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Old 07-08-2007, 08:41 AM   #1
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Question on a fuel injector switch

With a vehicle that is a 4 cylinder and has sequential fuel injection, I know that most of them utilize a common ground or a common positive wire. That's easy enough to tap into and add a switch.

My question is that if I tap into it and run a switch inside the cabin, would this give a slight delay in injection times thus throwing off the effeciency of the motor?

If this is in fact the case, I would think that connecting the switch in the cabin of the vehicle and running to a relay next to the valve cover would be a more effecient way of switching the injectors on and off.


Am I on to something here or am I thinking too much into this?
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Old 07-08-2007, 09:17 AM   #2
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No delay, you would need some serious length to worry about that. You will need some current carrying capability though and I'm "pushing" my luck with just a microswitch and have considered a relay for that reason.
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Old 07-08-2007, 09:56 AM   #3
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How much current carrying capability could you need? It will only be carrying the ground from one cylinder at a time. On my car there are four grounds that I wirenut together and two 18 gauge wires carry the ground to and from a regular toggle-type switch. There is no "delay" the engine rpms immediately begin to fall off. If you use a momentary switch instead, it has to stay off until rpms=0 or the engine will restart. I love the switch because all the accessories stay on, turns signals and the like. It simplifies the EOC process. I believe CO ZX2 came up with this, at least thats where I got the idea.
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Old 07-08-2007, 11:06 AM   #4
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re: how much current? It varies, I figure 10 amps is reasonably safe target. More would be better, I think my microswitch is rated at 5. I got the idea from metrompg as a way to keep the scangauge connected and not lose info.

ref: http://www.gassavers.org/showthread....ht=kill+switch

more info:
My metro stumbles on a little with the injector kill switch as it is TBI and there is noticable fuel left in the manifold to burn after turning off the injector.

My MPFI saturn stops almost immediately, though it now takes longer to spool down since it has a heavier flywheel (used to be an automatic).

I may go to an ignition based kill switch on my metro, though I had some strange behavior with my first attempt. It would almost die, then find some spark somehow and rev up a little bit and do that ad-nauseum ?!? I will try the other wire next time, and try something else after that
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Old 07-08-2007, 06:13 PM   #5
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Once you kill the injector signals there will be no more fuel injected. I don't see how an ignition based kill would make any difference. Then again mine is for a manual so I have my foot in on the clutch when I hit the toggle and kill the gas. the engine will still spin from momentum, but you aren't losing any gas, its not sucking it out of the injectors or anything.
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Old 07-08-2007, 08:39 PM   #6
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Killing the spark on a fuel injected vehicle is likely going to result in the injectors continuing to run until it stops spinning. That seems like the wrong way to go about it IMO.
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Old 07-08-2007, 08:54 PM   #7
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88HF, the injector in a TBI is quite a ways from the intake valves and that volume of air with gas in it will keep the engine stumbling on for a little bit + the gas that has deposited on the walls of the intake manifold.

On MPFI the injectors are right by the valves and so there is less fuel laden air to draw upon and the intake manifold walls are dry, so the engine shuts off right away.


Snax, it is a driveability thing, having to hold the switch in for 5 seconds or more and having it still not die occasionally is annoying. Others use an ignition kill switch with TBI cars to good effect (i.e. metrompg). In a TBI it leaves the fuel in the manifold where it can be available on a bump start, but it is of no use when it is stumble-stumbled away waiting for the engine to stop (I've tried).
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Old 07-08-2007, 09:42 PM   #8
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Ok, I follow that. Perhaps it would make sense to go the dual headed approach of killing both?
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Old 07-09-2007, 03:00 PM   #9
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Yah, a relay would also be useful for killing both at the same time. Or convert it from TBI to MPFI http://www.teamswift.net/viewtopic.php?t=6697&view=next
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Old 07-09-2007, 06:37 PM   #10
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I've found that while driving highway speeds in my crx, a bump isn't even necessary to restart the engine. The computer is still on and when you turn the injectors back on and put the car back in gear, let off the clutch, the car starts and pulls until you cut it off again without any trouble.
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