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Injectors pulsing when engine is OFF?!

Old 01-04-2019, 10:06 PM
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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?

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Old 01-05-2019, 01:37 PM
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Re: Injectors pulsing when engine is OFF?!

injector stuck open? injectors should only pulse when they get a signal from the distributor, ie: cranking. i would get a noid light and confirm the ecu isnt pulsing the injectors. IIRC, the injectors always have 12V on them (with key ON) and the ECU pulses the ground.

I would recommend getting a set of FACTORY injectors professionally cleaned. I recommend witchhunter and i used them a few times in the past. Really need a scan tool (or lap top) with FI, troubleshooting is much easier/faster than guessing.
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Old 01-05-2019, 10:33 PM
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Re: Injectors pulsing when engine is OFF?!

I don't think they're stuck open, they pulse normally and they do not leak fuel at all when the engine isn't running. Randomly, without any repeatable pattern whatsoever, they will pulse just like they do when the engine is running when it in fact isn't .

You are correct, the injectors have 12v+ when the key is on, and the PCM provides the ground to close the circuit and pulse the injectors.....

The injectors I'm running are decently clean, otherwise I'd throw them on my Arduino flowbench. Pulsing injectors through a few cycles with a decent solvent does clean them well. I learned that one well when I was cleaning and flow testing literally between 30 to 40 LS truck injectors for my turbo LQ4 truck.... I have a 7427 PCM that I swapped in this truck a few years ago and I have it shifting the 4L80 I put behind this SBC. I kind of wish I had just done an LS swap like my other truck but I wanted to do a cool kinda traditional SBC build for this one. I'm definitely better at using HPTuners for the OBD2 side of things vs TBI and TunerPro, but, I'll get it figured out one way or another.
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Old 01-06-2019, 01:54 PM
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Re: Injectors pulsing when engine is OFF?!

Wire shorted together or a wiring mistake? perhaps PCM is no good. try flashing in a stock calibration and see if it still does it.

Last edited by morgsie; 01-06-2019 at 01:57 PM.
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Old 01-06-2019, 02:04 PM
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Re: Injectors pulsing when engine is OFF?!

I think the PCM might be the culprit. Going to slap another 7427 in there later!
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Old 01-12-2019, 12:22 PM
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Re: Injectors pulsing when engine is OFF?!

I think the 7427 PCM that WAS in there was bad or funky. Stabbed another JY PCM in there and that weird issue seems resolved.
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Old 01-13-2019, 01:52 PM
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Re: Injectors pulsing when engine is OFF?!

Glad you found the fix...that was a weird deal.
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