99 GT No Idle

So I have a problem. I have been troubleshooting a start idle stall issue. Today I decided to block off my air intake with duct tape while my MAF was connected and she starts up and idles perfectly. Possible vacuum leak??? She must be getting air from somewhere but can't locate it. But when I connect the cold air intake back up she starts then immediately dies and wont run unless I hold the gas peddle down. Can the MAF have incorrect settings so when its metering the air its throwing it out of loop. I'll post a picture. I just replaced the maf because it was doing the same thing before and thinking maybe the maf went bad was the issue. So replacing it isn't the fix. I'm now stuck to thinking its the IAC causing the headache.

wmburns said:

The fact that the motor runs at all when the intake is blocks "proves" there's a vacuum leak somewhere. Consider a smoke test.

But does it seem reasonable to blame the MAF when there is proof that a large amount of un-metered air is entering someone else.

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I've been told there is always a vacuum in high performance cars by tuners and car enthusiast. The tune that was done all the car got corrupted and the car had to put back to stock with all those mods. So I believe the maf tables are incorrect along with the spark advance causing it to not idle. I bought SCT Advantage 3 tuning software and already received the base tune which I plan on loading onmy pcm today. I'll report how that goes but was told to not run wot until I fine tune the stang. Anything newer than 99 and up when performance mods are done can mess with the tables inside the pcm hence the reason why needing it tuned. On top of it the stock tables wasn't reading a stage 3 cam in their which really throws spark advance. If tuning it doesn't work I plan on tearing the motor down(Ill post Pics) and checking the piston rings. I use a lot of oil and I think I may have a leak. When I do I will rebuild with forged pistons and rods.

wmburns said:

IMO you may be over thinking this.

The MAF is installed in such a location that the MAF is able to meter all of the air that the motor is using. In this case there's a motor that continues to run even when the intake air tube is TOTALLY blocked. Just like a human being chocked at the neck, there is no way this motor should idle. But it does! The vacuum leak must be HUGE.

If a smoke test isn't available, try:

• Spray water bottle
• propane wand
• carb/choke cleaner.
• Brake clean

I've tried the carb cleaner trick. I've sprayed around everything that has vacuum but nothing is appearing out of ordinary or making the idle flunctuate. I'm back thinking the IAC could be stuck open. When I had the intake blocked air was being sucked backwards from the IAC not through it into the intake. And it ran at low idle, which would be obvious since the set screw allows a small amount of air into the intake. This leads me to believe the IAC is stuck open or closed. If the IAC was stuck open then the PCM would be confused since so much air is entering into the block un-metered by the IAC. The PCM obviously making the incorrect adjustments and therefore the start and stall since the stoichiomistry is off. I'm going over diagnosing idle air control issues from a previous post again. I believe it has to be with the IAC, either electrical or just got a lemon part when I replaced it and needs replaced again.

Click to expand...

Is there a fuse just for this part or relay could check?
What exactly does the black box on the IAC do? Its empty so why have it!

So it idles up at start to around 2500 rpm. This leads me to believe the IAC allows the air to bypass the throttle body. More air in the beginning at start with proper fuel and spark ignites the gases (proper stiochimetric mix) then the rpm drops the IAC closes which prevents air entering the intake causing the car to stall since the improper amount of air in entering the intake. I believe the mixing(air/fuel/spark)is incorrect in the tune at idle. Its correct at start since it has more air. The MAF reads back to the CPM since it reads up to 7 g/s then falls which should be correct since the IAC closes and the throttle blade is barely open. Hope this makes sense. I'm using the advantage software from SCT.

I resolved this issue by downloading the forscan software burns recommended along with buying the elm adapter. You can find the link below.
forscan-odb2-scanner-w-elm327-usb Adapter.
The tune for my maf sensor was incorrect. This can happen after upgrading to a different maf sensor which in my case that's exactly what I did. Or by drastically changing the airflow or fuel system, which would require a new tune. Remember maf sensors are used for measuring the amount of air coming in to the intake. My tune got corrupted in my case and I didn't realize this until after replacing my IAC, TPS, and maf sensor. I went through weeks of testing each individual piece according to my ford mustang manual thinking I also had an electrical issue. If I had only downloaded the software earlier and reviewed the maf airflow to voltage and a few other graphs I would have had this problem solved much earlier without throwing money at it. Ill post some pictures of my before and after using the software to diagnose my maf sensor issue. It just needed calibrated which any tuner can do that owns the software.

Below is a picture before the tune. Notice how much distortion there was on the bank 1 and 2 fuel trims. They were super high and super low. Once I posted this on this forum @a91what was able to tell me my tune was off. All for 30 bucks I solved my problem and knew what I needed to do to fix it.

The actual forum of my problem and how I resolved the start and stall issue can be seen below.

https://stangnet.com/mustang-forums/threads/sct-advantage-iii-racer-pro-software.907433/