Problem?

[Hib Halverson]

Given the engine's calibration at idle, low speed and "tip in" is correct for the cam profiles you have and that the ignition system is in good condition...

First thing I'd do is make sure the IAC and its seat in the TB are clean and that the IAC is working properly.

Next thing I'd do is inspect the TB. If it's dirt inside, remove and clean it.

If the cal is not right, I'd still look at the IAC and the TB but after that, I'd have someone who knows calibration redo the idle and part throttle cal...someone like Marc

[XfireZ51]

I've had the opportunity to provide a tune for several motors, including mine, with larger cams. I always start tuning from the premise that the ECM is an "aid" to how the motor operates. In the case of setting idle, that means the motor idles as it should without the IAC. In other words, the motor should operate properly without the ECM as a crutch. Setting of Min Air is one of those critical baselines for getting to a proper tune.
In another thread, I outlined how I go about setting Min Air Throttle. In summary, I block off the IAC and adjust the throttle opening to just UNDER my desired idle RPM. That way I invoke a small level of intervention by the IAC in order to reach the desired rpm. By small level of IAC, I mean something around 10-15 counts even for larger cams. At lower rpms, I want to get as much air going through the throttle bore (higher velocity and vacuum) rather than bypass air from the IAC. Makes for a more stable and smoother idle. At higher rpm, you want to take advantage of the IAC opening for additional air. That pretty much matches what I have in the IAC Throttle Follower tables. However, particularly on a cammed motor, I "slow down" the reaction of the IAC to throttle changes. This keeps motor from dipping below the Stall Speed and shutting down.

[Hib Halverson]

Originally Posted by XfireZ51

I've had the opportunity to provide a tune for several motors, including mine, with larger cams. I always start tuning from the premise that the ECM is an "aid" to how the motor operates. In the case of setting idle, that means the motor idles as it should without the IAC. In other words, the motor should operate properly without the ECM as a crutch. Setting of Min Air is one of those critical baselines for getting to a proper tune.
In another thread, I outlined how I go about setting Min Air Throttle. In summary, I block off the IAC and adjust the throttle opening to just UNDER my desired idle RPM. That way I invoke a small level of intervention by the IAC in order to reach the desired rpm. By small level of IAC, I mean something around 10-15 counts even for larger cams. At lower rpms, I want to get as much air going through the throttle bore (higher velocity and vacuum) rather than bypass air from the IAC. Makes for a more stable and smoother idle. At higher rpm, you want to take advantage of the IAC opening for additional air. That pretty much matches what I have in the IAC Throttle Follower tables. However, particularly on a cammed motor, I "slow down" the reaction of the IAC to throttle changes. This keeps motor from dipping below the Stall Speed and shutting down.

I think your philosophy is faulty.
Without the engine controls, the engine will not run so there is no way to "tune" the engine in a manner that, without them, it would run.

"Minimum air" is not an idle setting, that is, it should not be understood as a throttle opening with which the engine can idle if the engine controls didn't exist.

Up to now, I've not heard of using the minimum air adjustment at a tuning aid.

I've always thought the reason IACs existed is the throttle bores are too large an area to use in metering bypassed air used by the engine at idle. My understudying is that some of the air the engine needed at idle came though the throttle plates' minimum air adjustment but the rest of it, and in some cases the majority of it, came though the IAC.

[mike100]

Originally Posted by Hib Halverson

I think your philosophy is faulty.
Without the engine controls, the engine will not run so there is no way to "tune" the engine in a manner that, without them, it would run.

...

I think he was referring to the idle control scheme and not the fuel injection and ignition functions.
Assuming you could disable the iac control in software or even just removing the component and blocking the air bypass, the engine should be able to maintain a low idle speed on just mechanical means with the min throttle opening set on the throttle stop screw.

[Pete]

I've been tuning GM stuff for 30 years.
What Dom is saying is correct.

If i remember correctly (man going back 30 years) on the old TPI motors you had to disconnect the IAC to set idle speed.

Holy cow when you guys type you write dictionaries my two typing fingers would fall off.
I like short and to the point.

Have fun.
Pete

[Pete]

Originally Posted by mike100

I think he was referring to the idle control scheme and not the fuel injection and ignition functions.
Assuming you could disable the iac control in software or even just removing the component and blocking the air bypass, the engine should be able to maintain a low idle speed on just mechanical means with the min throttle opening set on the throttle stop screw.

BINGO!!!!!!

[Pete]

Originally Posted by NVMYRC51

I have a 16,000 mile, 90 Z. It has a Haibeck chip, B&B Catback, K&N with open airbox lid, and other than that she is stock. Runs like a dream....most of the time. The last two times I have driven her, she has died unexpectedly when letting off the gas and coming to a stop. Starts right back up and it does not happen all the time. Any ideas? Clogged or dirty injectors maybe?

Craig,

If your injectors are the original factory units, i would replace them.
I can 99.9% guarantee you they are bad.

Good luck
Pete

[XfireZ51]

Originally Posted by mike100

I think he was referring to the idle control scheme and not the fuel injection and ignition functions.
Assuming you could disable the iac control in software or even just removing the component and blocking the air bypass, the engine should be able to maintain a low idle speed on just mechanical means with the min throttle opening set on the throttle stop screw.

Thank you mike100. And if you read my other post, you'll see that the IAC passage can be blocked without removing the IAC. No need to disable ECM IAC controls. I tuned a Xfire TBI for 8 years, and it had 2 IACs, one for each TBI.
AND you had to balance the two TBs w each other.

[NVMYRC51]

Ok, quick update and more info. My injectors have been replaced, they are not the stock units. My car has had a chip in it for quite sometime and never had a problem. I have driven this identical set up; chip, catback w/x-pipe, K&N with open airbox top pretty much since I bought the car about 18 months ago. I drove it with zero problems during the Caravan this year to Bowling Green and back without a single stall. Now it has not gotten any worse and only happens coming down from a higher engine speed. From a lower speed or coasting easily it does not shut down and when it does it always starts easy. I am under the impression, as somebody mentioned, that it could be the IAC and/or dirty throttle body. I have only driven my car twice since I last posted this question and they were both short drives. I will look into the IAC suggestions today if possible and report back. I do hope to attend the ZR-1 gathering in May so maybe I can put some faces to names. Thanks guys.

[Paul Workman]

It sounds like you're in good company. My motor is ported and afterward I started encountering the same occasional stall you and others mention. It only happens occasionally, AND when coming down from high speed (HOT) mostly. Sure is annoying when you're coming into a hairpin turn on a mountain run and suddenly the engine shuts off when I shift gears as I enter the turn.