Last year I managed to acquire the Northstar 6 spd Fiero seen so much on this site. While it has some amazing attributes built into the car it has given me a couple issues that I have been unable to solve.
Probably the most annoying has been it stalling when letting off the gas, especially when approaching slower speeds. It will often drop down to 500 rpm and recover or just drop down to zero and stall. I have cleaned the intake and replaced the gaskets, replaced the crank sensors, MAF, TPS and it still happens.
I am not sure where to go from here, there are no codes to read and it does not happen all the time. I am not overly familiar with N* engines and their finicky ways but I am sure there are a few experts on this site to lend some expertise.

Does it start up easy after it stalls? If so It probably needs more fuel in the low rpm high kpa ranges.

You may also need to look into the minimum IAC setting if that's available in the code.

Do you have 2 idle speeds? One when you are coasting and one when you come to a complete stop (which is a few 100 rpm lower).

What ECM ar you running?

A trick that normally works, is to reset the base idle RPM. They normally suggest you set this in the 400 to 600 rpm range, but I would suggest to set this to 50 rpm below your desired hot idle speed. This adjustment needs to be done while hot. This adjustment will increase your cold start idle until it reaches operating temp, but past that it should idle like normal when hot... with less of a tendency to stall.

Depending on your ECM, there should be some throttle follower functions that you can adjust. Manual transmissions are much more sensitive to the throttle follower settings.

quote Originally posted by 87ecotec: Last year I managed to acquire the Northstar 6 spd Fiero seen so much on this site. While it has some amazing attributes built into the car it has given me a couple issues that I have been unable to solve. Probably the most annoying has been it stalling when letting off the gas, especially when approaching slower speeds. It will often drop down to 500 rpm and recover or just drop down to zero and stall. I have cleaned the intake and replaced the gaskets, replaced the crank sensors, MAF, TPS and it still happens. I am not sure where to go from here, there are no codes to read and it does not happen all the time. I am not overly familiar with N* engines and their finicky ways but I am sure there are a few experts on this site to lend some expertise.

Who built it (where's the build thread)?
And, as above, what computer are you using?

The stall is a normal part of using an ECM calibration designed for an automatic and using it with a manual transmission. There are work-arounds, but the fundamental way to solve it involves changing the idle follower settings in the calibration.

quote Originally posted by Joseph Upson: You may also need to look into the minimum IAC setting if that's available in the code.

If he has OBDI controls, then he'd have an ISM instead of an IAC.

https://www.fiero.nl/forum/Forum3/HTML/000030.html

Do you have 2 idle speeds? One when you are coasting and one when you come to a complete stop (which is a few 100 rpm lower).

What ECM ar you running? [b] It is a reprogrammed Cavalier ecm, Ryan Hess 7730 ECM

A trick that normally works, is to reset the base idle RPM. They normally suggest you set this in the 400 to 600 rpm range, but I would suggest to set this to 50 rpm below your desired hot idle speed. This adjustment needs to be done while hot. This adjustment will increase your cold start idle until it reaches operating temp, but past that it should idle like normal when hot... with less of a tendency to stall.

Depending on your ECM, there should be some throttle follower functions that you can adjust. Manual transmissions are much more sensitive to the throttle follower settings.

As far as the settings it has all been set and tuned on a dyno as per zac88gt threads and videos. the car ran great for a couple years but has since started this stall issue, I did a little research and took the advice on many Northsatr threads regarding the possible cause, so far none have worked

When it gets into the reprogramming of the ECM it gets a way over my head. I know this ECM is not directly reprogrammable, a chip has to be burnt with a separate program then reinserted into the ECM.

Ah, you bought Zach's car.

Is your EGR active? Have you checked the cleanliness of the idle air passages through the throttle body?

EGR has been removed, Throttle body has been removed several times and cleaned. Intake plenum removed with new gasket and resealed. Started replacing the obvious problems, like sensors, MAF, TPS and Crank sensors x2. Cannot find any leaks or vacuum issues.

There is no such thing as "obvious sensor". You wasted money on all those things.

Ryan's 7730 tune is MAP based. There shouldn't be a MAF sensor.

Have you done any actual troubleshooting or just thrown parts at it?

Have you tried an IAC?

Does the car start up normally after stalling?

Yes car starts normally after stall. And runs great otherwise.
Yes there is no maf sensor, I was wrong.
Did the recommended things, cleaned throttle body, replaced intake gaskets and checked and replaced the iac valve.
Still stalled. Took it to the shop, they checked for codes and leaks and checked the prom with the laptop and program that was included with the car. It all checked out fine. Suggested it may be an intermittent sensor such as crank sensor. So replaced them to rule it out.
It does not happen all the time, it is an intermittent problem.

So it's stalling lean. The same thing happened to me when I installed an aftermarket cam. I had to take out fuel in the low rpm's and it started stalling. It's probably worse now that it's getting cooler out. So what I think happens is the rpm's drop and the engine goes into a stall save mode, The iac opens all the way, kpa goes super high because there is too much air and it stalls because there isn't enough fuel to save the stall. I would email Darth or your favorite tuner and have them add fuel in the 70-100 kpa range at 800rpm and lower. You don't drive in these ranges anyway so it won't hurt anything. The other thing to check is if the fuel tables match between the idle and main VE. If they are way out it could explain why the problem is intermittent.

quote Originally posted by 87ecotec: As far as the settings it has all been set and tuned on a dyno as per zac88gt threads and videos. the car ran great for a couple years but has since started this stall issue, I did a little research and took the advice on many Northsatr threads regarding the possible cause, so far none have worked

If you let the car idle warm at a dead stop for a while, does it eventually stall?

Problem solved. I took off the TPS and the return spring was gone and it had a lot of play in it.
Replaced it and the idle is good again.

I run a Northstar throttlebody and think I'm having a similar problem with occasional oscillations at idle and sometimes a brief flat take off. Watching the log dash also appears to be telling with an intermittent oscillating closed thorttle TPS voltage so I'll go ahead and change it out since I also recently replaced the CPS. I have read stories about drive-ability problems that turned out to be not so obvious bad throttle position sensors similar to the type used with these throttle bodies.