Injector behavior on engine start? - Fuelly Forums

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Old 08-04-2006, 05:38 AM   #1
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Injector behavior on engine start?

Anybody know what the fuel injectors are generally doing when I turn the key to start the engine? My assumption is that they are dumping lots and lots of fuel until the engine catches, correct?
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Old 08-04-2006, 06:11 AM   #2
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From what the SuperMID shows, I would say yes during a key start but no during a bump start. I will have to pay attention for sure. I had the same concerns that you stated in the other thread and I have stopped doing the shorter duration EOC's. Cant say what the effect on FE is but im doing pretty good so far. I would say that anything less than 10 seconds isn't worth it. That is my current guideline. Also on some stoplights I will let it idle when I am confident the light will change soon.
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Old 08-04-2006, 07:06 AM   #3
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Thanks for the feedback. In theory, one could calculate exactly how much fuel it takes to start the car based on the average time to catch and the fuel injector capacity if they really are staying open. Compare that to the idle fuel flow rate and you're there.

I prefer to bump start any time I can (I think it's actually easier on the car) but have found myself coasting ICE-off up to just about every single stop sign on my route. That may change.
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Old 08-04-2006, 07:25 AM   #4
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AHHHH! I think I found my missing half gallon!

I found the stock injector flow rate of 270cc/min in a thread at H-T. Assuming wide open injectors for an average of 2s to start the engine, that works out to .0024gal/start. Puny, right? But wait! The ScanGauge reports ~0.3GPH at warm idle, or 0.000083gal/sec. That works out to 28.9 seconds of idling per engine start!

Now check this out: My last tank was "missing" roughly 0.5 gallons of fuel. Well, I calculated that my ridiculously hyperactive EOC/key starting should have produced ~220 starts for that tank. 220 starts x 0.0024gal/start = 0.528gal!

The "30 seconds worth of idling" 'myth' may not be a myth!
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Old 08-04-2006, 10:00 AM   #5
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I think I found a possible dumb mistake in my calculation. I assumed that the injectors would be wide open but still firing one at a time. Does that make sense?
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Old 08-04-2006, 10:05 AM   #6
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I don't think the injectors will run at 100% duty cycle on a stock fuel injection system.
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Old 08-05-2006, 01:17 PM   #7
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no they wont. even at WOT. at WOT there like 80 or 85% duty cycle. i jump one of my injectors to ground to make my car look like a old beater by it smoking white a ton. but when ever i do that i lose 3 mpg evreytime.
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Old 08-05-2006, 01:28 PM   #8
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You're such a nut.
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Old 08-05-2006, 04:06 PM   #9
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Keep in mind I'm not talking about normal driving, only during engine start. My understanding is that the ECU assumes a tremendous fuel demand to get the thing to catch. The question is how much?
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Old 08-05-2006, 04:48 PM   #10
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I heard that stock injectors can have flow rates of 10% + or -. There all not the same.
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