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Thread: Engine dieseling??

  1. #1
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    Engine dieseling??

    The last two times I drove my car the engine dieseled for several seconds when I turned off the key. What is the likely cause?? Nothing has been changed otherwise. Always ran very well without issues.
    1969 911S Targa
    1970 911S Coupe
    1973 911T w 3.2
    1972 Alfa GTV 2000

  2. #2
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    You didn't say what car you have the trouble with, but typically dieseling, or run-on can be caused by an idle speed that is set too high on a carbureted engine or by low octane fuel, (fuel vapours continue to burn from heat in the cylinders). Higher octane is less volatile.
    Cheers,

    Jones...
    early S # 2626
    '71 911T
    '05 BMW 330ci
    '22 Wills Sainte Claire
    '96 S10 truck

  3. #3
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    Quote Originally Posted by Jones View Post
    You didn't say what car you have the trouble with, but typically dieseling, or run-on can be caused by an idle speed that is set too high on a carbureted engine or by low octane fuel, (fuel vapours continue to burn from heat in the cylinders). Higher octane is less volatile.
    Sorry--1970 911S Coupe
    1969 911S Targa
    1970 911S Coupe
    1973 911T w 3.2
    1972 Alfa GTV 2000

  4. #4
    Serial old car rescuer Arne's Avatar
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    I'm betting on a bad tank of gas. Too much ethanol, or lower than advertised octane.
    - Arne
    Current - 2018 718 Cayman, Rhodium Silver, PDK

    Sold - 1972 911T coupe, Silver Metallic; 1984 911 Carrera coupe, Chiffon white; 1973 914 2.0, Saturn Yellow; 1984 944, Silver Metallic

  5. #5
    Senior Member majordad's Avatar
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    Same often happens to my 1972 S, and I’ve been told it’s due to the engine running rich.

  6. #6
    Would love to hear from Ed on this interesting topic...

    Here’s a generic article...

    https://axleaddict.com/auto-repair/W...-Keeps-Running
    Peter Kane

    '72 911S Targa
    Message Board Co-Moderator - Early 911S Registry #100

  7. #7
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    Quote Originally Posted by Arne View Post
    I'm betting on a bad tank of gas. Too much ethanol, or lower than advertised octane.
    I think we have a winner. Filled up with fresh tank of Shell premium and no more issues--not even a hint of run-on. Added a can of Seafoam to remove any excess carbon from cold weather driving, etc. Took a long drive. Runs great again.
    1969 911S Targa
    1970 911S Coupe
    1973 911T w 3.2
    1972 Alfa GTV 2000

  8. #8
    Moderator Chuck Miller's Avatar
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    Great advice, great outcome.... congrats Frank !!!
    Chuck Miller
    Creative Advisor/Message Board Moderator - Early 911S Registry #109
    R Gruppe #88

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    '73S cpe #1099 - Matched # 2.7/9.5 RS spec rebuild
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