E38 economy and tweaking AFM

[harascho]

I took the opportunity to take my tahoe on a longer trip. I had to drive to the south of Germany. It was about 200mls, a forth of it through some small towns and country roads, the rest a longer stretch of our autobahn. I set the cruise to 70mls/h and watched the AFM working. It only kicked in when the road went downward (very little was enough to get V4 mode) .As soon as the road was flat I was back to V8 mode. Without the cruise it felt that the slightest movement of the throttle and I was back in V8 mode. I think there is room for some refinements so that the V4 mode is kept longer even with a little throttle applied.
From the exhaust noise and vibration side all was ok. With the V4 mode at higher revs there are small vibrations but nothing big. V4 mode at lower revs changes the things a bit, more vibartions and a strange exhaust tone.

The overall fuel consumption was 13,7mpg which is what I think very high for that very careful right foot.
what could be done to get better milage out of a e38 controlled LMG 5.3?

Is there a way to get a kind of lean cruise mode? Could I tweak the calibration to get more miles out of a gallon?

I've got my laptop and the EFI V2 with me... what logs could be interesting on the way back to get a close look what causes my engine to drink that much?

Harald
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[gmtech16450yz]

Just looked at an '09 GMC 5.3, it looks like GM changed AFM to go in and stay in more below 2k rpms, and come out much easier (sooner) above 2k rpms. I'm betting what that means is they're trying to up the mpg's at 60-65mph light load to get the ratings better, or at least not lower. And maybe they realized it shouldn't be in 4 cyl mode over 2k if there's any amount of load.

What I have found is if you have a super light, steady foot and you have AFM stay in longer it will help mileage below 65mph. Trying to pull a load over 2k in 4cyl mode ends up not helping mileage, and I don't think it's a good idea either durability wise.

I have mine set up to only go into 4 cyl on decel in 3rd gear, and on decel and very slight load below 2k in 4th. Over 2k rpm (roughly over 65mph on my truck) I have it kick out pretty quickly when going from decel to accel. The truck is much nicer to drive on a steady freeway run when it's not doing the 4cyl loosing speed, 8cyl pick back up, 4cyl loosing speed, 8 cyl pick back up constant cycle. I also have AFM only come in on decel below 1500rpm in 3rd and 4th gears to keep it from lugging on accel.

You don't want to disable AFM all together because then you loose your decel fuel cut-off, which will hurt mileage.