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The Free Mod Series - Part 4: A/F Ratio Checking

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:D There are soo many ways you can have fun with cheap electronics!

Reading and deciphering what your car is doing


So you're running stock, have some upgrades, or maybe even have a low power Turbo, or S/C system and you're still running the stock ECU. How can you possibly tune the engine without hundreds of dollars of expensive 1 time dyno tuning???


A tuned engine is a happy, powerful, and economic engine! The OEM doesn't understand this.

The Vf signal. Our lifeline into what the ECU is doing to the base fuel map!!!

The Vf signal is the ECU's way of letting the world know what it is deciding to do with the fuel mixture. When all the sensors have sensed, the ECU has picked a map to run in, and everything else is said and done, the Vf signal shows you what the ECU has decided to do. (This is your long and short term fuel trims in an OBD-I car)

By connecting any type of volt meter to the ECU/diagnostic port, you can read the votlage of the Vf signal.

0V = Rich mixture 11-20% from normal (ECU is leaning the mixture)

1.25V = Slightly rich mixture 4-10% from normal (ECU is leaning the mixture)

2.5V = Within 3% of the basic map

3.75V = Slightly lean mixture 4-10% from normal (ECU is richening the mixture)

5V = Lean mixture 11-20% from the mixture (ECU is richening the mixture)


I have two wired up. One reading each Vf signal (Vf1, Vf2) One is for the front bank, while the other is for the rear bank. (Obviously I4's will have one Vf signal) If you feel like figuring out what the new voltages will be, you can bridge Vf1 and Vf2 and read the entire thing at the same time, but I wouldn't do it.

The o2 sensor. The only way to see the end result of the ECU's tuning!

The Vf signal shows you what the engine is doing in an attempt at perfect tuning. The o2 sensor shows you the end result o f what *actually* happened. By comparing the o2 voltage to the Vf voltage gives you a fairly accurate representation of what your engine is doing in the current conditions, and where you need to go.

This works with older lambda (narrow band) o2 sensors... You just can't accurately tell the A/F ratio.

If you can find a 1v, 1.5v, or 2v volt meter, you can wire those up to the oxygen sensor's themselves, or the oxygen sensor simulators from the ECU, or the diagnostics port under the hood.

450mV should correspond with 14.7 air/fuel ratio. The ECU will try to achieve this basically any time you are not more than 80% throttle. The smaller the voltage, the leaner, the larger the voltage, the richer.

*note* there is the *real* o2 sensor voltage, and there is the *simulated* o2 voltage delivered to the diagnostic port. The simulated o2 sensor is a stead, averaged signal that will like a tiny bit behind current conditions (.1-.5 second). The o2 sensor, while live, fluctuates quickly (a good sensor will fluctuate at least 8-10 times a second @ 2500rpm, and between 400-550mv at idle/cruise)

(bank 1 sensor 1 is the front bank o2 sensor, bank 2 sensor 1 is the rear bank o2 sensor)

The combination of the three allows you to see what the ECU originally wants to do with the A/F ratio, what the ECU is trying to do with the A/F ratio, and if it is successful in doing so. From that, if you have any idea about tuning you can interpolate fairly well how to do something.

Now! Someone will ask if it matters that you do both sets of signals. Not really, considering only the ECU itself can tune every individual signal. *However* I have all ready found that the signals do not always match each other... Often times during, and immediately after transition periods, the sensors will not match by a setting or two.

CEL's are annoying

You can wire up a switch in your cabin to check, and clear CEL's!!! This is very easy to do since above the driver's left foot is a diagnostic port which contains the second set of E1 and TE1 terminals. Simply splice the two wires together with a switch/hold button, and whenever you want to check/clear a code, simply hit the switch which will connect (ground) TE1 to E1!!! Instant basic diagnostic mode!!!


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No the stock o2 sensors in my car are heated lambda sensors (1 volt). over the years Toyota went to newer sensors, on a 3.3 volt scale, that are... IMHO 95% as accurate as a true 5 volt wideband sensor.

The vF data Toyota outputs is a 5 volt scale. It's our link to what the ECU is actually tuning.

Ever heard people (wrongly at times) say that an ECU can tune for roughly a 20% change from what it's programmed to expect? The vF voltage shows you what the ECU is having to do to th A/F ratio.

You, owning an OBD-II ECU, can simply buy one of the programs for somthing like a Palm Pilot, and not just scan codes, but see all of the ECU's data, just like a tehc can.

*Very* handy information to know if you're doing anything perofrmance related.

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