I need some help with tuning my idle and light throttle cruise.
I have an 89 vette running the modified ARAP bin on the $6E mask. I have been tuning with TunerPro for a while and got my BLM's very close to 128 at idle and light throttle. I thought I was good until I installed my LM-1 wide band and saw that my a/f ratio hovers around 17:1 at idle and light throttle cruise. At first I didn't believe the LM-1 suspecting some o2 intrusion into the header but then I realized that it is probably a true reading as my headers get very very hot at idle. Hot enough to crack my plastic power brake booster over a few months time!

My question is : How do you change what the ECM thinks is stoich? Can the O2 sensor be calibrated? I realize some O2 sensors may put out 450mv at 14.7 AFR and some may put out 550mv at 14.7 AFR. How would one correct for O2 sensor differences?

I have changed the following:
1. Adjusted the rich/lean threshhold voltages vs. airflow(raised them all the way to 700mv)
2. O2 error reduction gain values vs. airflow (raised all to 1.00 and also lowered the idle down to 0.65).
3. Also reduced the rich lean offset value from 16 to 13.

None of these changes reduced the AFR on the LM-1 wideband under 17.0 AFR.
Changing the injector seeded to lower the AFR slightly from 17.0 to bounce between 16.4 & 17.0 AFR.
Its very lean at idle. I did notice that when I made the changes the BLM numbers moved away from 128 but the value on the LM-1 stayed around 17.1 AFR.


It appears to me that the 128 BLM that the ECM thinks is Stoich 14.7 is actually approximately 17.0 AFR.
How do I fix this????
Please Help!
John

 

Sounds a little funky.
Have you watched the EGO voltage in the datalog? It should swing back and forth in closed loop from below 300 mV to above 600 mV at about once per second or slower.

Do you have both sensors in the same bank?

Does it read 17:1 at cruise?

Are you reading WB from the tailpipe - that'll always read lean at low flow without special care with some adapter pipe.

Have you set the base timing to whatever the ARAP bin says (retarded ignition will make EGT SOAR at idle).

Good luck
Jeremy

 

Yes it is very strange. The timing is set at 6* btdc. The LM-1 wideband is installed in the opposite bank. I have true duals with headers. Yes it cruises around 16.5 -17.0 also.
The o2volts seems to sway back and forth constantly so its toggling between rich and lean frequently.
I have a 3 wire heated o2 sensor that is from a BMW. I wonder if I should try a different part number? It is fairly new but I know that doesnt mean much. I think I've had this problem for a while.

Has anyone else ever experienced blm's at 128 but the afr wasn't 14.7?

John

 

Another thing to keep in mind is you are monitoring 2 different banks of the motor with your true dual exhaust and sensor placement. It might be worth the effort to swap the 2 sensors between the two banks of the engine and see if you still get the same readings. If not, it could be that you've got a misfire or injector problem on one bank or the other.....

 

There's a thread on open loop driving

https://www.thirdgen.org/techbb2/sho...hreadid=250949

where it was mentioned a narrowband O2 sensor was giving "false" readings relative to the wideband. It might be interesting to review to see if anything there is similar to what you're experiencing. However, I believe the problem described there had to do with the NB O2 sensor reading differently at different MAP/RPM levels.

I'd agree with vernw that it would be interesting to swap the sensors around to see if they still read the same. While I'm not very familiar with the LM1, I'd also try to see if it might be reading incorrectly. If there's a way to calibrate or "bench check" the sensor, I'd give that a try. Without a third method of determining AFR, you don't know which sensor is *really* lying to you. (ASSuming there isn't a bank-to-bank variance for some reason. Clogged injector, failing injector, etc...)

 

I believe the wideband over the NB. Like I said I was initially tuning for 128 blm which is a fictitious value that the ECM thinks is 14.7 or what ever value is programed as stoich. It does this all on the values that the O2 sensor puts out. If these are off then the 128 is off. And that's what I'm seeing 128BLM is 17:1 AFR.
I believe its lean at idle because the headers and exhaust are very hot and you can smell them also.
Its not a different bank issue the engine isn't skipping and has plenty of wot power which by the way I can tune to anything I want from 10.5 to 13.0 or leaner. Its presently tuned for 12.5-13.0 afr at wot depending on the rpm.

I am surprised than know one else has come across this.

Thanks for the replies!
John

 

I have a WB and NB within several inches of each other, heated NB.
And at times, the BLs are 128s, and ~14.7, at times they're high 13s.
I trust the WB.

I've even quit using the Bosch NBs as paperweights. They're junk, IMO. It was documented years ago at DIY-EFI that they admitted having a problem.

 

Is there any way ??? ???
Wondering in my happy place...

 

Thanks for the link to open loop driving. I think it hits right on with what I'm experiencing.

John

 

Thanks for the info Grumpy! My O2 sensor is a bosch. So is there a nb thats more dependable than the bosch? I dont mean accurate. I have the wide band for that.

Please Let me know of a better heated wb. I have the bosch 3 wire presently.

 

Delco AFS-74 3 wire heated O2 sensor.

 

Thats part of the code I havn't dived into.

Isn't the functionality based on cross counts, with a high and low mv threshold? I'd imagine you'd change that threshold.

-- Joe

 

That's a good thought.
I'll try changing the values if I have time this weekend and see if the range shifts a bit.
Thanks