Hello, I'm having a bit of idle ugliness in closed loop, and it appears that the the ECM pulls back on PW bit by bit, trying to hit stoich, and then all of a sudden there's not enough commanded PW for the injectors to open.

I think part of the root of this problem is having the universal 61lb 369uS injector offset still in the table. I'd like to put a more "correct" number in there. I've been told to just "fudge" it up, and I've been told it's in the neighborhood of 650uSec.

I finally tried to use my oscilloscope to find at least the electrical delay part of the offset and I'm confused about what I found. "regular" saturated, MPFI injectors, show a little "bump" of back EMF as the current flows in that occurs when the spool actually moves and opposes the magnetic field. I'm seeing no such thing with these TBI injectors.

1) Does anybody else have "known good" 454 injector values at 13 PSI?
2) What am I missing here?

Here's data from my truck. I loaded it against the converter to get the injector PWs this long.


The red trace is current. I set it up to be 1A/box. Voltage across the injector is in yellow, at 10V/box. I see everything I expect to see: ECM 'turns on' injector, Current ramps up to 4A Peak, Hold mode kicks in at 1A, I see the flyback voltage when the injector is turned off, I just don't see the little "bump" that tells me the injector has actually "moved". I see a little bit of a knee of sorts where the current data intersects the voltage data, but that's like 1.2msec!

Here's more like what I was expecting to see (shamelessly stolen off EFILive forum member joecar):



Interesting but unrelated, I wouldn't have thought that it takes 1.6msec just to get to the "hold" mode- interesting.

 

The knee in your TBI injector trace is when the injector is fully open. That is it took 1.2 msec to open all of the way. The injector started to flow fuel well before that point.

What I did was to use a variable voltage power supply to power an injector. Then use a pulse generator to activate the injector. I did this on the vehicle so that I could also vary the fuel pressure.

Then slowly increase the pulse width until the injector started to dribble. That basically became the offset value. Ran the voltage up and down as the EBL has a table of offset values based on vehicle voltage.

The other side of this is the PW multiplier table. This is also based on vehicle voltage and 'makes up' for the injector getting sluggish, or slower, as the voltage drops. And vice-verse of course.

RBob.

 

I do not know how this applies to EBL. However, I spent a lot of time investigating this with the 8746 ecm. I came to the conclusion that the 'Injector Bias' is NOT simply the turn on delay, but instead is the diference between the turn on and turn off delay.
I am using GM#17084304*RPD (80 lb/hr) injectors at approx 16 psi. For this I found the injector bias to be about 425usec.

 

Thanks for your reply! I didn't realize that number actually represented the difference between the black and red measurements above- I thought it was just the delay between "injector circuit closed" and "fuel begins flowing".

I have the same engine, trans, and rear gears as you- in a half-ton GMC, though.
Meanwhile, in my basement...

 

Setup is MOSFET driving the injector off a battery in the bottom of the cart. Signal provided by PWM signal generator on laptop, connected to old home stereo amp to get enough voltage to make the FET gate happy. Here's a quick trace I snagged off a junk carbon cannister purge solenoid I used for testing during setup of this rig.


Gonna go try it on my 454 injectors against real fuel pressure.

 

That is the conclusion that I came to. I had always seen everyone trying to use the turn on delay, however that never seemed to work out. It always left me with severe problems when going from sync to async mode.

 

I get a much better TBI idle eliminating async.

I only use async on the top end.

The offset that works for me is high enough to make for double takes and head scratching.

 

Tried that with my setup, however the pw becomes too narrow and the injectors become very non-linear and eventally start to drop out.

Offset at top end is NOT critical because the percentage of error it can cause verse pw is very small.

 

Don't you also run pretty high pressures, relative to stock?

If I understand this right, TBI injectors take a lot longer to open against higher pressure, which would increase the offset.

 

Just thinking outloud?

Yup, and the offset eats up time,..the reason I use async on top.