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Thread: MFI Ping at Idle

  1. 03-01-2008, 01:43 PM #1
    jcarpio
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    Jorge Carpio
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    MFI Ping at Idle

    Hi everyone,

    I'm having a strange pinging while the engine is idle and at temp and was hoping someone could shed some light on the problem.

    I have a stock 1972 911T engine (2.4) with MFI, and about 50K miles on it. At idle, when the car is at temp, I can hear a loud pinging coming from cylinders 4 and 5. I decided to take the air cleaner off and put a synchrometer on the stack (I was getting ready to sync them as part of CMA) and each time the cylinder pings, the reading on the synchrometer drops to 0.

    Any idea what this could be, when a ping causes air intake to stop at idle?

    Thanks,

    Jorge
    911S Registry #837
    1972 911T Sepia
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  2. 03-01-2008, 02:43 PM #2
    Davz912/911
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    David Sperow Davz912/911's Avatar
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    Pinging at idle

    Jorge,

    It sounds like pre-detonation. Usually caused by carbon build-up. You can try an "Italian" tune-up (take it out on the freeway and run it up to redline a few times). Or you can try dribbling small amounts of water down the carb-the theory being it "breaks up" the carbon, and cleans the combustion chamber.

    Good luck
    Dave
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  3. 03-01-2008, 03:12 PM #3
    jcarpio
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    Jorge Carpio
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    Hi Dave,

    This was just after a spirited run, close to redline for a good bit of time. I'll check out pre-detonation on the forums and see what I come up with.

    Thanks,

    Jorge
    911S Registry #837
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  4. 03-01-2008, 04:36 PM #4
    boxster03
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    what idle are you running? how do those plugs look? did this just start happening? any change in fuel? Have you run FI cleaner lately? It sounds like you are running lean which is unusual for mfi, any change to the dist setting/dwell? whats the advance? Is the idle staying dead on or is it running up or down ?
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  5. 03-01-2008, 05:17 PM #5
    jcarpio
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    Jorge Carpio
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    Hey boxster03,

    Idle is at about 900. The air correction screws are at their default setting, 3 half turns open. I haven't sync'd them yet. Maybe that's the problem??
    Plugs are new, about 150 miles on them. Bosch copper ones. I did just change fuel from 87 to 91, two tanks ago. I've never used FI cleaner. Timing is 5 degrees ATDC, with vacuum line connected. And, finally, idle is dead on at 900, no bouncing.
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  6. 03-01-2008, 05:44 PM #6
    boxster03
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    that idle is the holy grail do be happy you are there. maybe centrifugal spring is broken
    Early 911S Registry #750
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  7. 03-02-2008, 01:54 PM #7
    jcarpio
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    Jorge Carpio
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    What's the centrifugal spring?
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  8. 03-02-2008, 04:16 PM #8
    boxster03
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    I think it has something to do with the distributor
    Early 911S Registry #750
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