OK, so now I'm playing with my brand new AutoXray 3000 scanner and graphing software and I've come up with a general question, sort of!
One of the probs I'm trying to solve is the fact that when I drop the throttle on my automatic in D or OD, for instance at 63 mph, the scan data shows the injector pulse doesn't go to zero for 4.5 sec after the TPS is at .52 V (closed throttle). That's an awful long time when there's a cop comming at ya in a 50 zone and you don't want to light up the brakes.
It drops from 4 mS to 1.4 mS in the 0.5 S it takes for theTPS to hit zero and then dribbles to 0.9 mS, 4 sec after TPS hit zero, then drops to zero in the last 0.5 sec. Anyone out there know if this kind of time delay is a normal thing with these TPI systems?
The IAC acts like a dashpot to prevent stalling; that extra air needs fuel.
Most AutoXrays are slow (realitively speaking); so you're not seeing real time data when you log. If you have a driveability issue, post what it is. Otherwise, capturing and saving info when it's good, is good for when it's bad.
If it immediately dropped to zero it would quit.
The IAC acts like a dashpot to prevent stalling; that extra air needs fuel.
Most AutoXrays are slow (realitively speaking); so you're not seeing real time data when you log. If you have a driveability issue, post what it is. Otherwise, capturing and saving info when it's good, is good for when it's bad.
Thanks for the comments. I'm still digesting them. I've a few more things to do in diagnosing my engines' operation before I'd be ready to post the probs and all I've done so far. In the meantime, exactly what "slow" time are you referring to related to the autoxray; time to capture a complete set of data such that the later parameter values read have changed since the initial parameters were read, time between data sets during a capture sequence of multiple sets of data, or other?
what "slow" time are you referring to related to the autoxray; time to capture a complete set of data ........., time between data sets during a capture sequence of multiple sets of data, or other?
Either or.
The X ray is not being updated as fast as the data is being produced which you can see in more detail with a laptop , ALDL cable and datalogging software. Consider what you are seeing a snapshot
Exp
The '85 ECM transmitted diagnostic data at 160 BAUD ; about a 1.25 second interval between the 25 Bytes of data
The 85 - 89 ECM transmits diagnostic data at 8192 BAUD. The data stream of 64 Bytes cycles through every 62.5 milliseconds (16 per second) at this rate.
The injector BPW going to zero is DFCO, or Deceleration Fuel Cut Off.
There is a delay timer and disable/enable LV8 thresholds for DFCO along with throttle, rpm and velocity conditons.
With factory tuning, the LV8 to disable/enable DFCO is rather low (40/33 or 40/30) and the delay time is 1.75 seconds (manual) or 0.25 seconds (auto).
So it takes time for the IAC to reduce the airflow to bring the LV8 down into the DFCO enable range, and another 0.25-1.75 seconds delay time with conditions fulfilled before fuel cut occurs. 4.5 seconds sounds reasonable.
I've gone as low as 300 ms for the delay and raised the LV8 disable/enable thresholds to roughly 80/70, for nearly instantaneous DFCO on throttle release. Tuned in this manner the powertrain wind up/wind down and subsequent engine braking and DFCO fuel cut occur almost simultaneously for a smooth transition from accel to decel.
Feels good for long periods of coast down, but can be annoying in traffic, if I lift the throttle a hair. For increased comfort, I've added some delay back in (approx 1 second), so I get a sort-of bimodal engine braking response.
Last edited by tequilaboy; 11-04-2009 at 01:44 PM.
The injector BPW going to zero is DFCO, or Deceleration Fuel Cut Off.
There is a delay timer and disable/enable LV8 thresholds for DFCO along with throttle, rpm and velocity conditons.
With factory tuning, the LV8 to disable/enable DFCO is rather low (40/33 or 40/30) and the delay time is 1.75 seconds (manual) or 0.25 seconds (auto).
So it takes time for the IAC to reduce the airflow to bring the LV8 down into the DFCO enable range, and another 0.25-1.75 seconds delay time with conditions fulfilled before fuel cut occurs. 4.5 seconds sounds reasonable.
I've gone as low as 300 ms for the delay and raised the LV8 disable/enable thresholds to roughly 80/70, for nearly instantaneous DFCO on throttle release. Tuned in this manner the powertrain wind up/wind down and subsequent engine braking and DFCO fuel cut occur almost simultaneously for a smooth transition from accel to decel.
Feels good for long periods of coast down, but can be annoying in traffic, if I lift the throttle a hair. For increased comfort, I've added some delay back in (approx 1 second), so I get a sort-of bimodal engine braking response.
OoooooK, but I don't pretend to understand all of that. Like, what's LV8? I'm wondering why the IAC is doing anything when the throttle (TPS) is off idle. I think it's time to get a book on this GM system 'cause it sure doesn't sound like it's supposed to act like a carbed engine. Did you ever wonder what the world would be like without acronyms?
Corvette Store
'89 probs-typical scan data
