stumble at 1200rpm

[cmysyfly]

i have a stock headed syclone with 55's. with speed pro it seems to be stumbling on light throttle starts. no matter how i richen/lean it out on the ve table, adjust iac....etc i cant get rid of it . i have gone through the ignition and its still there. where should inj pulse width be at? it also wants to start up and die but if i feather the throttle a few times it will go into fast idle and run fine. the stumble almost reminds me of a bad accelerator pump on a carb. PLEASE HELP!!!! IM GOING BALD!!!

 

[bobc455]

First of all, you shouldn't be tuning using the VE tables. The VE table is a calibration table, not a tuning tool. Once the VE table is set, leave it alone unless your target A/F does not equal your actual A/F. Do your tuning with your A/F ratio table.

Off my soap box.

It kinda sounds like your AE enrichment is not set high enough- if it feel like the accelerator pump in your carb doesn't work, that's what AE enrichment does.

-Bob Cunningham
bobc@gnttype.org

 

[cmysyfly]

hey bob the stumble is about 90% gone now using the ae enrichment. at 70 mph o2 corr. at 0. when giving more accelerator slowly the correction wants to go -11, -15, -25. do i keep tuning using a/f table or is there something else that i should be doing. i did try to tune using a/f table but wasnt making much progress. could this also be ae enrichment related?

 

[bobc455]

I have to plead ignorance on tuning AE enrichment on a turbo motor- I drive an NA car and haven't had to tune an NA motor. On my NA motor, I have AE enrichment based on TPS only- I have my AE enrichement for MAP all zeroed out. On a turbo motor, you need some AE enrichment for MAP because of the turbo slowly building up speed (even thought the TPS doesn't change)- you may have to play with both AE parameters. Hopefully someone else can chime in and talk about tuning the AE / MAP parameters?

It doesn't surprise me that you see a bit of negative O2 correction just after moving the throttle- you are intentionally putting a bit of extra fuel into the motor because of AE enrichement, the O2 will see this and pull some fuel back out. If you let the motor settle back out for a few seconds, your correction should go back towards zero (I'd think). This is one of the reasons that if you are trying for good gas mileage, you should try for as little throttle movement as possible- every time you move the gas pedal down, you're squirting a smidge of extra fuel into the motor.

What I would do is drive around for a while watching your AE enrichment graph- most 2D (two dimensional) graphs will show the actual activity real-time, which will allow you to see where on the graph you're still stumbling. Then you can correct that point accordingly.

I would think your A/F ratio should be around 14.7 or so for just about all of your street driving, idling, etc. (unless you need to run a bit rich to cover up a rough idle or something, which is unusual with the FAST). If you still have a stumble when you move the throttle, then the problem still lies somewhere in the AE enrichment (either TPS or MAP based).

If your correction stays at -25 without moving the throttle, then you need to change the VE number at that location so you get 0 correction.

-Bob Cunningham
bobc@gnttype.org

 

[cmysyfly]

so when i run at wot lets say at the strip, is o2 correction supposed to stay pegged at -25? or again is this something that may have to do with ae map adjustment?

 

[bobc455]

AE only affects engine operation when you change the position of the gas pedal. When the gas pedal doesn't move (at WOT), then that isn't really an AE situation.

If you have O2 correction that high, then you need to change your VE numbers.

The way that the ECU computes how much fuel to put into the motor is based on how much air it thinks is going into the motor (the VE number). In other words, a pseudo-formula would look like this:

X (how much air is going into the motor) times Y (the air:fuel ratio) equals Z (how much fuel to put into the motor).

The ECU controls how much fuel to put into the motor, and it reads the A/F ratio from the A/F ratio table, so the only "unknown" to the ECU is how much air is going into the motor. The only way for the computer to know how much air is going into the motor is by the VE table. This table needs to be accurately calibrated so that it has a very close idea how much air is going in, so that it can put the correct amount of fuel in.

If your VE table is perfectly calibrated, you will see 0% correction under all circumstances. If the VE table is wrong, then the ECU will put in the wrong amount of fuel, and the oxygen sensor will tell the ECU to make an adjustment, but this is only a band-aid for improper VE numbers!

What you have to do is record a run, then play it back overlayed on the VE table. For every square where the O2 is correcting for a too-rich condition, lean it out by reducing the corresponding VE number. For every square where the O2 is correcting for a too-lean condition, richen it by increasing that squares VE number. You'll have to do this several times until your O2 sensor is making hardly any correction.

Once the VE table is set correctly, you shouldn't have to change it again (unless you change a camshaft or something)- your tuning should then come from your A/F ratio table.

Generally if your O2 correction is less than about 6% most of the time, then youre VE table is in pretty good shape. It's impossible to get it to 0% all the time.

AE is only like an accelerator pump, which doesn't really enter into the equation when you are just staying at WOT.

-Bob Cunningham
bobc@gnttype.org

 

[Craig Smith]

All I think I can add to the info Bob has already presented is to set the MAP AE limit to just over the vacuum/boost threshold. Once the ECU starts building boost, you probably don't want AE fuel being added as a result of the natural increase in boost with RPM and engine load. Let the ECU do that on its own - the AE fuel will generally throw you off. MAP AE limit is commonly set to 110-120 for this reason on forced induction engines.

 

[cmysyfly]

thanks guys for the info i will try these adjustments soon. after i get the trans fixed again............