ECU Air Fuel Ratio?

[0xfded]

Sort of a technical question, but perhaps someone knows or recalls seeing the answer:

On an unmodified 5VZ, is the ECU trying to keep the AF ratio around 14.7, or do we have a different value? And generally speaking, if you've ever spent time staring at the readout from a wideband sensor, how close to it does it usually manage?

A more optimistic question would be if there's any way to ask the ECU what the AF ratio is via OBDII, but from what I understand, wideband sensors are only stock on 99's and later, so my 98 is SOL.

Appreciate any leads.

 

[98OzarksRunner]

Don't know specifically on a Toyota, but with narrow-band O2 sensors, which simply read rich or lean, typically the ECU 'toggles' the A/F around the desired A/F, usually a couple of times a second. For example, it might read 13.9 and jump to 15.4 and back every second or so. The actual A/F will be about the average of the numbers.

The target A/F will also vary under load. Under acceleration it will be richer, maybe as rich as 12:1, and while cruising on the highway it will lean out a bit to save gas, running, say, 15.5:1 or more. Under deceleration it might go to 60:1 since the ECU cuts off almost all fuel under those conditions. But usually under normal driving or idle, it will be near 14.7:1, but with narrow-band O2s it will 'toggle' between slightly rich and slightly lean.

 

[JayRolla]

Like stated the motor is going to shoot for stoich (14.7:1) but variables like air density, temp and altitude will affect the afr so the ecu is constantly trying to hit stoich using the fuel trims to adjust when in closed loop.

When you hit the throttle past a certain percentage and rpm the ecu goes into open loop and it eliminates the o2 completly causing the ecu to switch to the wide open throttle fuel map giving the 12.1-12.5 afr so the motor is safe under load.

I dont think any 4runner has a stock wbo2. If you want to tune you need a piggy back or new ecu like haltech, megasquirt, aem, ect...

 

[Ed_C]

I dont think any 4runner has a stock wbo2. If you want to tune you need a piggy back or new ecu like haltech, megasquirt, aem, ect...

Toyota A/F sensors operate similarly to wideband sensors http://www.autoshop101.com/forms/h37.pdf

 

[0xfded]

Thanks for the info guys. It's not for tuning anything yet (no engine parts are on the line), just getting better input values into a realtime MPG estimate for a DIY dash module I'm building. The numbers I was seeing on a test run last night were on the high side, and I figure it's because I'm using a constant 14.7 ratio, which doesn't reflect the actual average (especially in stop-and-go situations.)

I'll have a read through that autoshop PDF. If I can read the sensors and get AF ratio information from them, that'd be ideal.

 

[TheDurk]

Thanks for the info guys. It's not for tuning anything yet (no engine parts are on the line), just getting better input values into a realtime MPG estimate for a DIY dash module I'm building. The numbers I was seeing on a test run last night were on the high side, and I figure it's because I'm using a constant 14.7 ratio, which doesn't reflect the actual average (especially in stop-and-go situations.)

I'll have a read through that autoshop PDF. If I can read the sensors and get AF ratio information from them, that'd be ideal.

Just be aware that your '98 does NOT have the A/F sensor, just a narrow-band o2 sensor. The A/F's are only on '99 and '00 models with Cali. emissions, and everything from 2001 and later. Also, you can't upgrade the sensor without upgrading to the 2-cat system and a new ECU, and you will confuse the hell out of the State's computer at inspection time--probably a non-starter.

 

[0xfded]

Just be aware that your '98 does NOT have the A/F sensor, just a narrow-band o2 sensor. The A/F's are only on '99 and '00 models with Cali. emissions, and everything from 2001 and later. Also, you can't upgrade the sensor without upgrading to the 2-cat system and a new ECU, and you will confuse the hell out of the State's computer at inspection time--probably a non-starter.

Yeah, I'm not going to bother trying to work in an AF for this. I'll see what I can get the narrowband sensors to tell me, and perhaps use that (or engine load) to bias the AF ratio when I'm calculating instant MPG.

Worst case, I stick with the constant and enjoy the lies it tells me.

 

[Robb235]

I have a '99 with Cali emissions and Air/Fuel sensor. How do I know what the AFR is? My scanner gives me voltage value for the A/F sensor, but it's a voltage value that I'm not sure how to equate to AFR.

Going off memory, I want to say that the reported voltage value for the front sensor is typically something like 0.6XX volts or something.

Not sure how to figure out the AFR from this?