at light throttle cruising (less than 4% tps) i have a situation where fuel almost seems to be cutting out intermittently. you can feel a slight bucking as fuel cuts out for a fraction of a second then restores to normal and keeps repeating intermittently. the o2 volts drop to around 50mv then come right back after a few milliseconds each time. fuel correction either doesnt have time to respond or isnt attempting to add fuel. i thought it might be something to do with dfco but i set the tps threshold to 1% and it still did it. if you throttle to 4% tps or higher it doesnt do it at all. maf reading appears normal. iac and spark are stable and normal. i have checked for vac leaks over and over and have found none. i am using $6e and arap. decel enlean fields are all set to stock. blms are close to 128. has anyone encountered something like this?

 

What your injector pulsewidths, if they drop to 0.0 it is the DFCO affecting it , if it occurs at the same time.

Also going too lean in overrun areas, like 20-30 or less kpa when a speed and letting off the throttle, is something else that could describe what you are seeing. Try adding a little fuel too overrun areas and see what happens. Timing can also do this as well, but in my experieince, only if there is a farily large difference from overrun to throtle areas.

Try moving fuel first, u sometimes get a tip in stumble when lean in overrun. Also u said your 02 was dropping to 50mv or less. Double check the dfco and make sure its not that first before proceeding.

edit: Another thing to check, real lite cruise cnoditions with an unheated 02 may be on the verge of dropping out and going open loop also, see if u can find any type of correlation between the two. Some headers/combos are more prone to this than others. but it will also do exactly what u describe.

later
Jeremy

 

its not dfco because i have isolated that and it seems to be functioning correctly. inj pw is not dropping when the bucks occur. id say its a lean miss but i am running a $6e and can't really just add fuel there unless i change the maf tables, which i have done already and getting about a 124 blm at cruise. i increased timing in the overrun areas but that hasnt helped. not sure what else to do at this point. its not a huge problem, but i notice it and its annoying. i need to correct it.

 

Too much timing. Try removing the timing in the low LV8 and the specific RPM or range that the bucking is occuring. It's a 383 thing . Let me know how it works out.

 

thanks alot guys for your great suggestions. however, i tried backing the the timing off considerably and it did not remedy the problem. its still occurring in the 45-65 lv8 range at 1500-2200rpms

 

Could be some other form of misfire. My car surged or bucked a little with a burnt wire. Sometimes I wonder if the arcing resets the ECU or does something crazy like that. EGR problem maybe??

Calibration wise, could it be going into single fire mode? How about into a block that's learned poorly? How small are the PWs?

 

i have same issue but different ECU and engine specs. at slow speeds in first gear and in second gear(of 5/manual) at 1% tps accellerating to 5% TPS my WB spikes lean to 25/1 from 12/1(running open loop) or same from 14.5/1 (CL). and bogs. the slower i accellerate the worse it is. i can prolong bog if i wish. i recently pulled out some AE and engine likes it cept for this stumble or buck at clutch engage or crawling in parking lot. if i depress pedal to allow a quicker TPS movement AE minimizes stumble. so if i drive aggessively bog is minimal. at idle at 21 d A and trailing off on SA to higher map. i also tried low SA at idle and 400-1200 rpms(all map) and made it worse. i did pull out VE at all higher map cells beyond 800 rpms/40 map(idle) so that i get more resulting effect from AE. i have 0% tps and 0 map with values of 0? if anything i have more AE in map than tps? maybe i should move more into tps 0/4/8% areas?

also it may be my intake volumes(TB/manifold/head) are large compared to my 350 CID engine size. a small tps% may allow significant enleanment. i am about to disable DFCO as it goes to like 50/1 on prolonged deaccel. exiting x way engine sometimes dies. will load up tunerpro rt and check PW at idle. seems engine liked rich (OL) idle to clutch engage?

19 lbs FP 80 lbs injs.

not datalogged VE tables for a while, will enrichen them in that area.

 

hey, what about increasing spark plug gap? im running an msd coil but the current plug gap is stock at .035, perhaps going to a .045 might help. also have a crane hi-6s thats not currently hooked up. maybe that would help ignite the mixture when the cyl pressures are extremely low... just a thought

 

i am running an aftermarket Accel coil and hei. i think about 50,000 v with new wires and new cap/rotor. using plug specs from edelbrock. suppose wont hurt to open up gap. think it is a fuel/timing issue however.

 

Dom had a bucking issue with his crossfire that sounds very similar to yours. I think he had a lean condition, might try adding some fuel on the MAF tables.

 

Not sure sure if this is relevant with $6E, but I've experienced a similar problem running $58 when fueling was switching between sync and async modes under very light loads.

 

i am looking at my last logs i took from WB. at office so cant attach. BUT they show a normal drive through my somewhat hilly subdivision. show when i open throttle blases and accellerate in first with clutch engaged i go immediately lean to 15.4/from tip in to accelleration. that takes .40 secs until minimal AE comes in and i go to 10/1 with blades then already closing. there is this delay on AE. as i stated before if open loop rich seems a whole lot better. i need to read up on throttle follower as there may be knowledge to be gained there.

point of interest. when i ran OL level road i could see on WB the A/F changing as i hit different VE cells without AE contribution. at steady throttle some cells were as lean as 14.50 and others as rich as 12.50/1. idle cell was about 12.50/1. i just upped FP from 16/1 to 19/1 prior to these logs.

 

egr is disabled. pw are about 1.20 - 1.80 when this is occurring. plugs, wires, cap all new and no misfires at any other rpm. how do i tell if it is going into single fire mode? single fire and double are set to the same constant.

 

can you explain this further? what is switching between asynch and synch and how do you tell if this is happening? alot of good info here btw.

 

Take a look at your AE stuff as well. With the stroker on the 6e, it's pretty tough to get it dialed in there especially when stepping into it lightly. Lots of chance for lean coughs. You can crank up the tables, but also look at the AE constants and make sure you have enough room there.

 

last night i added some AE to TPS at 0(was 0 in table) 3 6 9 and pulled out in MAP 40/60/80 same amount as my net AE. seems to be better this AM . in 2nd gear, low mph, i can stomp in it and car has no hesitation. 1st gear in normal city driving was improved. not sure if it could be made perfect???

OBTW it seems car runs better in mornings(50F). i think the manifold has a wet condition that helps hee A/F somewhat rich. PM driving fully warmed manifold (72F) that stumble is greater? just a TBI/large intake runner thing?

 

actually stepping into the pedal at all stops the stumbling so ae doesnt appear to be a factor. car has near perfect throttle response. it only seems to be happening at a steady throttle at 1-4% tps. on decel, dfco stops it from bucking and on acceleration over 4% tps it stops as well. i have the dfco threshold set at 1% tps.

 

It certainly sounds like a lean misfire condition. The AFR that is border line combustable . Detonation can be a "bucking" and it usually appears at cruising speeds with low throttle openings (high manifold vacuum).
If it's the lean misfire it'll compound itself in closed loop since the o2 signal won't be accurate of all the cylinders. If open loop (normally richer than stoich) doesn't have this bucking problem then it's definatly too lean in the cells or the ones bordering the cruising cells. Good luck.

 

i agree. thats exactly whats happening. so how does one trick the ecu into giving more fuel in those areas when its in closed loop?

 

Datalog the problem and it'll give you the MAF grams/sec (sometimes shown as gps). Go into the MAF tables and increase the value. A modified intake track, descreening, cam, change will require a modified MAF scalor/table modification. If you want to try and just richen up the whole area of that part of the MAF reading use the scalor and see if that is noticably better at part throttle. Test and tune. You can't just shoot from the hip with these kinds of issues.

 

i have hours and hours into tuning my maf tables. when it occured tonight the blm/int were 128/128, just like the majority of my cells. the tune is basically very good, great throttle response, 22mpg hwy, plug cuts show everything is doing well in the cylinders. after taking out some timing it seems a tad better. she was running decently tonight. i had to really pay attention to notice any subtle roughness in the hwy cruise. but...check this out...
at one point, after accelerating from a stop to cruising speed, o2 volts dramatically dropped almost to nothing and stayed there for several seconds. at first there was no noticeable change in the cruising but it was lean enough to throw a check engine light and she gave a nice buck before recovering. then, light went off, everything returned to normal and by the time i checked the code there was no dtc.
so...i am now thinking this is probably two different issues. one may be minor tweaking of the tune, the other more dramatic. but what could it be? cannister purge? i have egr disabled in my bin. i dont know what else would do this.

 

I think running $6E its equivalent is single fire(quasi-async) versus double fire(sync).

What I've experienced with big TBI injectors at low loads, is that the pulse widths get so small in sync mode that the ecu changes to async. So in a low load condition in sync mode, closed loop is sensing a rich condition and the INT & BLM start dropping to compensate. Once the pw threshold for async is met, fueling changes to from sync to async firing. When this happens, the o2 volts plummet and hang real low....you can often feel a bog. Now closed loop senses a lean condition and INT & BLM rise until sync firing threshold is met. As long as the low load condition exists, the cycle repeats.

You mentioned the pulse widths are 1.2 - 1.8ms when this occurs. If your code allows single mode firing, I'd bet this might be happening. Also, if single mode is disabled, then the saturated injectors are not getting a long enough commanded pulse width to operate properly.

In this thread there's some excellent commentary on this by RednGold86Z.

 

interesting. what about this: would decreasing the single fire inj consant slightly keep it from going so lean when in single fire mode? just a thought...

 

nope, that sucked. bad idea. messed up my nice tune. but hey, it was worth a try.

 

this quote reminded me of something i had read a while back somewhere else. on further analysis i'm convinced this is whats happening. there is a very slight missing going on when the inj pw drops below 1.80 and only when that happens. so thanks to RednGold86Z for that info and thanks to 89C2500 for pointing me in the right direction.


Now, i found a major problem which was seperate, but related. DECEL ENLEAN. it was compounding this situation dramatically and aggrivating it so much as to throw a lean O2 code when i decel after a heavy throttle run. i disabled decel enlean and voila, no more code. my decel is apparently already too lean to begin with, i didnt need this feature.

i also cleaned up my maf tables a little bit more to try and get everything close to perfect. car is really running better thanks for your help guys.

 

ok i just eliminated the problem completely. i found the table in the arap hac named INJ OFFSET FOR SMALL PW'S. it allows you to recalibrate your injectors in the pw range below 4ms. i increased the values by 2% in the areas where i was experiencing lean misses to widen the pw below 2ms and sure enough the problem disappeared. i was able to restore highway fuel and spark and there is no bucking whatsoever. i did have to redo some of my maf tables because they were running rich afterwards, but ill tell ya this was the correct fix for this problem.