WhtFast01

iTrader: (1)

Ok so here is my problem, my maf does not seem to be working properly but i think its a problem with the wiring harness. When I have the maf hooked up at the track at time it will go nuts a cut 40% of my fuel but when we run it in SD it does not have any issues. So I changed my Maf thinking that would take car of the problem and same result at the track. Oh anf one other thing I am now getting O2 codes aswell with new O2s 33's and 53's on both O2's.

HumpinSS

iTrader: (1)

To add to his post both the maf and the ve table was tuned so there isnt a tuning issue there. The maf shows no sign of being bad on the dyno but at the track the car will run consitant 12.01 with it hooked up (it studders and breaks up when plugged in) and then run 11.6 when it is unpluged.

jimmyblue

iTrader: (11)

Log the MAF frequency across a run where this
is going on; do you see it drop out or "chatter"?
Note that the scanner output is decimating the
data and you only see a fraction of the real stream
in time, so you may miss 7/8 of spikey events. But
MAF frequency should be perfectly continuous at
higher airflows. Similarly look at the MAF airflow
and the dynamic airflow values for anything like a
dropout or going lower when they should be climbing
monotonically with RPM.

MAF frequency can be corrupted by EMI from other
underhood wires' current pulses; injectors and the
ignition coils/wires in particular, but any other full-
time-busy signals can impinge on the MAF signal and
the PCM sees the composite. If they beat together
such that the MAF crossings are pushed around or
missed, the frequency will be misread by the PCM.
If you see a frequency error that comes and goes
across narrow bands of RPM this may be the deal.
GM even notes this in the fat green book as one
MAF problems diagnostic to check.

Anyway, any discovered inconsistency of behavior
in the frequency or the airflows can point you to
the culprit (it could be that, for example, the
cut-over from SD to MAF puts him across a step
in dynamic airflow because the two models, while
individually reasonable, disagree some about that
point). Find the thing that changes just before
the suck rolls on.

HumpinSS

iTrader: (1)

Yes there is a good bit of chatter on his maf 75mm but so was the 85 mm mafs Ive seen

Welp his wires are VERY OLD!!!!!!!!!!

jimmyblue

iTrader: (11)

I rummaged through my old logs and found one that
had the MAF frequency, airflow and CylAir (which I
turned into Dynamic Airflow by calculation).

A couple of interesting things to note. One, my car
has some noise in the frequency value (this is a rising-
RPM pull so airflow is in fact increasing monotonically).
This could be electrical noise or it could be that high
rates of change expose a marginally-stable internal
MAF sensor loop, challenge its settling time (which
might have been tweaked for response time and not
stability).

Two, a small (%-wise) change in frequency makes a
big change in computed airflow owing to the exponential
airflow/frequency characteristic of the instrument.

Three, the Dynamic Airflow does a pretty good job of
smoothing it all out, but in there I also see the value
has a slight down-step right about 4500RPM (again,
RPM running up cleanly). If this were more pronounced,
if the Dynamic Airflow were getting confused worse, a
worse transient underfueling would certainly result.
Breaking high would be bad for performance as well.
You tune for "normal" but have to live with these
"freaks" if they show up.

Interesting how far apart the MAF number and the
DynAir are, in the lower RPM. This was while I was
working on truing up the VE table. Plotting the three
airflows together might be a good way to see who's
driving and who's off base.