I'm running $6E code on my MAF car and have finally got my block learn values all close to 128. I burned my first chip that enables Highway Mode and went for a drive.

I watched as the ECM Learn Control was disabled for a while, was enabled for a few seconds (about ten), and then was disabled again. It was definitely going into and out of Highway Mode as predicted.

I also watched the block learn and integrator values. While in HM, the block learn was constant and the integrator was locked at 128. While out, these values moved around as usual.

...which got me to thinking. It has been confidently stated that while in HM the ECM is in open loop, therefore your "tables" must be right on or it won't work right (you're mixture won't be what you're shooting for). I wonder if that is really true. I wonder if since it is a "long term" correction factor, the block learn is used to calculate fuel while in HM. And perhaps the reason it drops out of HM for a few seconds every minute is to update the block learn, giving it the most accurate adjustment available.

Knowing that the "tables" can be off a fair amount, even on a stock car, supports my theory. Highway Mode would be useless if they had to be right on. However, it seems that it is disabled on all stock chips. Some have said that this is because of the EPA. On the other hand, if it really won't work right without perfect tables, perhaps GM realizing that very few cars would have such, disabled it. Maybe it was a nice idea that they couldn't perfect in time for production.

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'87 IROC 5.7l TPI - original owner!

 

EPA considered it a work around to get better CAFE numbers. Most gm codes have blood lines, and so the code just sits there, cause that's the way the original code was.
Also has to go rich ocassionally to keep the converter happy.

 

PJ, glad to see that you are working on this area. I think the reason that you dropped out of HM, then went back in, was because of the timers. There is a ECM constant for "highway mode fuel enable delay"; and also one for "max time in highway fuel mode".
I think the defaults are 10sec for the delay, and 60sec in HM. Just set the delay timer to 0sec, and she should stay in HM. It's more effecient that way. Also, there are several other criteria for HM fuel, like speed, temp and LV8, so don't forget those numbers. I notice on my scans that learn control goes back on, indicating going out of highway mode. Like when I get behind a slowpoke, and my speed goes below 50mph!! Not sure what a safe enable speed really is.

Your observations are valid, IMO. Glenn has spoken (posted) several posts on this subject. I am still amazed at the gas mileage he reports!!!

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Best ET 14.413 @95.57 without
pulling valve covers or manifolds.
Also with stock 2.77 rear end!!!

 

Well, now that it appears that I've closed a deal on a Miniram I suspect my gas mileage will be dropping. Hopefully, I will have my Miniram, heads and cam installed sometime in the Spring. I am still hoping to get better than 25 US MPGs and while capable of hitting 12s. I will let you know come this Spring.

 

I'm aware of these constants and the fact that you're supposed to drop out of HM periodically. I haven't messed with them, yet. I'm not convinced regarding why it drops out. Is it to take care of the cat or to update the BL values? The gist of my theory is that BL is actually used when in HM. Because the O2 sensor is inaccurate away from 14.7, tables have to be relied upon at other A/F ratios. The BL could (and might) be used to adjust the calculations...



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'87 IROC 5.7l TPI - original owner!

 

Glen- your goal should be fairly easily attainable. My brother's 87 GTA has a miniram sitting on top of a 409ci monster with the usual go-fast stuff. 700R4 Auto trans and a 3.27 rear. Just a chip (burned by TPIS) to make everything work reasonably well together, although I'm sure their tuning is not going to be as sharp as what you burn for yourself.

Currently: 22-23 MPGs on the highway, which is about the same as the original (worn out) TPI 350 got in that same car. FOr comparison, my stock 94 Formula only gets about 26 or so and it's got that massive 6th gear double overdrive helping it out.

I don't think there's any reason to assume a loss of mileage due to the miniram at all. The cam change is a different issue entirely.


[This message has been edited by Damon (edited December 04, 2001).]

[This message has been edited by Damon (edited December 04, 2001).]