With a wideband on a 87 blown coupe running a FMU what kind of air/fuel ratio should i see in closed loop at a steady speed, lets say 55 MPH?
Any ideas?
I think i am having a problem here.
Thanks Greg N

 

depending on your cycle rate of the WB sensor (they can read faster than the computer updates...) you'll bounce around from maybe 14.5 to 15.0 AFR. The ECM is trying to stay at stoich (14.7) under those conditions but it does that by "swinging" back and forth around stoich. It is designed to only recognize rich OR lean and then compensate - so you will bounce around on real-world AFR readings this way. In other words, it will cycle very quickly through a certain range, and this averages out to be your AFR. Interestingly, under WOT the ECM ignores the oxygen sensor input and you'll notice your WB readings become more consistent there (the ECM is not constantly correcting for AFR but relying solely on other pre-programed variables in the ECM).

Also, this is all assuming that your Wideband has an output voltage that mimics the narrowband range (14.0 to 15.0 AFR represented by 0.100mV to 1000mV with a switch point around 450mV representing stoich). If that has not been setup (if you get more than 1000 mV from the WB sensor) then it's a calibration issue and you need to get that fixed ASAP before you burn a hole through a piston...

"stoich" is modifiable in the ECM as well as the target AFR under highway mode, etc... so if someone has tinkered with your programming that may be an issue.

What kinds of problems are you getting?

 

My wideband is installed at the y of the y pipe and i am getting rich readings, from about 13.8-14.2 at warm cruise.

On my year (87) it only has one single wire O2 in the drivers side bank.

The last time i looked at the BLM it was about right on.

I either have a o2 going south, (still swings good) or i have a injector problem with the passenger side bank, i think.

To me it seems like if i had a air leak it would read lean.

Any ideas?

Greg N

 

that actually sounds about normal given your blower and stock O2 sensor - but could be a few things: the NB sensor is not necessarily as accurate as the wideband (esp if it's old) - so it can be off a little skewing the ECMs perception of stoich explaining the difference between the two. Also, the wideband could be artificially reading rich if there is a voltage offset with the sensor grounds (meaning, a voltage gradient through the sensor ground is skewing the volt output of the sensor and thus you're reading it wrong).

Lastly, 13.8 or so is not THAT bad for your described loads - some ECM tweaking could bring it more into line for economy but you want to verify the accuracy of your WB first (if you program the WB to output a constant voltage, you can compare it to what you datalog and see if there is a discrepancy). Changing the OEM O2 sensor may also affect things if your stock sensor is "off" a little (they can change output as they age and become contaminated).

Worst comes to worse, you're obviously not running too lean and I wouldn't tear the car/wiring apart to change it - the only downside is slightly decreased MPG. The blower changes things a little (I do N/A only) so being a tad on the rich side is not a bad thing. I would check the WB accuracy first, then replace the OEM O2, THEN consider tweaking the ECM to lean it slightly out for better economy. I shoot for 15-ish to 1 AFR for highway in my N/A application.