Great thread. I'm new over here, and don't remember if I saw a link from another Mustang forum or not, but great thread.
I have been fighting with some idle and throttle issues for some time now. (as alot of us have) Recently, it started when I went to a new tuner and he reset my tune. At that time, I had also made some fuel system changes, but the idle issues didn't really happen until the tune was reset.
We were trying to fix a long-time fuel lean condition above 5500 rpm, and ended up fixing it by making a few fuel system changes: replaced the GSS307/T-Rex combo with a GSS340 (ditched the T-Rex), swapped out the 42's with 60's, and added a Kenne Bell Boost-a-Pump.
I had an old Autologic chip, which doesn't have alot of tuning parameters. So, I upgraded to a newer chip, and my tuner spent alot of time trying to get the issues resolved. It had a high idle, was hanging way up during shifts, surged a bit, and was very slow to come down. But, if I put the clutch in, then it would come down. Very frustrating especially considering that we had finally figured out my top end lean condition.
Here's what I did:
Fixed my grounds - I had relocated my battery to the trunk, and the gound was connected back there to the chassis. I had also hid my wires, had the computer ground connected to a spot under the fender liner, and the screw was loose. ^%$#@!~ I ran a new ground wire from the battery to the tranny, and also extended the computer ground to that same spot on the tranny so it is in contact with a ground wire coming directly from the battery.
Did the base idle reset - I have always experienced that when I did the base idle reset (
Ford Mustang Base Idle Reset), as soon as I reconnected the IAC, the idle would go up a few hundred rpm. And, since my idle was high lately, even after doing it, I decided to set it very low with the IAC disconnected. I set it to about 500, and then used the idle bypass plate to raise it a little bit so that it would keep an idle. When I reconnected the IAC, it went up, and is now around 850-900 or so. The custom cam doesn't seem to like stock idle rpm, so that is about as low as I can go.
Reset the tps - which is, of course, required after resetting the idle. I was only able to get it to about .93 volts without elongating the holes, but after tens of hours of research on this in the past should be fine.
Cleaned the 10-pin connectors -
Ford Fuel Injection 10-pin Connector Fix Nothing to add here
Used the idle bypass plate - I know that alot of people think this is a band aid, and it probably is. But, I spent ALOT of time tweaking the allen screw with this after the above things, and have it idleing very nicely now! It is actually a pleasure to drive, doesn't die, the idle comes down nicely when driving, and it idles great at a stoplight. I think this is what really helped, as it still had alot of surging and would die even after the other things listed above.
The car is a joy to drive again. I haven't been driving it much because it was annoying to drive with the hanging idle.
Sorry for the book.