Delay in Engine speed decrease when throttle is disengaged.
I need some help, here. I have 95GT, last year of 5.0. It came with the problem when I bough it just over a year ago, but I have not addressed it until recently. I am still having the same problems, I don't know this is because, I have became paranoid or it is, actually, worse than it used to be. But, here are things I have done and found.
First symptom: Enormous MPG decrease, I mean down to 12MPG. Averaging about 19mpg combined at normal working condition.
The problem: I have noticed the engine speed didn’t decrease fast enough when I change gears, especially going from 4th to 5th. I would have to wait for about 2 seconds before I can shift it to 5th so the car doesn't judder from clutch engaging at higher RPM than 2000RPM at 50mph. (Normal shifting RPM from 4th to 5th is around 2700 ~ 3500RPM for my usual driving)
Usually, I would think my problem would be a vacuum leak somewhere in the intake. But seeing this has an electronic engine movement system, I have decided to fiddle with sensors and valves, etc.
The findings: I started to disconnect sensors one by on to see which one would make differences. So I started with Throttle bypass air Idle speed valve. And there is was RPM dropped right back to idle after engaging and disengaging throttle. So I purchased new Throttle bypass valve and new Throttle position sensor. They are now replaced with the new ones and, yet, the problem still exists.
The result: I have been driving the car without the throttle bypass valve connected after setting idling RPM at 980RPM. It seems to be doing about 18MPG and noticed differences in acceleration, much better. The exhaust is making a nice race car like puff noise during engine braking (engine speed deceleration: possibly, retarded timing [set at 20degrees]). The only drawback is that the engine stalls at idle when I am stopped for the first 1-mile or so. And when A/C or heater is on it wants to stall at around 7 ~ 10 second intervals depending on heater/ac fan setting at idle.
The question: what would cause the throttle bypass position sensor to stay open all the time or delay it for it to close at disengaged throttle. I mean, what would control the voltage of this bypass valve to stay open.
What your describing is not actually a "problem", its normal for the mustang to hang the rpm's a little. Its even more noticeable in the auto. Its just part of the way it was designed. Theres not a lot you can do about it.
You can try using a restrictor plate at the IAC which is supposed to help drop the rpms a little faster, but thats about it.
Stonecold is right. Ford did that for two reasons. One, to make shifting easier on the tranny. Two, to help burn all the gases inside the combustion chamber (tree huggers).
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97 Cobra #777 Blk/Blk
ProCharger P600B 7psi w/3 core intercooler, off road x-pipe, flows, pro kit, oil separator mod.
That was the general disucssion, I mean ECU programming. It sucks though, while the ride quality is good, I am not gettting the engince responce I would like. By removing the power(connector) to IAC and adjusting the idle speed by mechnical means, works for me for now, I guess.
What is the Ford spec on ignition timing? I thoguht it was 20 degrees retarded? I had it at 10 degrees once, it pincs like a b**ch, even at 2500 rpm.
Stock timing is 10* before top dead center. If you set it to this and you are having problems, look for something faulty in the ignition system or EGR valve. I had a problem once with my egr valve causing my 95 GT to ping so I cleaned out the egr valve and it fixed it. My computer threw a code that told me it was malfunctioning.
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97 Cobra #777 Blk/Blk
ProCharger P600B 7psi w/3 core intercooler, off road x-pipe, flows, pro kit, oil separator mod.
That was the general disucssion, I mean ECU programming. It sucks though, while the ride quality is good, I am not gettting the engince responce I would like. By removing the power(connector) to IAC and adjusting the idle speed by mechnical means, works for me for now, I guess.
By removing the power connector and manually adjusting the idle, did your fuel economy increase?
Have you experienced any problems with this set-up. Because I might do the same.
Delay in Engine speed decrease when throttle is disengaged. 
I am still having the same problems, I don't know this is because, I have became paranoid or it is, actually, worse than it used to be. But, here are things I have done and found.
