Stalling issue - Ford Mustang Forums : Corral.net Mustang Forum
 
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post #1 of 6 Old 09-26-2014, 09:28 PM Thread Starter
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Stalling issue

I'm having a problem with my car stalling at lights and stop signs. I am running the Pro-M EFI system.

When I set the idle per the instructions that come with the Pro-M kit, I am setting the idle at the RPM that results in a smooth idle, which is about 1000 RPM's. The process is to set the idle with the IAC disconnected. If I do that, when I rev the engine, it will die as the RPM's come down.

I have to crank the idle set screw another 2-3 turns to keep it from stalling.

Chris from Pro-M had told me in the past that the EFI system uses timing more than IAC to control the idle, and that I should be looking for about 20 degrees of timing at idle with the spout connected. When I set the idle set screw per the instructions, I am seeing timing right around 20 degrees, which is exactly what the EFI system should be doing. However, the car dies on return to idle when the stop screw is set up this way.

If I crank the screw in another 2 turns or so, the computer will drop the timing to 12 degrees or so in an effort to keep the RPM at the target idle of 1000 RPM. With the stop screw set in this manner, the car runs fine, idles at 1000 RPM, and has no issues.

I am wondering if I have a damaged or lazy IAC motor. Or maybe my BBK throttle body is causing the issue?

I'd like to get the timing at idle back up to the target of 20 degrees or so. The car seems to be a tick more responsive and easier to drive when it's set up this way, and it doesn't smell of fuel as much at idle.

Any thoughts or advice are appreciated.


Quote:
Originally Posted by hondatech
One does not simply collect wins at the dragstrip. Victories there are guarded by more than just rear wheel drive... There is torque there that does not sleep. Not with 10,000 RPM could you do this, this is folly...
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post #2 of 6 Old 09-27-2014, 12:11 AM
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what is the DESIRED idle rpm set in the ECM?

the ECM can only command the IAC if it knows how much air mass is supposed to be at idle

throttle plate airmass plus IAC airmass = idle airmass

therefore idle airmass at your rpm that is measured, it knows how much the TB flows (because you told it how much) then can conclude how much DC to command the IAC via the IAC transfer function.


therefore idle air mass and throttle body airflow needs to be accurate.

if not then decay and pre position will not be very helpful...the things that bring the idle down slowly into closed loop idle.


89 GT gr-40, KB2200(10psi), tweecer R/T,LC-1, 22gal cell, w/tailpipes.
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post #3 of 6 Old 09-27-2014, 02:07 AM Thread Starter
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Quote:
Originally Posted by indy2000 View Post
what is the DESIRED idle rpm set in the ECM?
1000.

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the ECM can only command the IAC if it knows how much air mass is supposed to be at idle
Right now, the MAF meter is reading 7-10 grams of air at idle. I do not know the air mass that the ECM is looking for at idle. I can command any idle RPM that I wish, but if there is a way to command air mass directly (as opposed to calculating the requirement based on bore, stroke, number of cylinders, and the idle RPM I've commanded) I am unaware of it.

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throttle plate airmass plus IAC airmass = idle airmass
Sure, that's pretty easy to understand.

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therefore idle airmass at your rpm that is measured, it knows how much the TB flows (because you told it how much) then can conclude how much DC to command the IAC via the IAC transfer function.
I'm not sure that I follow you when you say that I told the ECM how much the throttle body flows at rest. I did not input any kind of data that directly represents throttle body flow. I did unplug the IAC and adjust the throttle stop screw until the engine idled well - I guess the ECM could assume that during that procedure the IAC was at full close and come up with the air mass that the throttle plate allows to pass by while at rest - but since the Ford IAC doesn't completely close, I'm not sure how the ECM could come up with throttle plate air mass based on that since various throttle bodies and aftermarket IAC components would undoubtedly yield different amount of air mass when at full close - that is assuming that unplugging the IAC motor moves the unit to full close in the first place.

Quote:
therefore idle air mass and throttle body airflow needs to be accurate.

if not then decay and pre position will not be very helpful...the things that bring the idle down slowly into closed loop idle.
Metered idle air mass is accurate, as accurate as the Pro-M MAF meter can be anyway. Throttle body air flow, I'm not sure how the ECM comes up with that - or if the Pro-M system used that information at all.

IAC decay and pre-position are not parameters that I am familiar with, nor am I familiar with whether or not they can be adjusted in the Pro-M software.

It doesn't seem that others with the Pro-M EFI system have this issue - this is what leads me to think I have a hardware failure, as opposed to a software / calibration / tuning problem.

My question was more geared towards the IAC, specifically is it likely or even common that an IAC would not operate correctly. Do they get lazy, do they have problems not returning to closed position, etc... I have taken the IAC apart and cleaned it, verified that it is operating in some fashion (it makes noise, and I can feel the unit engage in some manner or another by resting my hand on the case), but I still don't know if it's working properly.

The second part of my question was whether or not the BBK throttle body on the car is likely a culprit.

The problem seems air flow dependent, because I can cure the issue by tightening the throttle stop screw. However, by the time I get the screw to the point that the engine doesn't die on return to idle, the computer is actively pulling timing to bring the idle RPM down to the commanded 1000 RPM. At that point, can I assume that the IAC is being commanded to full close? Or as closed as it gets on a Ford set up?

If that is the case, then my first thought is that the IAC can't respond quickly enough to keep the engine from stalling. Since no one else with the Pro-M system seems to have this problem, that makes me think the IAC is lazy or otherwise not working. Again though, I'm not sure.

Maybe a better question is:

Is there a good test procedure for the IAC valve? Something that will verify both operation and response time/rate/frequency?

Quote:
Originally Posted by hondatech
One does not simply collect wins at the dragstrip. Victories there are guarded by more than just rear wheel drive... There is torque there that does not sleep. Not with 10,000 RPM could you do this, this is folly...
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post #4 of 6 Old 09-27-2014, 02:15 AM
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I am not sure what logic pro m uses.

What I am telling you, is from Ford strategy.

I cannot see how prom could be much different.

Why are you commanding 1000rpm for idle?

89 GT gr-40, KB2200(10psi), tweecer R/T,LC-1, 22gal cell, w/tailpipes.
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post #5 of 6 Old 09-27-2014, 02:16 AM
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Can you not log IAC in real time, with the prom?

89 GT gr-40, KB2200(10psi), tweecer R/T,LC-1, 22gal cell, w/tailpipes.
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post #6 of 6 Old 10-06-2014, 06:39 PM
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Wouldn't you want to set base idle without the IAC a little lower than what is set in the tune.. Also remove the spout so the ecu isn't trying to compensate with timing?

Thinking back to my A9L tuning days. May not even apply here. Just throwing some things out there.... From what I remember these idle flow settings are there so the ecu knows before hand how the engine is going to react with application of the IAC.. A surging idle is a good example. The ECU expects one thing and the engine delivers another..
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