Good morning all. I've gotten the tune pretty well figured in the car at this point in time. Now I am just fine tuning the drivability stuff. Started playing with lean cruise a couple months ago before it was really honed in and it just didn't play well. Now it seems okay in some areas but off in others. Basically what I am finding in my logs is that the non lean cruise AFR is running about what it is commanded at cruise, near stoich. But when lean cruise comes in it goes fat. Right now I have it near 16:1 and it drops off to 13:1 when it comes in. This is in the 50 mph range. At speeds of 70-80 mph it seems to run about right, bouncing from 15.8 to 16.4 or so. The problem is with cruise locked out in those areas the AFR is about right... So I'm not sure if I should just stop using it in the range I am (does seem to help tip in at steady state at higher speeds) or if I am missing something.

My understanding is that it reads BLM, goes in, stays in and then comes out to re-read, then goes back in if conditions allow. It just doesn't seem to actually change anything. I took a ~10 mile drive out 195 and ran at 50 mph in the middle for about 3 miles and it re-entered 3 or 4 times and each time it was in it was about 1 AFR richer than when it was out. At 70 mph it seemed to iterate closer and closer to the 16:1 with each re-entry. Which is what is confusing me. The other issue though is that may not have been the case as temperature and humidity dropped off a lot on that part of the drive.

I guess what I am wondering is if it is just working normal and the tune needs more work, or if it just can't compensate that broad a range, or if I am missing something else. Thanks in advance for any help or insight guys.

 

If I am reading your post correctly, it sounds like lean cruise is working as it should. The main reason for dropping out and re-learning the BLM is that the engine will stabilize with different fuel trims.

Be sure that the BLM is stable among various throttle positions and loads. Then just let the lean cruise work.

RBob.

 

No that part I get and I agree appears to be working, BLM stays right around 128 plus or minus 3 or so I would say. But according to the wide band on the lower end of speeds it never reaches it's commanded 16:1... It hangs around 13:1 when in lean cruise unless I am doing 65-70+.

There are some back roads I can hang around at 50-55 mph for miles without stopping and the wide band reports in lean cruise an AFR of say 13:1 even though it is commanding 16:1. Doesn't seem to matter how many times it drops out and back in, it never seems to satisfy and the BLM stays about the same... If that makes sense. The highway speeds are perfectly fine though.

 

What is the NB O2 showing at this time? That is the sensor that the ECM is using for the fuel trims.

RBob.

 

500-600 mV range at lower speeds, 40-100 mV in the higher speed range. In both circumstances it is relatively stable voltage. I meant to data log yesterday as I added a little more timing (for highway mode) when I went for a ride thinking it may help a little once in but my device froze and never saved the log.

 

> 500-600 mV range at lower speeds,

Definitely on the rich side.

> 40-100 mV in the higher speed range.

Correct for when in lean cruise.

I would tackle this by looking at the log (in the time of interest), checking that the ECM is in both closed loop and BLM learn is active.

If so, then checking that the INT is dropping. And does the BLM follow it. If the INT isn't dropping then check the O2 window values in the calibration at the airflow that is being reported.

RBob.

 

So I tested it out today. First I didn't make any changes, other than the fact it is like 78 and low humidity compared to the last time I drove it. Logs show it looking a little better. So before I went and drastically changed anything I lowered the in time for lean cruise and shortened the lock out time too. So now it stays in for ~50 seconds, comes out for 10 then back in. This seemed to do the trick. It iterates closer each time it does it. BLM only shifts a couple numbers from 128 but at steady state I doubt it needs much more.

I'm wondering if even though it is trying to work properly, the longer in times were letting the O2 sensor cool enough that when it went back out it wouldn't quite read correct. The NB in the car has been through the wringer and has given me issues in the past with closed loop. I've had it just randomly start adding fuel everywhere making it pig rich and then just stop. I have a brand new GM AFS74 3 wire in the box ready to go so it may be wise for me to swap that in. Especially after the engine hydro locked with fuel a few weeks ago, that made a mess and I still don't really understand what happened. But for now, it appears to be okay. I wish I had the datalog from last outing to see if it was working properly at that time.