Improved fuel economy?

[skylark]

This thread has ALOT of bad information regarding lean cruise on GM applications. I have yet to see an OEM calibration inside the US with the lean cruise enabled from the factory although many ECMs and PCMs have the coding to run it. I have seen Export vehicles such as Holden and GM Mexico calibrations have it enabled. They do not use any form of a switch and also do not require cruise control for it to enable. It is usually MPH and MAP based qualifications. If you are above say 40 mph and below 70 KPA MAP the lean cruise will activate. When it activates it disables the 02 sensor by switching to Open loop, then commands an air/fuel ratio that is leaner while advancing the timing at the same time. If you are tuning your own chips or flashing your own Gen3 PCMs it is actually fairly easy to enable. On my Express van I was able to enable lean cruise in the 0411 that Holden used in Australia. You can set your AFR to whatever you want it at. In the 30-50 kpa range I run 16.5:1, 50 kpa 16:1, 55 kpa 15.5:1, 60 kpa 15:1, 65-80 kpa 14.5:1, and 80-100 kpa 14:1. I set it to work like a power valve and make it richen up at higher loads so that I do not have to exit lean cruise under higher loads. At the lower kpa ranges I add as much as 8* of timing to keep the engine responsive because that is what my setup likes. On flat land at 70 mph, with 5.13 gears and 31.5" tall tires with the 4L80E, I am turning 2,850 rpm, running about 18% throttle and a MAP reading of 45-50 KPA.

What @DerekTheGreat is talking about is where the early non overdrive stick shift trucks had lean cruise. It had nothing to do with cruise control as most of the trucks that I've seen with it were 2500 and 3500 without cruise control. They have the old sm465 with an aluminum shift tower. There is a switch in the tower that lets the ecm know that the tranny is in 4th gear.

That is the information that I know for sure. I would be extremely surprised if they only did it in export trucks. How they did it I honestly couldn't tell you.

 

[DerekTheGreat]

Bingo.
Referred to as "lean cruise" because that's what you're doing, cruising at a steady speed, which the ECM can then lean out the mixture I presume.

I never hooked my scanner into my 454 truck to see what it was doing and just how lean it was running.. It did have cruise control, was an SLE 1 ton, had everything except power locks & windows and tilt wheel. My 350 truck, well, it always shows block learn of 128 but I'm not sure what to make of that. Some forums simply say "128" is the mid point between 0 & 255 which is what the ECM's range is. I have no idea what AFR 128 means though, and if it changes based on conditional things, such as MAP, vehicle speed and throttle positon for example..

Anyway, I'd like to know if the 350 trucks with OD had a similar feature or if they are already tuned to run leaner when cruising and just how much leaner or if they always try to run 14.7:1 or whatever stoich is when cruising. Does anyone know?