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Old 05-14-2007, 06:46 PM   #1
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Engine AutoOff Switch

I have been toying with the idea of having an automatic engine shutdown when coasting, i.e. accelerator not pressed and when idling at intersections. Now to do this I am either going to manipulate the fuel pump (easy as there is one wire) or fuel injectors (harder if I can't find a common ground wire).

Now turning off the fuel pump gives the engine 1-3 seconds running time before it runs out of fuel which is like a delay, however that might mean a longer start up time which is a NO NO for intersections; I am going to test this shortly. If the start time is longer due to no fuel pressure than one should go with fuel injection as and the engine off delay can be accomplished with a capacitor attached at second 12V bosch like relay when gas pedal is no pressed.

Now the engine will also be turned off when stooped, but when one presses the gas pedal it starts the car. This is accomplished by another relay that activates the ignition on when a warning light that turns on when engine is off, in my car when I start it I find the battery not charging light will work best; it is the last one to turn off right after engine starts. The design is for a manual transmission but should work in a automatic by letting the car start in gear???.

The relays in question are super easy to find at scrap yard or can be bought rather easily i think.

Link about the relays: http://www.bcae1.com/relays.htm

Attached is the simple design.
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Old 05-15-2007, 03:17 AM   #2
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It would seem to me that an injector shut off switch would be better, Because it is immediate, but when you shut off the FP, some fuel is still injected, at a lesser pressure, which is less fuel, and causes a lean condition, (brief knocking).

Do you have manual or automatic transmission?
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Old 05-15-2007, 05:17 AM   #3
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It would seem to me that an injector shut off switch would be better, Because it is immediate, but when you shut off the FP, some fuel is still injected, at a lesser pressure, which is less fuel, and causes a lean condition, (brief knocking).

Do you have manual or automatic transmission?

I have a manual transmission which makes this auto off/on switch rather easy. I am also leaning toward the injector shut off, it is cleaner and quicker. However I do not worry about the lean burning and knocking. As the fuel pump is turned off when there is no acceleration i.e. at idle only, i figure it would be more worry some at higher throttle speeds.

Has anyone tried shutting off injector with just one common ground wire for all of them ? If that is not possible than I require two 12v relays per injector, which is 8 total relays.

I won't be able to work on my car for a few days it seems, it would be good to know it can be done with one common ground wire for all injectors.
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Old 05-15-2007, 09:28 AM   #4
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First of all, it would be very helpful to know what type of vehicle you have, before someone gives you a bunch of information that is not really relevant to your car.

For myself, I have an 89 Honda, which I had been driving. It is not operating at the moment, because I haven't been able to figure out what is wrong and I haven't had the time to work on it.

In any case, on that vehicle, here is the deal. The Honda automatically turns off the injectors anytime I take my foot off of the throttle and the engine rpm is above 1000 rpm. This means that if I am coasting/slowing down, in gear, the ecu automatically shuts does not turn on the injectors. This is a verified phenomenon, on my car, because I have my Oxygen sensor wired so I can monitor it.

I do have a cutoff switch wired into the car. I chose to cutoff the power going to the injectors, but leave the fuel pump turned on. The injectors don't have a common ground, but they do have common power and the ecu pulls the other side of the injector to ground, to turn it on. It is sort of a negative logic, in the sense that when the voltage to the injector is low, from the ecu, then the injector is on and spraying fuel.

My biggest problem with this system is that when I shut the injector voltage off, the engine stops ok, but the cel light comes on and although the car will start right back up, if I turn the injectors back on, the cel light stays lit. I think it is just a flag that goes up when the ecu detects that the engine is not running, even though the ecu thinks it should be. The cel light goes out if I shut off the ignition switch and restart the engine. However, I don't like doing this because of the steering wheel lock. Therefore, on my car, I took the ignition switch off and used a cutoff tool to remove the wheel lock.

All in all, this accomplishes what I believe you are describing, so I hope this answers some of your questions.

One thing I have concluded is that I need to somehow get the $ to get a scantool type of device, for my car. The difficulty is that without some type of reasonably instantaneous feedback, it is hard to tell how much effect a change is making. This is compounded by the fact that the car gets pretty decent mileage, if driven reasonably, so it takes a fair amount of driving to use enough fuel to verify or validate an idea. Additionally, it seems like their are always additional variables which enter in to make it more difficult to make a totally repeatable, clearcut conclusion.
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Old 05-15-2007, 03:23 PM   #5
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Was just thinking that if the fuel was shut off and you kept it in gear as you came to a stop it would engine brake all the way down to 0 mph but should the light turn green you could just turn the fuel back on and shift gears. I would still like to figure out a way to do that and shut off the reading going to the ScanGauge so that it would know that there was no fuel being used. I down shifted today on a downhill exit ramp with stops at the bottom - yeah big waste but I have to avoid the better mileage road because it is too bumpy with frost heaves - and downshifted to 3rd at 45mph 3000+rpm which shows 0.6gph burning with fuel shut off. It ends up throwing off the fuel used a little but really distorts of the MPH for the trip when that happens. I decided to take a more direct route home and back lately and find that the fuel consumption (MPG) is still reasonable even though there are downhills that waste fuel.
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Old 10-12-2008, 09:09 PM   #6
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Just a quick update. I've been running my auto off switch for over a year and 3 months now. The engine does not run when the gas pedal is not pressed. The car auto starts if stooped, or simply resumes the fuel injection process when coasting in gear. The system that is shut of is the power to the injectors as per the car's wiring manual diagram, however I think it shuts the car's computer off as the engine light that is normally on when car is not started but is off when car is running is actually off when the gas pedal is not pressed; so there is no annoying engine light problem if the injectors are shut off as Gary Palmer pointed out.

The starter is still the same even though on an average drive to work it will re-start I guess about 5 times so either a starter can take it or I've been very lucky or it's manufacturing in this Hyundai Accent 1996 manual transmission. I get about 5L/100km driving in the city as opposed to 7-8L/100km, I use high octane 98 fuel. On highway I get about 5L/100km or less if I drive 100km/h (I calculate my economy by the amount of fuel I put in and km driver). There is an override switch that is needed when I take it for an oil change other wise I can see the mechanics getting very confused. I actually love it that I just simply touch the gas pedal to start the car, kind of like a hybrid.

The actual design used 4 relays rather than 3 as in the original diagram.

So in a nut shell this is a success!

It has been a very busy year for me as I have bought a rental property and got married. If there is interest please send me an e-mail rather than private message on this site. More interest will motivate me to post and get involved in having other people try it!
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Old 10-13-2008, 04:07 AM   #7
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Can you describe better how it starts the engine?
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Old 10-17-2008, 06:06 PM   #8
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Can you describe better how it starts the engine?
There is a micro switch on the gas pedal. If it is pressed and the 'charging' battery light is on it activates the starter (as if turning the key to start which runs the starter) and the injection computer, if the 'charging' light is not on (car is moving, tranny is engaged, and the alternator is charging) than it just starts the injection computer. If the gas pedal is not touched it turns power off going to the computer; the engine is not running.

I'm using bosh like relays from older cars; I pulled these from a scrap yard. Quick search found this link: http://www.bcae1.com/relays.htm

I believe the above setup can only work with a standard transmission.

Hope that helps, anything else I can clarify?

I'm thinking of adding a relay to turn off the load off the alternator if the gas pedal is not pressed (i.e. when braking). I have read no alternator load increases gas millage.
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Old 10-17-2008, 06:09 PM   #9
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So, to "activate the starter", do you use a configuration like this?
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Old 10-17-2008, 06:21 PM   #10
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No no no, That's 10 relays in that diagram. I only use 4. The idea is this:

1. Gas pedal not pressed
- no fuel going to the engine
2. Gas pedal is pressed
*- we are stopped because; alternator light is on
- start engine; act as if the ignition is starting the engine; this stops when alternator light goes off!
- provide power to injectors
*- we are not stopped but alternator light is not on (tranny is engaged and we are cruising but we are not touching the gas pedal)
- provide power to injectors

When I am parked I just touch the gas pedal to start the engine. Kind of like the 'ON' button in commercials I've seen a while back for a toyota.
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