Cruise AF and VE tables

[Telco]

2000 Chevy Silverado 5.3L auto

After playing with my computer settings, I've been able to limit how rich the truck gets, it now goes no richer than 12-1. But, after a long cruise with the scanner going, I've found that it also goes no leaner than 14.68-1. Is there a setting that will allow me to go leaner in cruise?

Also, in the VE table. What is the effect of a cell getting a larger or smaller number? I've searched all over and not been able to find anything that clearly says "if this number goes up X happens, lower and Y happens." Any help would be appreciated. The work I've done so far has worked wonders for responsiveness and WOT power, but I've not been able to increase mileage by a single drop per gallon. And, I promised my dad I'd get him from 19 hwy to 23 hwy when I return his truck. I'd like to do it with programming but am fully prepared to start slapping in hard parts if necessary .

I will admit that I've not had time to read the whole tuning manual (I did print it out ) so if anyone says that my specific questions can be answered by reading the manuals I'll buckle down on them this weekend.

Thanks for any assistance.

[joecar]

Increasing VE table means the PCM calculates a greater cylinder airmass, which causes the PCM to calculate a greater fuelmass to meet the same specific commanded AFR... so if your engine's actual physical VE has stayed the same, then increasing the VE table causes the actual measured AFR to go richer.

If you run OL:
If you get the VE table dialed in (commaned AFR equals actual AFR), then you can run leaner by making the commanded fuel open loop table (B3605 or B3647) leaner in the cruise regions... (but do a safe amount of richness in the load/wot regions).

[Mr. P.]

I am repeating Ben here from the EFI101 class this summer... he shared that he has a 5.3L EC Silverado and has been playing with lean cruise ideas in his tune; his observation/suggestion was to first use a dyno and tune the vehicle's AFR & timing for max power/efficiency, then after your normal tune was complete revisit the half-dozen or so cells in the VE table that the vehicle operates in during a typical cruise situation and reduce their values (lean it out) until you see a measured 5% drop in HP on the dyno. He said that with his truck it turned out to be about 15.4:1 and if my memory is correct I think he said he gained an honest 2-3 mpg without adversely affecting driveability. But only modify the cells that are low-load, the motor will not tolerate being run that lean if there is any load present (you'll build up a lot of combustion chamber heat).

Mr. P.

[Lextech]

Joecar---I am not understanding something. If we want to run slightly leaner in low load cruise cells--How does B3605-Commanded Fuel Open Loop help us? I am not disagreeing, I just don't understand. My dad has a 99 5.3 Silverado and I have lowered the O2 switch points in the 0,8,16 boxes. He doesn't care about performance at all, only low load cruise fuel mileage.

[joecar]

Joecar---I am not understanding something. If we want to run slightly leaner in low load cruise cells--How does B3605-Commanded Fuel Open Loop help us? I am not disagreeing, I just don't understand. My dad has a 99 5.3 Silverado and I have lowered the O2 switch points in the 0,8,16 boxes. He doesn't care about performance at all, only low load cruise fuel mileage.

You're right, I asssumed OL... ...I don't know what happened, I'm in OL by habit...

my bad, I'll retract/edit the 2nd part of my post (see my edit)... ... I need more Monster/Jolt... :idea:

[Telco]

Outstanding. I'll leave the factory VE alone then, aside from the few cells for cruise. Unfortunately I can't get that much dyno time and don't have a wideband, so I won't tweak far. I've seen the results of a lean engine, and that's something that has been scaring me about doing this. Luckily low mile 5.3s can be had in my area complete for 900 bucks, from salvage. And I'm talking computer, harness, everything needed to put one into a non-computerized rig .

[ScarabEpic22]

Good info, Ill do some logging this week on my TrailBlazer 4.2L and see if i can reduce the VE in cruise areas for more mileage.

[Telco]

Have a question on the fuel/air mix now. There is a field where you set the fuel air mix stoich number, and on my truck it was set to 14.68. I adjusted it to 15.5 at the same time as adjusting the VE tables on the assumption that since my data log didn't go over 14.68 in lean that this might be stopping it. But, after doing this and setting the truck up with a lower VE number in the cruise cells, I'm seeing a little worse mileage.

What affect is there to the computer if the stoich number for the fuel used is adjusted? My assumption was that it would allow it to run leaner, but now I'm thinking that it's assuming that an O2 voltage of .450 is 15.5 instead of 14.68, and is still trying for a 14.68 mix due to other tables in the computer. Does this sound logical, or am I interpreting the results wrong?

[Telco]

Anyone? Anyone? Bueeeeeeeeeler? Heh heh...

Well, if nobody knows I guess I'll find out as I'm going to change ONLY this parameter and see what happens. Last mileage check was 15.5, but I'm not sure if it's due to the program or the extremely cold weather we've been having. I'm not one to let the engine idle, but since it's been so cold I've been very easy on the engine until the temp needle deflects, and the danged thing doesn't get warmed up until I'm almost to work so I'm afraid that the last 2 weeks it's been in warmup mode more than it's been in closed loop operation.

[5.7ute]

Changing B3601 to 15.5 tells the PCM that the AFR is 15.5 to 1 at an EQ of 1.0. This will stuff up all your fuelling calculations & make the engine run richer not leaner.