MAP sensor keeps adjusting my timing
03-04-2012, 04:28 PM
#1
MAP sensor keeps adjusting my timing So I just finished my s10 LS swap, it runs good but the problem I'm having is that the MAP sensor keeps advancing and retarding my timin so at idle is goes from 400 rpms to 1500 rpms and will do consistently do that. It's a 5.3 been bored to a 5.7, tv2 cam, ls6 heads milled about 8 thousandths, with a FAST 102 intake and 92mm FAST TB. HPT said it was irratic. Anyone else experience this, and how did you fix it? Thanks for any info.
03-04-2012, 07:27 PM
#2
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Timing is going to respond to MAP (cylinder air mass
follows intake pressure) but I think you've got the
order reversed. MAP is just doing its job and reporting.
If you're just overfueled at idle, then the IAC will be
following the idle RPM loop and that loop easily can be
unstable, motor responds too slow to step-air and the
IAC swings wide, too late, then overcorrects the other
way. Check this by shutting down adaptive spark and
locking the IAC counts (scan tool, bidirectional controls).
If it gets stable, then you either work on the idle-range
airflow fidelity, the idle RPM PID coefficients or both.
Good luck with the latter, although bring up the derivative
values I think helped me some.
Seen a few people reporting problems with the bigger TBs
and having to scale IAC area and so on. I had a similar
sounding surge problem from my ported TB where I had
epoxied the IAC port and drilled according to 'net advice,
but the 3/16" hole was just too small and any counts over
about 150 were falling away from the airflow-vs-IAC-counts
curve. This will make the idle RPM control loop "wind up"
and again you get unstable. But a locked IAC would sit
still RPM-wise.
Now if things are still not stable with air and spark locked,
that leaves the fueling. I've seen that there is a "fuel
surge" that I can make come & go by altering the VE table
slope in the MAP axis - this is going to determine whether
you are going to have an underfueled, overfueled or straight
situation as MAP increases. You may have heard of "lean
surge" in carbed cars, same kind of deal. Hard to hit the
load-points you need to, to get that right - not like you
have a lot of ways to put load on the motor at idle. A/C
on and off is about it. I use locked converter highway
cruise and a varying pedal to catch AFR at iso-RPM,
varying load, for tweaking the MAP-axis profile.
Hopefully your cylinder volume is up-to-date, what with
all the crazy bored-out combo action that's in the picture.
So break it down, see which of the various possibilities
is/are in play and tighten it up.
follows intake pressure) but I think you've got the
order reversed. MAP is just doing its job and reporting.
If you're just overfueled at idle, then the IAC will be
following the idle RPM loop and that loop easily can be
unstable, motor responds too slow to step-air and the
IAC swings wide, too late, then overcorrects the other
way. Check this by shutting down adaptive spark and
locking the IAC counts (scan tool, bidirectional controls).
If it gets stable, then you either work on the idle-range
airflow fidelity, the idle RPM PID coefficients or both.
Good luck with the latter, although bring up the derivative
values I think helped me some.
Seen a few people reporting problems with the bigger TBs
and having to scale IAC area and so on. I had a similar
sounding surge problem from my ported TB where I had
epoxied the IAC port and drilled according to 'net advice,
but the 3/16" hole was just too small and any counts over
about 150 were falling away from the airflow-vs-IAC-counts
curve. This will make the idle RPM control loop "wind up"
and again you get unstable. But a locked IAC would sit
still RPM-wise.
Now if things are still not stable with air and spark locked,
that leaves the fueling. I've seen that there is a "fuel
surge" that I can make come & go by altering the VE table
slope in the MAP axis - this is going to determine whether
you are going to have an underfueled, overfueled or straight
situation as MAP increases. You may have heard of "lean
surge" in carbed cars, same kind of deal. Hard to hit the
load-points you need to, to get that right - not like you
have a lot of ways to put load on the motor at idle. A/C
on and off is about it. I use locked converter highway
cruise and a varying pedal to catch AFR at iso-RPM,
varying load, for tweaking the MAP-axis profile.
Hopefully your cylinder volume is up-to-date, what with
all the crazy bored-out combo action that's in the picture.
So break it down, see which of the various possibilities
is/are in play and tighten it up.
