Idle issue still going on.......

ebk06

Member
Feb 23, 2007
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So Ive been fighting this idle issue for some time. I followed the idle surge checklist to a T a few times and pulled codes KOER and KOEO and get an 11. Did a cylinder check and everything turned out fine. I finally gave up and took it to a friend whose a mechanic and has been in the mustang scene longer than Ive been born.

The car was only surging at a stop and would drop down to 300 rpms and surge up to 900. He fixed that problem but not completely. Upon start up it surges for a couple seconds then levels out although he had to set the idle to 900 rpms, which is high. He spent a lot of hours checking things and figured out the car would only surge wit the IAC plugged in. We both check the voltage to all sensors and replaced a few of them including the IAC. The throttle body was worn out so we replaced that.

Literally everything we could think of was checked at least 4 times and we cant figure this one out. The car drives great otherwise. Does anyone have ANY ideas about why the car idles better with the IAC unplugged? The voltage measures perfect on the wires to the sensor.
 
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With the idle set so far above the commanded idle, the EEC can get confused when trying to get it lower, esp as it transitions from a roll to a stop.
 
The engine is completely stock and its an '87 just for reference. Jrichker, I've done that a few times and it would set at 600 just fine but once the IAC is plugged back in the surge reappears. Like I've said everything on the check list has been done a few times. The vacuum lines were all changed out a couple years ago but I did end up replacing them again just in case and still nothing works. Im 110% stumped.
 
Try the base idle and plug the IAC in without the SPOUT.
If the surge goes away, you have a problem in the PIP circuit.

If the engine still surges, then the problem is in the IAC.

IAC doesn't work: look for +12 volts at the IAC red wire. Then check for continuity between the white/lt blue wire and pin 21 on the computer. The IAC connector contacts will sometimes corrode and make the IAC not work. The red wire on the IAC is always hot with the engine in run mode. The computer provides a ground for the current for the IAC. It switches the ground on and off, making a square wave with a varying duty cycle. A normal square wave would be on for 50% of the time and off for 50% of the time. When the idle speed is low, the duty cycle increases more than 50% to open the IAC more. When the engine speed is high, it decreases the duty cycle to less than 50% to close the IAC. This is why a DC voltage measurement isn't very helpful in diagnosing IAC problems. Once you get past the 12 volts on the red wire, everything else is a pulse that needs an oscilloscope to measure it correctly

55762d1222607463-ever-seen-done-idle-air-bypass-solenoid-help-iab-iac-duty-cycle-explanation.gif


Probable cause:
Disconnect the computer main connector and the IAB connector. Measure the resistance between pin 21 on the computer connector and ground. My guess is that you will find less than 200 Ohms, which is a short that would keep the IAB fully turned on.

An old-fashioned dwell meter can be used to check the change: I haven’t tried it personally, but it should work. In theory, it should read ½ scale of whatever range you set it on with a 50% duty cycle. An Oscilloscope is even better if you can find someone who has one and will help.