Idle surge on my 402 LS2
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Idle surge on my 402 LS2 I've got a 11:1 402 LS2 with a 248/254 cam and a '98 LS1 PCM running it. Its got a th350 behind it with a 5000 stall.
A friend of mine did the initial tune and it is OL SD. I have tweaked the tune the past few years when changing stalls, gears, etc and I do my own tuning to the timing and secondary VE tables at the track (nitrous car).
The car has always had an issue with idle and it is driving me nuts. It will surge pretty bad at times and I'm not sure why. I've noticed that while scanning it the IAC will be swinging alot trying to control it but the weird thing is that if I unplug the IAC it will idle perfect.
Is there an issue with the tune? The IAC itself? The IAC circuitry in the PCM?
Thanks in advance for any help!
-Jeremy
A friend of mine did the initial tune and it is OL SD. I have tweaked the tune the past few years when changing stalls, gears, etc and I do my own tuning to the timing and secondary VE tables at the track (nitrous car).
The car has always had an issue with idle and it is driving me nuts. It will surge pretty bad at times and I'm not sure why. I've noticed that while scanning it the IAC will be swinging alot trying to control it but the weird thing is that if I unplug the IAC it will idle perfect.
Is there an issue with the tune? The IAC itself? The IAC circuitry in the PCM?
Thanks in advance for any help!
-Jeremy
I'd begin with bumping the 400RPM VE column lower to
lean out the idle. With that kind of cam even if you
tuned it by the numbers, indicated-stoich would be
rich in fact. And it's hard to get data in places you
can't hold idle.
IAC activity is because the loop is unstable, likely
due to being too rich (soggy).
lean out the idle. With that kind of cam even if you
tuned it by the numbers, indicated-stoich would be
rich in fact. And it's hard to get data in places you
can't hold idle.
IAC activity is because the loop is unstable, likely
due to being too rich (soggy).
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From: St.Louis, Missouri
Yeah, I just did some cold start datalogging and noticed a few things.
Base idle was at 850 from 133*F on up... I changed that to 1000rpms. That seemed to help a little bit but still surged pretty badly and randomly.
I then started thinking about something I read on here earlier while searching and someone was talking about basically having the proper base idle, timing, VE, etc and then tweaking a few things.
Well, I decided to take a look at my seconday VE table (all it runs off of in OL SD) in the idle area and noticed a large jump from the 800rpm to 1200rpm columns in the 60-80kPa MAP areas. I changed those and made them alot closer because I figured my idle VE tune was basically crap in that area and the surge was because the PCM was jumping between those values trying to "find" 1000rpm set idle and with the large difference in numbers it was surging. That seemed to help a bit but it was very lean on my wideband (18-19) so I added some fuel in those VE idle areas and the AFR got better but it is still surging. Not sure what to think.
I have a few log files I could post if someone wants to look at them.
*If I command 18-20* of timing and command 5-10 steps on the IAC it idles rock solid at about 1000.
Base idle was at 850 from 133*F on up... I changed that to 1000rpms. That seemed to help a little bit but still surged pretty badly and randomly.
I then started thinking about something I read on here earlier while searching and someone was talking about basically having the proper base idle, timing, VE, etc and then tweaking a few things.
Well, I decided to take a look at my seconday VE table (all it runs off of in OL SD) in the idle area and noticed a large jump from the 800rpm to 1200rpm columns in the 60-80kPa MAP areas. I changed those and made them alot closer because I figured my idle VE tune was basically crap in that area and the surge was because the PCM was jumping between those values trying to "find" 1000rpm set idle and with the large difference in numbers it was surging. That seemed to help a bit but it was very lean on my wideband (18-19) so I added some fuel in those VE idle areas and the AFR got better but it is still surging. Not sure what to think.
I have a few log files I could post if someone wants to look at them.
*If I command 18-20* of timing and command 5-10 steps on the IAC it idles rock solid at about 1000.
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From: St.Louis, Missouri
I'd begin with bumping the 400RPM VE column lower to
lean out the idle. With that kind of cam even if you
tuned it by the numbers, indicated-stoich would be
rich in fact. And it's hard to get data in places you
can't hold idle.
IAC activity is because the loop is unstable, likely
due to being too rich (soggy).
lean out the idle. With that kind of cam even if you
tuned it by the numbers, indicated-stoich would be
rich in fact. And it's hard to get data in places you
can't hold idle.
IAC activity is because the loop is unstable, likely
due to being too rich (soggy).
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You might need to touch 800 too since you idle high.
But the idea is, if you swing into it, and it's bogus
rich, then the PCM has to add air (more than it thinks,
so IAC-speed) to keep the thing lit and then it comes
out of it wound up and overshoots, goes too far and
cuts IAC ....
I idle below 800RPM so I am always interpolating
between the 400 and 800 columns. If your idle dips
below 800 then you'll see the influence of the 400
column though probably not as much as I do.
Tightening up fuel makes the motor respond faster
than the IAC will step through the range. That keeps
you out of instability. The other option is to tune the
idle RPM P,I,D params for how slow the motor wants
to be, but I had no luck with that.
But the idea is, if you swing into it, and it's bogus
rich, then the PCM has to add air (more than it thinks,
so IAC-speed) to keep the thing lit and then it comes
out of it wound up and overshoots, goes too far and
cuts IAC ....
I idle below 800RPM so I am always interpolating
between the 400 and 800 columns. If your idle dips
below 800 then you'll see the influence of the 400
column though probably not as much as I do.
Tightening up fuel makes the motor respond faster
than the IAC will step through the range. That keeps
you out of instability. The other option is to tune the
idle RPM P,I,D params for how slow the motor wants
to be, but I had no luck with that.
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From: St.Louis, Missouri
Awesome, thanks for the info... that makes sense.
Just to make sure... the 400 column controls 0-400, the 800 column controls 401-800, etc... right?
Also, I know it probably doesn't mean anything but what kind of AFR should I be targetting at idle? I'm assuming alot leaner than 14.7 due to the big camshaft and alot of mixture being "bled" off at idle.
Just to make sure... the 400 column controls 0-400, the 800 column controls 401-800, etc... right?
Also, I know it probably doesn't mean anything but what kind of AFR should I be targetting at idle? I'm assuming alot leaner than 14.7 due to the big camshaft and alot of mixture being "bled" off at idle.
Not exactly; you interpolate based on RPM, between the
columns on either side of you. So (say) at 700RPM you
would be 1/4(VE(400)+(3/4)*VE(800) and at 900RPM
you would be 3/4*VE(800)+1/4*VE(1200).
About all I can suggest for mixture is to play around with
the real time controls in the scan tool and find what
mixture gives you the minimum idle MAP at 800RPM and
1200RPM. Have to tweak the IAC to keep on point as
you go. The mixture may not be strictly stoich but it will
be "what makes Mama happy". When you find those
points record the average NBO2 voltage they make
and what's on the wideband, and use those for figuring
a correction factor. It'll not play well with the "believe
the histogram" approach (paste WB error) but you can
use the old eyeball technique and hand fit the table
just fine, so you get the same result in natural running.
columns on either side of you. So (say) at 700RPM you
would be 1/4(VE(400)+(3/4)*VE(800) and at 900RPM
you would be 3/4*VE(800)+1/4*VE(1200).
About all I can suggest for mixture is to play around with
the real time controls in the scan tool and find what
mixture gives you the minimum idle MAP at 800RPM and
1200RPM. Have to tweak the IAC to keep on point as
you go. The mixture may not be strictly stoich but it will
be "what makes Mama happy". When you find those
points record the average NBO2 voltage they make
and what's on the wideband, and use those for figuring
a correction factor. It'll not play well with the "believe
the histogram" approach (paste WB error) but you can
use the old eyeball technique and hand fit the table
just fine, so you get the same result in natural running.
I would really like to see the idle surge logs, please post up.
Or shoot to my email dale.h2omotor@gmail.com
I have a larger cam going into my next project & would very appreciate an opportunity to help tame it a bit!
Or shoot to my email dale.h2omotor@gmail.com
I have a larger cam going into my next project & would very appreciate an opportunity to help tame it a bit!
Does it surge sitting still? Or coming back to idle (return to idle)? Reason I ask is the TH350 may have something to do with it. Is there a speed sensor input to the PCM?
If it idles stable with the IAC unplugged, may need to tweak the airflow PIDs some. Spark trimming too.
Send me the tune and a log of it surging. edcmat-l1@msn.com
If it idles stable with the IAC unplugged, may need to tweak the airflow PIDs some. Spark trimming too.
Send me the tune and a log of it surging. edcmat-l1@msn.com
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Does it surge sitting still? Or coming back to idle (return to idle)? Reason I ask is the TH350 may have something to do with it. Is there a speed sensor input to the PCM?
If it idles stable with the IAC unplugged, may need to tweak the airflow PIDs some. Spark trimming too.
Send me the tune and a log of it surging. edcmat-l1@msn.com
If it idles stable with the IAC unplugged, may need to tweak the airflow PIDs some. Spark trimming too.
Send me the tune and a log of it surging. edcmat-l1@msn.com
What are the airflow PID's?
I worked on the tune a bit last night... basically lowering and smoothing the VE table in the idle area. I also changed base timing in the idle area and eliminated the over/underspeed timing tables. I need to flash that tune in and see what happens now. I know I shouldn't change multiple variables at once, but I did so on purpose.
Thread Starter
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Here is the tune and log file. This was after I changed target idle speed to 1000rpm. You might notice a few times in the log when I commanded a certain amount of timing and/or a certain amount of IAC counts. I was doing that to see how it would react.
Yikes!
So after re reading I'm assuming as Ed mention there is a speed sensor reporting & you are not actually driving? Thru me for a bit!
That would cause timing table changes & more.
The motor responded to the changes you were trying, that is a good thing.
I've not worked with a cable TB, however if you unplug & it's better I would think that is a big clue. I'm not sure which way it reacts unplug as in richer or leaner, whichever way is the way.
Then during the re read I see the WB is reporting very lean & the commanded is around 13.0????
Were is the WB in the HPT scanner?
The idle looks better for sure with the over/under speed timing table enabled, other than it flat lined to much.
Anyways I'm no help....with this...more experience is needed. To bad I would really like hands on to try!
My guess is that this would be sorted quickly, just need a pro!
By chance are able to still operate CL or is that gone, Same question reagarding the MAF
So after re reading I'm assuming as Ed mention there is a speed sensor reporting & you are not actually driving? Thru me for a bit!
That would cause timing table changes & more.The motor responded to the changes you were trying, that is a good thing.
I've not worked with a cable TB, however if you unplug & it's better I would think that is a big clue. I'm not sure which way it reacts unplug as in richer or leaner, whichever way is the way.
Then during the re read I see the WB is reporting very lean & the commanded is around 13.0????
Were is the WB in the HPT scanner?
The idle looks better for sure with the over/under speed timing table enabled, other than it flat lined to much.
Anyways I'm no help....with this...more experience is needed. To bad I would really like hands on to try!
My guess is that this would be sorted quickly, just need a pro!
By chance are able to still operate CL or is that gone, Same question reagarding the MAF
