I've decided I should write a post about this, because it might be useful to somebody, and we all know how annoying this problem is.
There are lots of reasons a Fiero can get a hunting idle, so this isn't the answer for everybody. But it may be of particular interest to people using Multec injectors from a 3.4L engine with the Fiero ECM.

My car is an 86GT auto with a stock (I assume) 2.8L engine and the stock 7170 ECM (1985 cars have a different ECM, but this info should still apply).

Since buying the car, it has had a hunting idle when warm. When I datalogged, I could also see the INT value behaving strangely at idle. I went over some mechanical explanations but had no luck. No vacuum leak has ever been found. Tried grounds, IAC.
I replaced the injectors with refurbished multecs, and in the process found that the old injectors were also multecs (but a different part number). So I've never had the original pintle injectors.
It continued to have a hunting idle with the refurbs. In the ECM, I adjusted some various things like the injector base constant, which it needed, but none of it addressed the hunting.

==
------------------ Background Theory
There's an issue with the way injectors behave at short pulse widths. When the ECM calculates how long to open the injectors, it assumes that the volume delivered through an injector will be a linear function of how long it's open. If it wants 10% less fuel, it reduces the pulse width by 10%. The problem is that when the pulses get short, an injector loses linearity. This confuses the ECM.
In order to compensate for this, there is a table in the 7170 ECM called "Low Pulse Width Injector Offset vs BPW" (as named by TunerPro). What this does is for a given BPW (pulse width), it adds a certain number of uSec to the pulse. This correction is meant to make the injector's flow appear linear.

Another issue is voltage. The higher the voltage driving an injector, the faster it will open, so the ECM needs to take that into account. There is another table for this. TunerPro calls it "Injector PW Correction vs Battery Voltage".

Every injector needs different values in these tables in order for the ECM to control them accurately. The stock Fiero chip is calibrated for the stock injectors.
------------------------------

I started trying to edit these tables using values I copied from other cars that had Multecs from factory. I hoped they'd be "close enough" but it didn't work.
I was too dumb to record the part number on my injectors before I installed them. After they were installed, the part I could see looked like 7609826 OR 928607
After some web searching, I decided they're probably 17109826. (Somebody please tell me if there's another possibility that I missed.)

As best I can determine, these belong to a 1993-1995 FBody 3.4L, among other cars. The FBody app is the most relevant because the engine is very similar to ours, and it's ECM uses speed density, just like our ECM does, not a MAF sensor. Unfortunately I couldn't find any data on this ECM.
A few weeks ago, RobertISaar came to the rescue with a post on gearhead-efi
http://www.gearhead-efi.com...M-Information-P66-V6

I looked at the factory tune for a 1994 FBody 3.4L auto. I copied it's values for the Low Pulse Width Injector Offset vs BPW.
This table seems to have done the trick. After over 2 years, the annoying hot idle hunting is gone, and the INT values while idling are stable. It's been working for 2 weeks so far.

At first I also copied the FBody values for the Voltage offset. However, the FBody ECM tune refers to it as "fuel pump voltage" and those settings were driving my idle BLMs very high. I found another car with these injectors whose ECM refers to it as "battery voltage", like our cars, and it's voltage offsets were a bit higher. Those values put my BLMs closer to where they should be, and the terminology is consistent with 7170 terminology, so I'm using them.

Bottom line, I'm using the Low Pulse Width Injector Offset vs BPW from a 1994 FBody 3.4, and the Injector PW Correction vs Battery Voltage from a 1995 A-Body 3100.

The tables don't transfer perfectly. The values have limited resolution so there's some rounding. The Battery Voltage table doesn't have entries for the same voltages between the A-Body and the Fiero, so I interpolated values that fit the Fiero's table entries.
I'm sure this setup isn't perfect. I just copied the injector offset tables from other cars that made sense to copy. There were other changes but those didn't relate to the idle hunting.

The key point I want to make is, if you have multecs and a hot idle hunt, look at the "Low Pulse Width Injector Offset vs BPW" table. Find values that match your injectors. If you want it to idle, this table is important. For the 3.4L injectors, I highly suggest copying from the FBody.

Thank you!
AFAIK, you're the first person to come up with a fix for this. Good show!

The "common knowledge" was that it was the ECM that was not compatible. Not the tune.
You've just shot that one in the head.

Can you share the values that worked for you?
Not everyone has all of those masks available to reverse engineer.

Edit - I forgot you're running a 2.8. Those figures may not be applicable to a 3.4. But still...

[This message has been edited by Raydar (edited 06-18-2013).]

I fixed a hunting idle on my 3.4L swap back in 2010 via the Fuel Pump Voltage vs PW table using $8F in my 7730.

"The surging can have a few sources. One is a bad ground. The ECM has, as part of it's code, an adjustment to the time the injectors are open due to voltage to the fuel pump."

This from 10/04/2010 : https://www.fiero.nl/forum/A...120111-2-111927.html

I thought I answered more than a few hunting idle threads with my fix. Guess I didn't say it well enough.

Great write up though. Especially the "Low Pulse Width Injector Offset vs BPW" table find. Plus to you.

quote Originally posted by Hudini: I fixed a hunting idle on my 3.4L swap back in 2010 via the Fuel Pump Voltage vs PW table using $8F in my 7730. ... I thought I answered more than a few hunting idle threads with my fix. Guess I didn't say it well enough.

Your fix appears to have gotten lost in the background noise, or perhaps lost in the translation. There was a lot going on in that thread.

In any event, it doesn't minimize your success. Thanks!

I'm using the $A1 with Mustang injectors. If you/anyone could publish your #'s so I can compare to mine, that would be much appreciated.

This isn't any kind of perfect setup, even for my own car, and I don't want to present it that way for anyone else's. But for what it's worth, below are the settings I'm using. Again, this is with 17109826 injectors from a 3.4L FBody engine.

This table is the Injector Offset vs Low BPW table for an FBody 3.4L
I checked these 4 BINs and they all had the same values in this table:
16203261 94 F auto federal
16203281 94 F auto federal
16217091 94-95 F manual national
16240891 94 F manual
This table is much longer than the Fiero ECM, giving it more detail. However, most values just repeat each other.

code:

---

TABLE: Injector Offset vs Low BPW  (1994 3.4L F-Body)
BPW (mSec)     Offset (mSec)
            0.00
    1.97-3.88      <all 0.00>
    1.53-1.95      <all 0.02>
    1.36-1.51      <all 0.03>
    1.07-1.34      <all 0.05>
    0.98-1.05      <all 0.06>
    0.96      0.08
    0.95      0.08
    0.93      0.08
    0.92      0.09
    0.90      0.09
    0.88      0.09
    0.87      0.09
    0.85      0.11
    0.84      0.11
    0.82      0.11
    0.81      0.12
    0.79      0.12
    0.78      0.12
    0.76      0.12
    0.75      0.14
    0.73      0.14
    0.72      0.14
    0.70      0.15
    0.69      0.15
    0.67      0.15
    0.66      0.15
    0.64      0.17
    0.63      0.17
    0.61      0.20
    0.60      0.20
    0.58      0.18  <strange that it drops here>
    0.56      0.18  <does the same on all 4 BINs I looked at>
    0.55      0.21
    0.53      0.21
    0.52      0.21
    0.01-0.50      <all 0.23>
    0.00      0.00

---

Below is the table I used for the Fiero ECM. It was based entirely on the above table from a 3.4L FBody.
The difference in the numbers is because of:
1) The 94 FBody ECM measures the offset in mSec. The Fiero ECM uses uSec.
2) Rounding error. TunerPro has to round off according to how the numbers are stored in this ECM.

code:

---

TABLE: Low Pulse Width Injector Offset Vs. BPW   (adapted to Fiero)

BPW (msec)             usec Added
    3.90      0.00
    3.66      0.00
    3.41      0.00
    3.17      0.00
    2.93      0.00
    2.69      0.00
    2.44      0.00
    2.20      0.00
    1.95     15.26
    1.71     15.26
    1.46     30.52
    1.22     45.78
   0.976     61.04
   0.732    137.33
   0.488    228.88

---

Notice how the entries have a resolution of 15.26uSec. That's an ECM limitation. A value like 20 can't be stored, it has to be rounded.
These values are all much lower than the factory tune. It leads me to wonder how accurate the stock ECM can be in this range - it is after all a slower ECM than 1990s cars.


Injector Offset vs Battery Voltage
=======
F94A is the values from an automatic FBody 1994, which refers to it as offset vs "fuel pump voltage". I've noticed that the manual trans values are slightly different for some reason. I didn't end up using either.
A95 is the values from a 1995 ABody 3100 "BPCL" chip. It refers to it as offset vs "battery voltage", like the Fiero ECM does. These are the values I decided to use, because it put my BLMs in a better range. The overall progression seems similar, but the ABody values are offset some from the FBody values, like one was anticipating more of a voltage drop than the other.
The voltage entries on the A95 table use a different scale, so I interpolated values that fit the Fiero table. I ignored the extremes though.

code:

---

Injector Offset vs Battery Voltage
Voltage	       stock (uSec)     F94A       A95                                  A95->rescaled (used these)
0                     671.4            400          915.53                             915.54
1.6                  671.4             640          915.53                             915.54
3.2                  671.4             640          915.53                             915.54
4.8                  671.4             640          915.53                             915.54
6.4                  5432.20          3080        2807.62 (6.20v)               2807.66
8.0                  2380.4           1530        1617.43 (7.74v)               1525.90
9.6                  1525.9           670          1068.12 (9.29v)               1007.09
11.2                1037.61          490          762.94 (10.84v)               701.91
12.8                 793.47           310          503.54 (12.39v)               457.77
14.4                 579.84           180          305.18 (13.94v)               274.66
16.0                 396.73           90           152.59 (15.49v)               122.07
17.6                 274.66           0            61.04  (17.04v)                 61.04
19.2                 213.63           0            0                                        0
20.8                 152.59           0            0                                        0
22.4                   91.55          0            0
24.0                   30.52          0            0
25.6                   0                0            0

---

The voltage offset table has a huge effect on idle BLMs. When I changed this table, my BLMs went from 120 to 150 (using F94 values), then to the 130s (using A95 values).
I'm not sure how much these affect the idle, since it would seem the voltage stays basically the same. However, Hudini has mentioned that he previously fixed a hunting idle with changes to this table, so maybe it's more significant than I thought.

There were some other misc fuel related changes I did but they don't affect the idle. These included changing stoich a bit (we have corn-gas), and scaling down the injector base constant and AE tables, in proportion to the size of injectors vs engine. It seems to accelerate without any hesitation, but I haven't given it a good workout.

[This message has been edited by armos (edited 06-18-2013).]

Thanks! At least now I have a frame of reference.

I've tightened the idle deadband with success too but it's a fix that shouldn't be needed if everything else is dialed in.

I've got a 3.4L w 24lb injectors and 270 cam, that has a hunting idle but it only happens while the engine is warm and in drive. Leads me to believe that the idle A/F may be rich. Will do some wideband tests this week and see what they show. Just love tuning with those ancient ECM's-not!

------------------
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Powerlog manifold, Northstar TB, LS1 MAF, 3" Flotech Afterburner Exhaust, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

So anyone with input on the stock 7170 (86-88) ECM and the stock 2.8?

The stock injectors should work with the stock ECM programming without a hunting idle right? New Fieros and newish Fieros didn't have a hunting idle problem right?

I don't have the 7170 in my Fiero anymore so I can't test it.

It does seem to make sense that this problem can be caused by more than one issue.

Also to toss this into the discussion - The 7730 ECM has a fuel pump voltage sense input. The 7170 only can sense the voltage input to the ECM. The fuel pump voltage runs through the harness and fuse box and relay/Oil Pressure Switch/harness again. Note also - The ECM runs on the constant voltage input, not the switched voltage input. The switched voltage input is there only to tell the ECM to come up. When you turn the key off the ECM stays on for a while and then turns itself off. That is why they couldn't power it from the switched input.

It might also be interesting to find a idle hunting engine and run the ECM and injectors off of a regulated 12v supply. An alternative would be to unplug the alternator plug and just let the engine run off of the battery. With no alternator the voltage would remain constant. Not sure if it would point to the root cause of the problem or not. Then again if the problem is caused from multiple root causes it might just muddy things even worse.

[This message has been edited by phonedawgz (edited 06-21-2013).]