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Thread: Injectors pulsing when engine is OFF?!

  1. #1
    Fuel Injected! CDeeZ's Avatar
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    Injectors pulsing when engine is OFF?!

    I built a pretty stout TBI and have not been able to work the bugs out until recently.... The engine will start easy and idle well and have good throttle response some times.... But other times it won't start, or if it does it is hard to start and runs poorly......

    I recently discovered that with the KEY ON but ENGINE OFF, you can open the throttle and the injectors go crazy and start pulsing..... That can't be right..... THe oil has gasoline in it and the one plug I pulled out was wet with gasoline.......

    At the 5:50 mark of this video you can hear that the engine starts right up and idles well and has good throttle response....... So the video shows what this thing SHOULD be like........ Then randomly without warning, it runs poorly..... I'm thinking the plugs being wet must be the problem and I'm also thinking the problem must stem from the issue I describe with the fuel injectors........ Anyone got any insight into this?

    https://www.youtube.com/watch?v=RBFBryKSTYs

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    I have never seen nor heard of that. IMO there should be an RPM signal before that pulsing occurs. Custom code or a setting that was incorrectly changed? Possible loose memcal? Hard to say if the connection between this and the good/bad running just might be related.

  3. #3
    Fuel Injected! CDeeZ's Avatar
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    Yeah me either. It has me stumped. No custom code, at least that I'm aware of lol. If I EVER find the cause, I will post back with the cause. Everything else checks out so I really don't know.

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    I installed a used 7427 into my truck a couple of years ago that whould show MAP sensor fluctuations of 15-20 kPa on the scantool during KOEO. The voltages at the sensor and at the ecm were steady. I put it in a pile of "fix me someday" ecm's and installed another.

  5. #5
    Fuel Injected! CDeeZ's Avatar
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    Did that make the MAP issue on the scantool go away? I will drag out another one of my 7427s and slap it in there.

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    Yes. After swapping PCM's the scantool MAP reading behaved and the engine stopped surging.

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    I once worked on an LT1 that had a bad wiring harness between the cam position sensor (optispark for you LT1 guys) and the computer. Just breathe on the wire and the injectors would all pulse and the ignition coil would start firing. We replaced the harness and the problem went away. Just my experience. But I'd try a PCM too.

  8. #8
    Fuel Injected! CDeeZ's Avatar
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    Thinking it was the PCM.... Was a junkyard find that probably got submerged in rain for a few months/years before I found it. Stabbed another JY 7427 in there and the weirdness seems gone.... For now....

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    Fuel Injected! CDeeZ's Avatar
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    Tonight, the pulsing with the key off came back.... Not sure what it could be. Maybe ANOTHER flaky junkyard PCM....

    I guess if it doesn't affect anything else it won't matter. Just will have to treat it like a carb and not flood the engine when it's off :)

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    Did you mean to say key off? The original report was key on, engine off.

  11. #11
    Fuel Injected! CDeeZ's Avatar
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    Whoops sorry. I meant to say key ON but engine off.

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    lol...

    And you thought no one was paying attention. ;)

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    fundamentally, injectors wont pulse unless it thinks the RPM is >0.

    so whatever you're using for primary positional reference during cranking (cam sensor, crank sensor, distributor base) is signalling falsely (interference? defective sensor?)

    key on engine off, you still have datalogging ability. lets see a log of when it's doing it, key on engine off. does RPM stay at zero the whole log?

    also, does the ECM actually pulse the injectors (does your BPW increase)?

  14. #14
    Fuel Injected! CDeeZ's Avatar
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    Quote Originally Posted by 1project2many View Post
    lol...

    And you thought no one was paying attention. ;)


    Quote Originally Posted by steveo View Post
    fundamentally, injectors wont pulse unless it thinks the RPM is >0.

    so whatever you're using for primary positional reference during cranking (cam sensor, crank sensor, distributor base) is signalling falsely (interference? defective sensor?)

    key on engine off, you still have datalogging ability. lets see a log of when it's doing it, key on engine off. does RPM stay at zero the whole log?

    also, does the ECM actually pulse the injectors (does your BPW increase)?
    Well it's a TBI setup using the stock sensors so no cam or crank sensors. You guys have a better working knowledge of OBD1 stuff than I do, so correct me if I'm wrong here but the positional reference in this case will be from the DRP right????

    The funny thing is, there is no repeatable or predictable pattern to when it does this. So, I haven't been able to catch it on a log yet. I will try to do that though.

    Again, you guys have a better working knowledge of OBD1 than I... But if the injectors are making a noise and actually spraying fuel then they are being pulsed by the PCM right??

  15. #15
    Fuel Injected! steveo's Avatar
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    . But if the injectors are making a noise and actually spraying fuel then they are being pulsed by the PCM right??
    you'd think. what if theres an intermittent injector short to ground?

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