So, after driving dad's C10 to get the a/c working from my previous thread, I noticed a stumble that happens if you're slowly accelerating (and I mean slow like stuck behind grandpa in his 95 buick). I can reliably repeat the stumble. I've datalogged the truck while driving and the BLM histogram seems to be around 124 to 132 consistently except 2200rpm/45kpa that consistently hits 136 no matter how much fuel I add to the VE table there.

The vast majority of the histogram is 126 to 130 in the area where the stumbling happens . But, glancing at the open laptop when it stumbles it appears the stumble happens right along blm boundaries and looking at the datalog seems to agree. I reviewed the log at a spot where I knew it stumbled mid way through 2nd gear and noticed the blm jumped from 129 to 138 just as the cell changed from I think 6 to 7. If I hold the throttle steady slowly accelerating it will stumble and pop in the exhaust a little like an ignition misfire almost but if I give it more throttle to move it on the ve table it smooths out even if it doesn't go into PE and it runs perfect if I drop into PE as well. WBO2 shows afrs appear normal like I'd expect. I put the truck on the old sun machine scope at work and the ignition doesn't appear to have any issues.

The current boundaries seem to be 700, 1200 and 2000 and 25, 50 and 80. I idle at 750 in this truck (manual trans) and It seems when it's stumbling it happens between looking at the laptop either around 70 to 80 kpa with moderate acceleration without hitting PE and the light slow acceleration right along the 50kpa boundary. Would this be something worth changing and if so what should I set the boundaries to? It idles between 700 and 800 rpm and 50 to 60kpa and the majority of the cruise area is 2000 to 2800 rpm at 35 to 55kpa (40 to 65mph)

Its a 7730 running S_AUJP v5 btw.

 

I've noticed this phenomenon before too, when tuning other people's cars with 8D. The BLMs vary wildly and it seems to happen when you cross BLM cells... it'll jump from like 126 to 138 or something. But I've never been able to figure it out. Most often the person doesn't report any odd behavior from the driver seat, so I do my best to smooth out and then call it a day.

I'm interested to hear what the experts say on this one...

One thing to check... there's a switch that has the parameter "Reset INT when BLM cell changes" or something like that (I don't have my laptop with me at the moment). Make sure that's unchecked. Years ago I was curious what would happen if I set that switch and the car developed stumbles and hesitation pretty bad.

Though on the flip side, you might try setting the switch for "Reset INT when AE active". That keeps the ECM from screwing around with the fuel trims while accelerator enrichment fueling is active.

 

Thanks for the response. I already have both of those switches set up like you said. I've noticed it doesnt seem to have the issues when I drive immediately after a cold start while its still in open loop. I'm considering locking the blms to 128 and letting int handle any adjustments to see if that helps. Is that as simple as setting BLM max and min to 128 or should I check something else. I have also thought about lowering the threshold to enter pe. The PE enable, min map is set to 8.1 kpa is that 8.1 direct or do I run the 8.1 through the formula in the notes on tunerpro? If I do that it appears 8.1 = 77kpa roughly and I've got some issues at heavier load 65 to 70 kpa that would probably go away if the PE came on around there.

 

Well I drove again today to work. When I got to work the tunerpro histogram was 124 to 132 everywhere except around 2200 at 45kpa, more on that to follow. The last few times I've run the BLM averages from the datalog through the excel spreadsheet calculator I have its basically swapping the cells from one side of 128 to the other, in other words not really making any changes. So I usually take that to mean the VE table is reasonably tuned. Since the VE seems to be mostly right, I decided to try pulling some timing. I wound up pulling 10% timing across the entire main SA table (so 2 to 3 degrees at idle up to 7 or 8 in the cruise area. Cruise SA dropped from 42ish to 36ish degrees, WOT and idle areas not so much. That actually seems to have smoothed out a lot of issues I've had, such as a surge at heavy half throttle starts from a stop in first gear around 2000 rpms and some bucking after shifting into 2nd (three on the tree in this truck) that I was assuming was all related to the skip/stumble I've been feeling at light throttle accel. Though the "normal acceleration" seems to be much better it still wants to stumble, skip and pop in the exhaust if accelerating very slowly, like in heavy traffic or a slow driver. I'm talking 10 to 15% tps at most just slowly accelerating in each gear and shifting around 1800 to 2000ish. This light acceleration seems to run rough at any vacuum level but most noticeable at 40, 45, and 50 kpa around 2000 to 2200 rpm. It will do it at 60 to 80kpa also around the same rpm as well but not as reliably. This means if you are running roughly 35 to 40 mph and slowly accelerating or pulling a slight hill in 3rd it will skip pop and jump too. When this is happening the WBO2 is jumping up and kissing 16 to 16.5ish afr (I dont have the gauge wired to log as I've never been able to get it to work, I just eyeball it, an innovate mtx-l that shows the digital number and an analog led sweep like a needle). I am now starting to suspect its an ignition misfire after all, even though the scope didn't show anything idling or free revving, and most misfires I've seen seem to happen under much more heavy loads.

With the lowered timing it did rev more smoothly on the drive home when I drove at my normal acceleration pace with no impeding traffic so I'll leave that alone for now and try sneaking up on it again when I get it figured out. I also tried setting the max and min BLM scalars to 128 but it didn't seem to have an effect or doesn't work like I thought on a different test drive. On that drive the blms still changed all around and it still skipped and so on. So i came back and tried changing the spark latency tables as well because I noticed mine didn't match the ones listed for aujp, Im not sure where it came from but it jumped from 214 to 308 between 1600 and 2000 right in the problem area and back to 214 by 3000, but the aujp latency table actually made it skip worse I think. I want to say I found the table I'm running online years ago in a mercruiser forum because the Ramjet 350 is suppose to be running a marine module and ecu but I swapped the ecu to the 7730 so I could tune for the different cam (LT4 hot cam). That may explain the difference there. Either way it runs better with the original table so I reverted back from both of those tune experiments.

The lean reading from the misfire could be why its constantly asking for more fuel in a block from 2000 to 2200 and 40 to 50 kpa. No matter how much fuel I add to the VE table in that block (and I now have a hump there) it still has BLMs between 134 and 137 every drive and datalog. My first thought was the misfire was due to a lean spot but now i think its got a lean spot due to a misfire. The plan is to pull plugs this weekend and inspect them and the cap rotor and wires even though they have less than 10k miles on all of them. Coincidentally the boundary between 4 cells is 50kpa and 2000 rpm according to my tune so that had me all thrown off.

 

BLMs are not accurate. They are late reacting to anything going on. You should rely on the Integrator (INT) to trouble shoot. BLM averages are not accurate either. Check the BLM Max and Min per cell and you'll see great variation. I don't use BLM at all.

My advice is: disable SAM, BLM and Prop Gain, and see what happens. If a change of cell goes from 124 to 138 instantly, no wonder why it stumbles. Disable that BLM crap

Peace

 

This past weekend I pulled the plugs and found 2 fouled I think from when the valve stem seals had gone bad a while back. I installed new plugs, cap and rotor and all the stumbling and such seems to be gone. We've been driving it all week just to verify, so I thought I'd provide some closure on this post.

SbFormula, What exactly is needed to disable the BLM and prop gain? I may investigate doing that later. Right now Dads happy and enjoying his a/c so I probably wont mess with it for a while, but I'd like to have an idea where to go next.

 

Thanks for the update. I use to say, and forgive me for not thinking about it, an ECM can not "untune itself", so if an issue arise, like yours, retuning is not the solution.

Don't mess with it... GREAT idea!

Peace