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Thread: Engine backfires

  1. #1
    Registered User zewald's Avatar
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    Default Engine backfires

    hey guys

    so i just recently got my 75 280Z up and running, and actually drove it around some, needless to say, i'm stoked. the only problem is that the engine backfires any time i try to give it more than a little gas. if i press on the gas slightly, to allow the rpm's to creep up, it's fine, but any time i step on the gas with any real force, the engine sputters, backfires, and loses power. I've checked all my vacuum hoses, and there are no leaks. any idea what the problem could be?
    thanks in advance,

    Zach

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    That's what my 76 did when I got it and the AFM was bad. Replacing it fixed it. But there is quite a bit of thought around about how to tell if your AFM is bad or not. If you give it full throttle will it go smoothly? At full throttle the ECU gives full enrichment and the AFM has less effect.

    Is your timing set correctly? You could also check the breaker plate in your distributor to make sure it's not bound up.

    Is the Z a manual transmission?

  3. #3
    Z geek at large FastWoman's Avatar
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    Be careful! Your '75 air flow meter doesn't have a backfire relief valve, so you can actually bend the vane.

    The short answer is that you're probably running lean, although it could certainly be a timing issue.

    A couple of things to check:

    Is there a rip in the boot between your AFM and throttle body? if so, you won't meter air correctly and will be running too lean.

    Is your fuel pressure correct?

    Several of us have been kicking around ideas/approaches/experiences with our EFI systems. Cozye and I worked through the lean running issues in our '78 engines by fooling the ECU into enriching the mixture. We and others believe that the ECU has drifted in its calibration throughout the years, delivering progressively shorter pulses to the injectors. The solution we used is to install a variable resistor in series with the coolant temp sensor and to adjust the fuel/air ratio with it. If you do a search for my "purs like a kitten" thread, you'll see documentation of my own long saga.
    My last three sports cars while I still owned all three:

    2001 BMW Z3 Roadster 3.0i soft/hard top (sold)
    1966 Ford Mustang Coupe (sold)
    1978 Datsun 280Z (enjoying very much )

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    Registered User zewald's Avatar
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    Zed, i don't think it's the timing since it idles perfectly. though i could be wrong. i have checked the breaker plate, and it looks fine. it is indeed a manual transmission. i haven't tried to full throttle it, but i will tomorrow and let you know.

    FastWoman, i don't think there's a rip in the boot, but i can easily get one to see if that has any affect. and i'm not exactly sure how to check the fuel pressure.

    maybe this is a dumb question, but my fuel damper is pretty much wide open, could that be effecting it?

  5. #5
    Z geek at large FastWoman's Avatar
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    Check the bottomside of the boot with a mirror. Stretch the folds gently apart to examine the creases.

    You'd check the fuel pressure with a fuel pressure gauge connected to a T-fitting inserted between the fuel filter and fuel rail. You can either buy a fancy one at your local auto parts store or cobble your own from cheap parts at your local hardware store (that probably won't be as accurate).

    As far as I'm aware, your fuel damper wouldn't do anything but dampen out noise. I imagine it would be rare for one to fail.
    My last three sports cars while I still owned all three:

    2001 BMW Z3 Roadster 3.0i soft/hard top (sold)
    1966 Ford Mustang Coupe (sold)
    1978 Datsun 280Z (enjoying very much )

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