Getting ready for a long road trip this weekend and wanted to get best possible gas mileage, so I decided to try out open loop cruise on my way home from work this evening. Seems to hit the target AFR pretty well during very steady cruise and at idle, but even mild acceleration seems to make the afrs shoot down to low 14's to high 13's. Not sure open loop is going to net better gas mileage than closed loop at 14.7:1, esp for around town driving.

 

doubt it.

 

I'm not sure why this doesn't work for everyone. I regularly get 23-25 average mpg depending on how much I stay off the throttle. It is true that the car will quickly add fuel if you accelerate but that shouldn't happen most of the time while "cruising". Of course even brief WOT runs will eat fuel, I find it actually helps to be in traffic as I'm forced to just move along with the herd.

 

here is another experience, with tps and load zeroed out.

I see an unusual thing, under moderate load and steady throttle the afrs will go to 14 or so, if I let off the throttle a little and then get back to the same load/tps as before, now the afr will ride where I have it tuned.

 

Same issue I was having months back.

I did a couple long trips with open loop cruise on and off (300miles +)

I found I got about 1-2mpg better.

Wasn't worth the trouble in my book.

 

I've setup a evo9 to do this.

I was having a rich problem above 50-60 load so I scaled the MAF and that seemed to fix the problem.

Spent a few hours on the road with the car and had no problems after that.

 

If the MAf see's an onset of airflow it will quickly overfuel because it's supposed to work in more aggressive throttle input.

Maybe pegging the accel enrichment to 1.98 in the area will delay it enough to take the edge off

 

I found the same thing. Open loop cruise was definately not worth the worry or trouble. However, changing the crossover point via wideband to somewhere around 15.7 picked me up a solid 5mpg and I can still use closed loop. That is worth it for me, expecially since I have the EGR blocked off.
Without an EGRV I still get nearly 25mpg w/ 350whp.

 

mrfred, open loop is making quite a few assumptions. The stock ecu can't account very well for differing air humidity which effects oxygen content. What is the specific gravity of your last tank of fuel? Also how perfect is your air seal after the airflow sensor? I have yet to meet an evo (or DSM for that matter) without a leaking throttle body shaft seal. Open loop cruising will always be something that needs monitored.

 

I'm sure the TB shaft seal leaks a bit. Fuel at the time of the observation was E85. Trims were spot on during closed loop driving. At any rate, this is good motivation to finish up my WB -> NB signal conversion soon.

 

I got 27mpg on the highway by just changing the narrowband switchpoint with a zt2.

 

So I started messing around with the open loop cruise stuff again a couple months ago.

Last year, or whenever we all first started messing around with this I wasn't too impressed by what was happening, mostly what mrfred mentioned in here.

I started tweaking some more stuff and I think I finally got it running how I want it.

The *biggest* thing that helped with it running how I wanted was tweaking the Open Loop Throttle high and low load tables.

Stock they look like:

3.00781
3.00781
3.00781
3.00781
3.00781
3.00781
3.00781
2.5
1.79688
1.81641
1.81641
1.77734
1.73828
1.73828
1.69922

It's 3.01 until 3500. I figured this is probably TPS volts. At 4,000 it drops to 2.50v and at 4,250 it is 1.80.

The main problem I had before was that any small throttle movements off "cruise" throttle would drop the leaner AFR to be stoic or even richer. This cause me alot of frustration and is why I shelved the idea moons ago.

Well, this time I raised the 4000 to 4250 rpm volts a bit higher, and walla it fixed the issue. I know I don't cruise at 4000, but I am sure some interpolation happens at 3500 - 4000. Also I of course I only saw this at higher then legal cruising speeds.

I also made both Open Loop Load tables the same, around 35% from 2500 to 3500, 40% at 4000, and smoothing it out to stock values after that.

I am still getting everything really tight but so far I am much more happier on how it is working than before.



and AFR close up:

 

razorlab, I am always impressed by your steady afr line.


however, on another line of open loop use: with all tps and load zeroed out I would get a jump in afr during cruise for no reason for short duration.
then I remembered I had reentered stock accel enrichment values. For whatever reason, accell enrichment does seem to have a consequence on openloop driving.
By lowering some values the afr jump has been eliminated.

I know mrfred and others have determined accel enrichment is not what it says, that it has to do with baro. Could it be responding to pressure in the intake manifold? The small range stock sensor? So accel enrich is in fact a vacuum / load enrichment pump based on intake pressure readings? Perhaps it is intended for extremely small variations in load, or?