Tricking the PCM/ECU into lean burn?
Hello -
I posted a question on saturnfans.com : https://www.saturnfans.com/forums/showthread.php?t=89959 ... but haven't gotten a response yet, so I am reposting the question here : Bigdaddy94sc2 - Quote:
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 |
From my understanding
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Red -
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https://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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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. |
Tricking the PCM/ECU into Fuel Cutoff?
diamondlarry -
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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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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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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 |
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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