Hello,

I've spent several months trying to tweak my throttle body system for more power and performance. I've finally gotten it to run half way right, but it has the bad trait of almost bucking at light throttle positions. This typically happens slightly above idle in parking lots or in slow moving traffic. It doesn't do it under load. This is annoying and will chirp the tires sometimes.

It almost feels like the ignition is cutting out and then the engine catches back up. My combo is such a mixed mess of parts, that I'm considering starting over... I want to blame the throttle body, but I can't specifically give a reason why.

Edelbrock heads
Holley ProJection intake
USED Holley TBI with GM 454 injectors & pod
New IAC, Delco O2, MAP, module, cap, rotor, wires...

 

Sounds like you VE tables still need some work. You might also make sure that your 02 is cross counting properly.

MAKE SURE YOUR BPWC IS SET CORRECTLY or it will surge in closed loop.

 

I'm still running the factory 120 BPC even though I've set the FPR at ~14psi. Funny thing is, I have nearly maxed out my VE in certain areas...I suppose that signifies something is wrong but I'm not so sure.

 

in my case i had terrible bucking. down right dangerous. it turns out adding fuel was helpful. lean. both in VE as well as tps-AE try tuning for a 122-124 BLM and add to AE and see what happens. my BLMs were 128-131 when i added fuel. i played around with spark in those rpm/map cells but that was no help.

 

Same thing, running open or closed loop,trailing throttle into a corner(in town) I think the ve table was close, lots of logging and correction,, I did get some relief by cutting back in the MAP vs AFR table in the Idle areas, 30 and 40 .. I think the issue is getting the IAC to pickup the idle before it drops way low,, (of course the sob won't drop under other conditions) There is a lot that I do not understand about the progression thru the fueling routine especially exit of DFCO and entry into idle. Also of mystery is when the open loop AFR vs temp. is modified by the MAP vs AFR. What are the qualifiers? There is also conflict between the ECU Mine (7747) and XDF 7747 on the MAP vs AFR, as the values are reversed to each other,, so far I'm too dumb to figure out which is correct for certain, I suspect the XDF version is correct. Have read Rauscher's dissertation on the IAC,and his revisit. I certainly applaud him and his efforts, but am still somewhat foggy on the throttle followers operation,, all of the definitions list as % of throttle follower, but related to what? You might also look at the mv. offset for idle. Hope some of this helps as I know it's a real greasy pig .I run a 4wd with a lock rite, the surge is hard enough to lock the rear end in a corner and with all the driveline slop it whips hard enough I think I'll have to get out and chase down the driveshaft. Thanks and good luck

 

The throttle follower (TF) percentage is based on the TPS%. As the TPS% increases the TF steps increase. There are two multipliers for this: one in park/neutral, the other when not.

The open loop AFR is the sum of the AFR vs. coolant and the AFR vs. vacuum (barometric - MAP). This is just taking the value from each table and adding them together, then divide by ten (move the decimal point). So the value of 139 is 13.9:1 AFR.

The surging may be caused by the ECM switching between sync and async mode. Try locking out async fueling by setting the min/max sync-async thresholds to 0.

These:
RBob.

 

Thank you very much for your reply, is that Barometric or MAP ( like as in the same, or present barometric minus engine MAP)? IS the AFR vs vac always active on the open loop AFR vs coolant when running open loop? Or are there qualifers.. SO is % throttle follower percentage of max allowed T.F. steps vs %of present TPS to allow for a value of more than 100%? The ECM (7747)is from an astro van with automatic now running a 350 with manual trans(originally run by an 8747,used the original harness) the p/n offset is set to 0,, are there other settings that must be changed to make the switch from auto to man. happy? As far as I can tell there is no neutral sw on the trans., I think only reverse for the back up lights, the bu light sw is on the top of the trans, cozy to the floor and tight to the probing hand( set the body before the loom "wheres all them snakes go?".). Will a lack of this circut cause a slow drop to idle if dfco was not entered? Thank you so much for your higher knowledge,, in my case it's a feeling of higher trees and more rope . Uh. does async effect acceleration enrich or PE ?

 

AFR vs vac is always used in calculating the desired open loop AFR. It's just part of the equation.

I believe AE is always async (unless we're talking about the EBL), even if you disable it by shifting constants around.
As far as PE, since async operation is for really small PWs, it would never have been used in PE anyway, since PE is going to be WOT and larger PWs.

 

vac = present barometric minus engine MAP

As for park/neutral, the input wire should be left open. This is 'in drive' all of the time (pin B10).

The TF starts once the TPS% is greater then the bias term. The TF steps then follow the TPS% until max allowed TF or max allowed IAC is reached. As for the slow return to idle, look at the decay and the delay terms. These can be used to speed up the reduction of TF steps. Doing this will also increase HC emissions on a throttle lift.

On some vehicles it is nice to reduce the TF% gain term(s). Too much can make the engine jumpy.

RBob.

 

Thank you very much for your responses,and sharing your knowledge . Would too early throttle follower action also contribute to a stalling problem (pulling away from a stoplight) when running a holly TB with fair radical cam.?

 

Why would this be the case? Isn't a high BPW the same as having VE too high across the board? Wouldn't the BLM's pull it in?