How to pull codes". Learn the procedure for entering your car's built-in Diagnostic Trouble Code scanner/reader system. Pull your codes, noting whether each is Current or History, and then go back to the thread and open the link with "obd2" (OnBoard Diagnostics protocol -II) in the URL. Click on the links in the main index box to get the meanings of any P,B,C, or U codes.

    Then, c'mon back...............

    BTW, Your car takes AC Delco Platinum plug #41-987, pregapped to .050", torqued to 13 ft/lb.

  4. #3
    Ranger's Avatar
    Ranger is offline Cadillac Owners Member
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    Re: Mpg

    Nothing to reset after a plug change.

  5. #4
    trainwreck68 is offline Cadillac Owners Member
    Automobile(s): 1995 Seville STS
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    Re: Mpg

    Make sure whoever did it did not cross up 2 of the plug wires. On a V8 engine, espacially a higher hosepower engine, such as the Northstar you may not notice too much in the way of power loss and the engine may still run fairly smooth but it will effect gas milage. I had this problem on my 95 Caprice.

  6. #5
    eyekandyboats.inc's Avatar
    eyekandyboats.inc is offline Cadillac Owners Connoisseur
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    Re: Mpg

    What plus wires? there is no plug wires to mess up.

    Quote Originally Posted by trainwreck68 View Post
    Make sure whoever did it did not cross up 2 of the plug wires. On a V8 engine, espacially a higher hosepower engine, such as the Northstar you may not notice too much in the way of power loss and the engine may still run fairly smooth but it will effect gas milage. I had this problem on my 95 Caprice.

  7. #6
    trainwreck68 is offline Cadillac Owners Member
    Automobile(s): 1995 Seville STS
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    Re: Mpg

    Oh, I'm sorry, my 1995 Seville has plugs and plug wires. I didn't know they made internal combustion engines that had plugs and no wires to get the spark to them.

  8. #7
    Submariner409's Avatar
    Submariner409 is offline If it won't run, slap in a big stereo
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    Re: Mpg

    The latest and greatest ignition systems use either "Coil-on-plug" or "Coil-near-plug" installations: each individual spark plug has its own coil, so the only "coil wires" are the individual 12V trigger leads from the Powertrain Control Module. The ignition "timing" is electronic, not mechanical, is not adjustable under most circumstances, and is triggered by Hall effect sensors which generate pulses by sensing tooth movement on a reluctor wheel or magnetic trigger wheel. Very similar to a multi-port fuel injection trigger system.

    Look at my post #2 - that's a 2000+ ignition coil cassette for the front bank of cylinders: 4 coils, each connected to its plug by an insulated boot with spring conductor in the center. The black connector is one of two Ignition Control Modules (one for each bank) which is triggered by pulses from the PCM and CranKshaft Position sensors (CKP) and piston position (TDC) is referenced by a CaMshaft Position sensor (CMP).

    Your 1995 uses a system called "waste spark ignition". Google it to find out how it works and why proper spark plug/coil wire connection is critical, as in your Caprice.

  9. #8
    trainwreck68 is offline Cadillac Owners Member
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    Re: Mpg

    AH HA, that explains it.

    Thanks.

  10. #9
    chubbyranger is offline Cadillac Owners Connoisseur
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    Re: Mpg

    Don't eliminate the possiblity that whoever did the plugs screwed up something else (vacuum leak maybe?) that could be causing the mileage problem.

  11. #10
    rjba5450 is offline Cadillac Owners Member
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    Smile Re: Mpg

    Thanks for all your responses. Found out the wrong plugs were installed. 41-601 instead of 41-987. Believe it or not, they are going to change them out today for free. I was amazed to say the least. Thanks again!!!

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