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Old 05-08-2008, 11:14 PM   #5 (permalink)
Master EcoModder
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Pushrod - '02 Chevrolet Cavalier
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JohnnyGrey- i would like to learn how to lean out my gas engines when I am not loading them heavy. On a '91F250 302 for example the ECU is not reprogrammable is it? I did a brute force lean burn by put a 0 to 10 ohms reostat in series with the injector pwr. And as I dial in more resistance the voltage to the injectors decreases, the ECU tries to compensate but evenually the 02 voltage starts dropping and I can get it down to 0.24 volts and the engine will still run. This is not a wide band 02. comparing your wide band to a stock sensor, if you had to quess how lean do you think I am. i am going to try it with a different mileage computer as the one I am using reads injector pulse width which of course gets wider as the ECU tries to compensate for low voltage.
Modulating the injectors in this fashion probably won't give you consistant results. They're "on/off" devices, so by supplying low voltages, the cylinder to cylinder spray pattern and volume could vary considerably. As long as your car is in closed loop mode, there's no point in attempting mods like this since the ECU will compensate as far as it will go, then throw a CEL fit.

Your narrowband sensor is a poor indicator of AFR. Treat it like a digital signal, where >0.45v is rich and <.45v is lean. What actual voltage you get will vary with each sensor, exhaust temperature, ECU bias voltage etc, so there's no real way to know. Plus, if the voltage stayed at .24 for long, that's probably the voltage floor of the signal. It indicates a lean mixture, but there's no way to say how lean you were running.

A wideband lets you do so much more. Mine is made by Bosch and the driving unit is made by AEM. It puts out 0-5v which represents linearly ratios from 11:0.1 to 18:0.1. The driving unit also has a narrowband emulation mode that does switch back and forth over that .45v point at 14:7.1. This is fine if you want to run at that ratio, but we don't.

So we need a box that puts out either 0v or 1v depending on whether or not that 0-5v wideband signal has crossed a certain threshold. If the switch point is set at 4v, the motor will run quite lean (around 16:1), but the ECU will believe it's running at a steady 14.7:1 and will not throw a CEL. Also, it will be training itself around that ratio, and will hold it steadier than you could in open loop mode.

With a narrowband sensor, you're working blindfolded. The hot rod guys prefer to tune with widebands, but in a pinch, they can tune with a narrowband. Their only concern is running rich so nothing blows up, so as long as the O2 reads high and the engine sounds happy, everything is fine. For economy though, you need tight control over the AFR.

As to whether or not the ECU in your truck is programmable, I would say not for closed loop operation, and we're not interested in the open loop tables, they're for WOT operation. Also, I doubt Ford would implement a lean burn system since it is not emissions legal. I have no idea how Honda got away with it in the HX/VX.

It is technically possible for an ECU to run a lean burn on an engine equipped only with a narrowband sensor. In fact, that's exactly what GM does on cars it sells outside the US. That narrowband sensor is only good for switching around 14.7:1, so the ECU establishes fuel tables for that mixture. Once you get on the highway, the ECU goes into lean burn mode, which is actually open loop. Since the ECU already has a well-established fuel table that produces mixtures very close to 14.7:1, it can scale that table to produce whatever fuel mixture it needs. For example, 92% to run at 16:1. Because the narrowband sensor will read consistantly lean, the ECU occasionally wanders back to 14.7:1 to confirm that its tables are correct, then goes back into lean burn mode. On US ECUs, the mode is disabled.

Last edited by JohnnyGrey; 05-08-2008 at 11:27 PM..
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