Tricking the PCM/ECU into lean burn? - Fuelly Forums

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Old 12-19-2006, 10:46 PM   #1
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Tricking the PCM/ECU into lean burn?

Hello -

I posted a question on saturnfans.com :

http://www.saturnfans.com/forums/showthread.php?t=89959

... but haven't gotten a response yet, so I am reposting the question here :

Bigdaddy94sc2 -

Quote:
Originally Posted by Bigdaddy94sc2
For STFT to drop when slowing down is normal, due to decell fuel cutoff. 25% is the max it can go to, then it goes into decel fuel cutoff when it sees brakes applied, speed decreasing, and no ABS activation. It doesnt totally cut fuel, but darn close.
I'm also a member of gassavers.org, so your post made me want to ask this hypothetical question :

Context : 1999 SW2 DOHC with Manual Transmission, Coasting in gear at 60 MPH (so RPMs are above idle) on a flat highway. Under these conditions, speed is decreasing but the brakes are not being applied.

Question #1 : If the PCM thought that the brakes were being applied, would it apply the rule you said above, aka "It doesnt totally cut fuel, but darn close". If yes, then I could conceivably install a "fake brake" button that would trick the PCM into reducing the fuel to the injectors.

Question #2 : What would be the possible negative side effects? Would the brakelights come on? If my speed is decreasing, I don't think this would be a bad side-effect.

Question #3 : Is there slack in the brake such that I can just barely press the brake pedal and trick the PCM into thinking the brakes are actually engaged?

My reasoning is that it appears that I can increase MPG within the rules of the PCM without risking safety. If I press the accelerator, my "fake brake" button signal would be ignored.

Thanks,

CarloSW2
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Old 12-19-2006, 11:21 PM   #2
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From my understanding

Quote:
Originally Posted by cfg83
Question #1 : If the PCM thought that the brakes were being applied, would it apply the rule you said above, aka "It doesnt totally cut fuel, but darn close". If yes, then I could conceivably install a "fake brake" button that would trick the PCM into reducing the fuel to the injectors.
The PCM sees deceleration as a signal change from the VSS or from an accelerometer. The PCM compares the signal taken at the moment and from the previous poll and determines whether or not the vehicle has sped up or slowed down.

Quote:
Originally Posted by cfg83
Question #2 : What would be the possible negative side effects? Would the brakelights come on? If my speed is decreasing, I don't think this would be a bad side-effect.
Brake lights are usually controlled by a switch mounted on the brake pedal lever. By sending a signal to the PCM of perpetual decel, your speedo will read a slower than actual speed if your gauges are PCM controlled. Modified signal in, modified signal out.

Quote:
Originally Posted by cfg83
Question #3 : Is there slack in the brake such that I can just barely press the brake pedal and trick the PCM into thinking the brakes are actually engaged?
If you had ABS you could pull it off my modifiying the signal coming in from the wheel sensors. But that my screw up any TCS based systems you may have.
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Old 12-20-2006, 12:36 AM   #3
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Red -

Quote:
Originally Posted by Red
From my understanding

...

If you had ABS you could pull it off my modifiying the signal coming in from the wheel sensors. But that my screw up any TCS based systems you may have.
Hmmm. Overall it doesn't sound doable. Officially, my car doesn't have ABS or TCS, but I do have lots of "ABS Fuse" markings and fuses in my fusebox :

http://www.saturnfans.com/photos/sho...0/ppuser/20905

Could this be dormant circuitry or just "normal brakes" fuses? Maybe there is a way to do it, but it sounds more complicated than just splicing a button into a PCM wire. I need a Saturn expert mechanic.

CarloSW2
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Old 12-20-2006, 03:41 AM   #4
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Quote:
Originally Posted by cfg83
Red -



Hmmm. Overall it doesn't sound doable. Officially, my car doesn't have ABS or TCS, but I do have lots of "ABS Fuse" markings and fuses in my fusebox :

http://www.saturnfans.com/photos/sho...0/ppuser/20905

Could this be dormant circuitry or just "normal brakes" fuses? Maybe there is a way to do it, but it sounds more complicated than just splicing a button into a PCM wire. I need a Saturn expert mechanic.

CarloSW2
Carlos,
I seem to have those dormant fuses also. I have fuses and, I think, a relay in spots marked ABS but do not have ABS. As for tricking the PCM, in my car I can touch the brake pedal and make the brake light come on without the brakes being applied.
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Old 12-20-2006, 08:52 AM   #5
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Tricking the PCM/ECU into Fuel Cutoff?

diamondlarry -

Quote:
Originally Posted by diamondlarry
Carlos,
I seem to have those dormant fuses also. I have fuses and, I think, a relay in spots marked ABS but do not have ABS. As for tricking the PCM, in my car I can touch the brake pedal and make the brake light come on without the brakes being applied.
Thanks for the info. I should have titled the thread for fuel cutoff, not lean burn. I know I can do the fuel injector cutoff mod like you have, but my traffic conditions are dicey for bump start restarts, so I am trying to figure out ways I can cutoff the fuel with the car in gear.

With your injector cutoff, if you stay in gear, and you release the injector cutoff, does the fuel "just start up" again as if nothing had happened?

Thanks,

CarloSW2
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Old 12-20-2006, 11:25 AM   #6
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Quote:
Originally Posted by cfg83
diamondlarry -



Thanks for the info. I should have titled the thread for fuel cutoff, not lean burn. I know I can do the fuel injector cutoff mod like you have, but my traffic conditions are dicey for bump start restarts, so I am trying to figure out ways I can cutoff the fuel with the car in gear.

With your injector cutoff, if you stay in gear, and you release the injector cutoff, does the fuel "just start up" again as if nothing had happened?

Thanks,

CarloSW2
I'm not real sure. I do know that my car sometimes won't re-start(by bump-start) after using the injector kill switch until I turn the key to accessory and back to run first. The really annoying thing is that I never know when it will decide to do this.
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Old 12-20-2006, 02:06 PM   #7
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Since your dealing with Saturn's, I cannot directly say whether the following things are true. However, I'm driving an 89 Honda Wagovan and I have done some things and made some observations which may be helpful, in your circumstance.

On my 89, I currently have a vacume gauge and I have a small DVM, which is wired up to the Oxygen sensor. The following behavior is what I have observed, between the two.
-When the slowing down, in gear, if the engine rpm is above 1250, the fuel is cutoff, entirely, to the intake manifold. The Oxygen sensor will rapidly go to very nearly 0 volt's, as soon as I remove my foot, completely, from the gas pedal.
-When idling the car goes into closed loop mode, as quickly as the oxygen sensor heats up and begins working. I replace the one wire Honda sensor with a 4 wire sensor. With this sensor, from a cold start it takes about a block for the oxygen sensor to begin working.
-When accelerating, the ECU will stay in closed loop mode, until the vacume is at about 5 inches, or less. It then goes into open loop mode, where the fuel values are pulled from the fuel table's. This is observable, via the DVM.
-In closed loop mode, the voltage on the sensor continuously fluctuates from approximately 0.85 volts through 0.5, to 0.25 and then back again. Each cycle takes approximately 3/4 second or so.
-On my Honda, I put in a cutoff switch for the injector supply voltage. It serves to shut fuel flow off to the engine. Unfortunately, the ECU is smart enough to recognize that the engine isn't running, normally and so it throws a fault condition and I have to turn the ignition switch off and back on, to reset the ECU, to get the engine to run.

I don't know how similar a Saturn ECU is, but I have to believe the ECU is at least as smart as my 89 Honda. Consequently, I doubt that your going to have any success at fooling the ECU into letting you shut off the fuel flow and I suspect the ECU is very likely already doing that, anyway.

Anyway, FWIW, Good Luck
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Old 12-20-2006, 03:07 PM   #8
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Quote:
Originally Posted by Gary Palmer
I don't know how similar a Saturn ECU is, but I have to believe the ECU is at least as smart as my 89 Honda. Consequently, I doubt that your going to have any success at fooling the ECU into letting you shut off the fuel flow and I suspect the ECU is very likely already doing that, anyway.

Anyway, FWIW, Good Luck
My Saturn only seems to be partially as smart as the Honda. It will usually re-start by popping the clutch but, every once in awhile, it won't and I have to turn the key off then back on. I haven't found a pattern to when it does or doesn't do this.
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Old 12-20-2006, 04:40 PM   #9
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Quote:
Originally Posted by cfg83
Could this be dormant circuitry or just "normal brakes" fuses? Maybe there is a way to do it, but it sounds more complicated than just splicing a button into a PCM wire. I need a Saturn expert mechanic.
Dormant circuitry usually. But that depends whether or not your Saturn uses a common wiring harness. Else you could be lacking the circuitry as well.

An alternative that I have been trying to figure out how to implement without damaging anything is to fool the ECU into thinking that it fired the injector. I am not sure how the circuit works for a Saturn's FI, but on my Jeep all the ECU does is supply a ground to the injector for X amount of time. If some kind of bypass circuit can be put in place between the injector and the ground, to intercept the ground, you'd get the same result as a fuel cutoff
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Old 12-20-2006, 07:46 PM   #10
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On my car there is a separate switch on the throttle body that tells the ecu when the throttle is fully closed. The switch is used only for coasting fuel cut.

I'd imagine other cars are using something similar. The fuel cut couldn't be dependant on the vehicle slowing down, otherwise you'd stall everytime you went uphill.

What about stepping on the brake and gas at the same time? Does it cut fuel on your car?
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