Yet again, some thoughts/questions on bucking...
12-08-2005, 08:50 PM
#1
Yet again, some thoughts/questions on bucking... OK, now that I have given the car time to do things on its own and get used to a tune, including myself, I have some new comments towards some of the bucking. Here are the conditions
3rd gear, 1600 rpm sometimes almost severe bucking where I just let out
4th gear, 1600 rpm slight bucking felt
5th gear, 1600 rpm sometimes it will buck but that only when going down hills very light throttle
So I am wondering if the timing needs to be changed in lower g/cyl ranges? It seems the lower the speed the worse the bucking, also if I chug along at about 1200 rpms the car does fine. I was thinking it was fueling related and it was, somewhat. Yes in these ranges the o2s will switch slowly, proportional fuel is zeroed out in these areas as well, BUT at 1200 rpms oscillation is still slow. That makes me wonder now if I need to look elsewhere. My timing values were at about 43-45 in the bucking area. I am going to try to lower them a few degrees and see what happens.
I am just looking for situations it takes your cars to buck (I know some guys on here have been going back and forth in old threads about it recently). Tell me what rpm it bucks at, load, your timing values in this area, and all that good stuff. The more data to compair to the better. I can try what guys do without bucking and I still have it. I guess certain cars are just moody and like odd ****, so the more data we can get to draw a conclusion maybe we can come up with something. Unless someone has an idea that they would like to share
Mike
3rd gear, 1600 rpm sometimes almost severe bucking where I just let out
4th gear, 1600 rpm slight bucking felt
5th gear, 1600 rpm sometimes it will buck but that only when going down hills very light throttle
So I am wondering if the timing needs to be changed in lower g/cyl ranges? It seems the lower the speed the worse the bucking, also if I chug along at about 1200 rpms the car does fine. I was thinking it was fueling related and it was, somewhat. Yes in these ranges the o2s will switch slowly, proportional fuel is zeroed out in these areas as well, BUT at 1200 rpms oscillation is still slow. That makes me wonder now if I need to look elsewhere. My timing values were at about 43-45 in the bucking area. I am going to try to lower them a few degrees and see what happens.
I am just looking for situations it takes your cars to buck (I know some guys on here have been going back and forth in old threads about it recently). Tell me what rpm it bucks at, load, your timing values in this area, and all that good stuff. The more data to compair to the better. I can try what guys do without bucking and I still have it. I guess certain cars are just moody and like odd ****, so the more data we can get to draw a conclusion maybe we can come up with something. Unless someone has an idea that they would like to share
Mike
12-09-2005, 09:18 AM
#2
Have you logged your IAC counts where the bucking occurs? I know that you probably have the effective area table nailed down pretty well, but I've noticed as I continue to dial my IAC table in that the bucking is reduced. I've also noticed that while shifting the IAC table to the right a couple columns was a good start for my FAST 90 set-up, the shape of the curve is not identical to stock so some areas require additional tweaking.
12-09-2005, 11:08 AM
#4
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There has to be something in the three key
elements (air, fuel, spark) that shows an
unstable / oscillating value with time.
One thing to try, is breaking the loop; feedback
loops are what sustain oscillation. If the car
ceases this when you jack the closed loop
enable temp (and mixture is otherwise open-
loop correct) then you may suspect either O2
sensor lag, or the poroportional fuel stuff maybe
having a time constant (perhaps w/ sensor lag)
that destabilizes fueling.
What, in time domain, is bouncing (besides RPM)
in sync with the surge characteristic frequency?
Another possibility is that some RPM or MAP based
table has a ridge, spike or divot that makes for
some PCM schizophrenia when this feature is
traversed. Repetitively falling in, and crawling back
out of a hole can also make a destabilizing lag
process. Just an eyeball inspection of the VE,
spark table, MAF surfaces / lines should show you
whether any uglies are lying about.
elements (air, fuel, spark) that shows an
unstable / oscillating value with time.
One thing to try, is breaking the loop; feedback
loops are what sustain oscillation. If the car
ceases this when you jack the closed loop
enable temp (and mixture is otherwise open-
loop correct) then you may suspect either O2
sensor lag, or the poroportional fuel stuff maybe
having a time constant (perhaps w/ sensor lag)
that destabilizes fueling.
What, in time domain, is bouncing (besides RPM)
in sync with the surge characteristic frequency?
Another possibility is that some RPM or MAP based
table has a ridge, spike or divot that makes for
some PCM schizophrenia when this feature is
traversed. Repetitively falling in, and crawling back
out of a hole can also make a destabilizing lag
process. Just an eyeball inspection of the VE,
spark table, MAF surfaces / lines should show you
whether any uglies are lying about.
12-09-2005, 11:21 AM
#5
only thing that changes with the surging in open and closed loop, is the dynamic airflow value. iac is constant, timing is constant, fueling is not in closed loop but it still surges in open loop in the same range, yet mcuh worse. i am going to recheck the VE with a WB but i have to get my friends efi live to do so since i cant afford EIO right now for hp tuners. last time i checked though the VE was right on...
12-09-2005, 12:14 PM
#7
Seems like the airflow must be constant if timing isn't moving nor IAC.
The IAC value can be constant but still be off. Let's say you observe your IAC count is 59 where it is bucking. It still could be adding too little air even if it is constant at 59 with the result being that you are rich. I'd do the idle test (log IAC and compare dynamic air to idle desired air) and see if your IAC is dead on at 59 counts. Just a thought...
The IAC value can be constant but still be off. Let's say you observe your IAC count is 59 where it is bucking. It still could be adding too little air even if it is constant at 59 with the result being that you are rich. I'd do the idle test (log IAC and compare dynamic air to idle desired air) and see if your IAC is dead on at 59 counts. Just a thought...
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12-09-2005, 01:50 PM
#9
Originally Posted by Ragtop 99
Seems like the airflow must be constant if timing isn't moving nor IAC.
The IAC value can be constant but still be off. Let's say you observe your IAC count is 59 where it is bucking. It still could be adding too little air even if it is constant at 59 with the result being that you are rich. I'd do the idle test (log IAC and compare dynamic air to idle desired air) and see if your IAC is dead on at 59 counts. Just a thought...
The IAC value can be constant but still be off. Let's say you observe your IAC count is 59 where it is bucking. It still could be adding too little air even if it is constant at 59 with the result being that you are rich. I'd do the idle test (log IAC and compare dynamic air to idle desired air) and see if your IAC is dead on at 59 counts. Just a thought...
12-09-2005, 02:09 PM
#10
Originally Posted by jimmyblue
There has to be something in the three key
elements (air, fuel, spark) that shows an
unstable / oscillating value with time.
One thing to try, is breaking the loop; feedback
loops are what sustain oscillation. If the car
ceases this when you jack the closed loop
enable temp (and mixture is otherwise open-
loop correct) then you may suspect either O2
sensor lag, or the poroportional fuel stuff maybe
having a time constant (perhaps w/ sensor lag)
that destabilizes fueling.
What, in time domain, is bouncing (besides RPM)
in sync with the surge characteristic frequency?
Another possibility is that some RPM or MAP based
table has a ridge, spike or divot that makes for
some PCM schizophrenia when this feature is
traversed. Repetitively falling in, and crawling back
out of a hole can also make a destabilizing lag
process. Just an eyeball inspection of the VE,
spark table, MAF surfaces / lines should show you
whether any uglies are lying about.
elements (air, fuel, spark) that shows an
unstable / oscillating value with time.
One thing to try, is breaking the loop; feedback
loops are what sustain oscillation. If the car
ceases this when you jack the closed loop
enable temp (and mixture is otherwise open-
loop correct) then you may suspect either O2
sensor lag, or the poroportional fuel stuff maybe
having a time constant (perhaps w/ sensor lag)
that destabilizes fueling.
What, in time domain, is bouncing (besides RPM)
in sync with the surge characteristic frequency?
Another possibility is that some RPM or MAP based
table has a ridge, spike or divot that makes for
some PCM schizophrenia when this feature is
traversed. Repetitively falling in, and crawling back
out of a hole can also make a destabilizing lag
process. Just an eyeball inspection of the VE,
spark table, MAF surfaces / lines should show you
whether any uglies are lying about.
12-09-2005, 02:18 PM
#11
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Originally Posted by GuitsBoy
Ive been chasing the same issues as WS6FBTA00. We even have the same cam. I tried running open loop the last few days and it hasnt helped the bucking any, so I no longer suspect it to be o2 sensor feedback fueling issues. I almost wonder if one side of the motor has better compression than the other. Perhaps Im grasping at straws, but there are a few of us that just can not get rid of this bucking no matter what we try.
We just got 7 inches of snow so winter is here and I won't be doing any testing till march(ish). 