dethanb

G'Day to you all,

Please forgive me for posting this, but I did a couple of searches and didn't see anything pertaining to "exactly" what I was looking for, so be kind in your flaming me...

I have a 1996 Mustang GT. A couple of questions I have is with the idle on the car.

I have installed on the car:

• K&N Cold Air Intake
• BBK X-Pipe
• Flowmaster American Thunder Cat-Back
• Reversed from 75mm throttle body/intake plenum back to a 70mm throttle body/intake plenum
• Replaced the IAC Valve
• Replaced Intake plenum gasket
• Cleaned the gunk out of the throttle body and intake plenum
• SCT SF3 Tuner

I've been trying for quite some time to get the Idle down below 1000, and preferably around 700-800 rpm. I've done the above changes to the car and tried using the SCT SF3 Tuner to get the idle's down to where I'd like. I was getting a CEL for the IAC, but once I replaced it, it went away and now have a clear CEL.

Can anyone tell me how to get my idle down to a respectable level of 700rpm(ish) instead of dropping to 1200 and sometimes as low as 1000rpm? I've called American Muscle (where I buy all of my parts) and spoke to the guys there but none of them will give me a straight answer as to whether or not the Tune will take care of this or not?

I "H-A-T-E" the hanging idle at 1500rpm until I come to a complete stop and then it only drops down to 1000-1200 (constantly bouncing between those RPM's) I'm planning on racing this car in Autocross/Solo II this summer, and I want to use the engine RPM to load the front end with weight during a deceleration, but with the hanging idle it will go into a coast, which won't work for me. Plus I love the sound so much better at the lower RPM's.

1) Is there another way to fix this other than the smaller hole in the IAC Valve gasket as I've already installed one

2) Will a BAMA Tune, or something else help me to get this car to act like a car should when using the engine to brake the car all the way down to 700 RPM?

3) How can I get the car to idle lower than 1000-1200 RPM?

I know you guys have probably answered this 1000 times on here, but pointing me to the correct posts would be greatly appreciated, and any help would be even more appreciated.

Thank you much...

cliffyk

Make sure the TB stop screw is properly adjusted--here is the procedure.

If you have access to an oscilloscope check the waveform across the IAC--it should look like this, with a duty cycle of 40% to 50% at idle with all accessories off and the battery charged:



If the duty cycle is low, 10% to 30%, then the TB stop screw is turned in too far holding the throttle plate too far open and the PCM cannot close the IAC valve any further to bring down the idle speed.

OTH if the duty cycle is too high, 60% to 90%, then the stop screw if not turned in far enough and the plate is too far closed. In this case the idel speed would be too low as PCM could open the IAC valve enough to control the idle.

dethanb

Okay, so I don't have an oscilloscope although my dad had half a dozen or more of them back in 70's when he was a field engineer.

Today, I was able to work out the set screw from the throttle body and tried to play with the idle. I pulled the negative battery cable and allowed the car to sit for a good half an hour before reattaching. I backed out the set screw and allowed the throttle body to rest on it's own. Once I put the battery cable back on, I fired the car up and...

Didn't change a thing!

I completely removed the set screw and still idled at around 1000-1200 rmp!!! I flashed the computer back to "stock" with the SCT Tuner and kicked it on, and still 1000-1200 rpm. Then I reloaded a box tune from the device and no change.

Any ideas?

cliffyk

If the idle speed was still high with the stop screw removed then either the throttle blade is not closing completely, or you have a vacuum leak somewhere.

With the TB stop screw backed off all the way or removed, what happens when you disconnect the electrical connector at the IAC? If the engine does not die, or at least slow considerably, then you have a vacuum leak...

dethanb

Ahhh, I'll have to try that when I get home tonight. Next question... How does one find a "vacuum leak" if that is the case? Is there a tool, or specific technique in doing so?

cliffyk

It the engine is running at 1.0 to 1.2k rpm, with the throttle closed and the IAC unplugged, then I don't know how you could miss it?

If it's not a blatant "in-your-face" leak, like a disconnected/cracked/missing vacuum hose at the upper plenum, then try disconnecting the vacuum lines on the upper plenum one at a time, and plugging the plenum port with your finger.

If the engine slows or dies then you will have found the bad vacuum system. The FPR, EVAP, EGR, and HVAC systems all use vacuum in their operation...