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Old 06-01-2015, 09:43 PM   #1
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Excessive High Idle

I have a 94 Mustang GT, that isnt exactly stock. I have: Cam, cold air intake, Pro-M MAF sensor, 70mm throttle body, 24 lbs injectors, Dyna Mod tfi module, Performance Distributors distributor, stock heads and intake manifold, and custom exhaust work with an off road xpipe and full 2 1/2 inch exhaust.

After driving the vehicle for about 5 minutes, the engine wants to idle at 2000 rpm. I unplugged the IAC motor, and did not change a thing. I eliminated the EGR valve and the EVAP purge valve to eliminate those as possibilities. Throttle body, MAF sensor, and Cold Air Intake have been changed with no difference in symptoms. I have a wide band air/fuel ratio gauge, and my readings are perfect, suddenly it leans out for a few seconds, then the idle acts up, but the air/fuel ratio goes back to normal. I connected my SNAPON scan tool, I have no codes, and verified correct tps reading at closed throttle (0.97v). My MAF sensor reads normal voltage at idle and when snaping the throttle open. Now, keep in mind, this is also before the problem occurs. I cant get the scanner hooked up when the problem occurs, because I dont want my car just sitting at 2000 rpms while setting my scan tool up. I smoke tested the intake and verified NO vacuum leaks. I have an SCT 4 Bank Eliminator Chip tuned by Bama Performance thanks to American Muscle, but I removed the chip to eliminate that as a possibility, and problem is still there.

I recently had this problem arise when I hooked my vehicle speed sensor up to the car, where I did't have it attached before because the speedo was way off due to me recently installing a different rear gear ratio.

I'm pulling my hair out, running out of ideas of what could be causing this... Other thoughts of mine are the Constant Control Relay Module due to it supplying the main power to the PCM, Injectors, Evap Purge Valve, EGR, and MAF sensor, or the TFI module and/or Distributor as a possibility of timing concern...

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Old 06-03-2015, 10:37 PM   #2
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Engine idle control. The EEC uses the NSS circuit to control idle strategy to be used by the EEC. If the system is not able to ID a "in-neutral" condition, idle surging and rolling will occur.
NSS input is also an engine load determiner along with other inputs (TPS, MAF, VSS, etc).Since NSS input is used to determine idle control strategy, its importance in the system is real.The IAC valve uses ect sensor input in order to know when it should open or close.If there is an issue with the ect sensor, it will affect the IAC valves operation.Make sure the ect sensor ohm values are correct when the engines cold & hot.Test the sensor on a cold engine then warm the engine up & test it again.Just disconnect its wiring harness and touch the +ohmmeter probe to one terminal & the -probe to the other terminal. Values should read as follows::
50°=58.75 ohms
66°=37.30 ohms
194°=2.80 ohms
212°=2.07 ohms
The fact that the problem arises after 5-10 minutes of the engine running, sounds like the issue starts once the ecm goes into closed up.Since numerous engine load determiners are wired inline with the 10 pin connectors (salt & pepper shakers)I would do the maintenance procedure on them.The link below will show you how.You should also do the wiggle test while the engines running to see if you can recreate the problem. In other words,wiggle every sensors wiring harness, tap on the sensors with a screwdriver handle.If the problem begins while wiggling a harness,the wire has a break or short in it.You should get a change in idle when disconnecting the IAC valve.Since the problem didn't start til you reinstalled the vss,check the sensor & its wiring too.
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Old 06-03-2015, 10:42 PM   #3
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Believe me, I'm a master technician, I do understand what you are taking about. However, like I said, the problem still occurs when the IAC is disconnected. I'm thinking it's actually an ignition timing causing this. Advanced timing will create a higher idle. Question is, why...

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Old 06-03-2015, 10:44 PM   #4
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Wondering if it's actually an internal problem in the distributor, a problem in the CCRM, or internal in the EEC-IV

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Old 06-03-2015, 10:45 PM   #5
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What is the NSS you are speaking of, though?

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Old 06-03-2015, 11:51 PM   #6
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Well first thing is you need an actual legit tune... The 94/95 computers are sensitive enough without an aftermarket cam, the bama chip is junk. Also if you are running 24s you will need a tune to get the fuel tables right in the computer even if you are running a "calibrated" MAF. Those MAFs **** with the voltage on the sensor to trick the computer which does not exactly play nice with the 94/95 computers and anything 96-up. If its a regular voltage maf that is not "calibrated" then there is a lot of your issue right there. Tuner will set it correctly and also be able to fix your speedo issue although that should not be causing any issues.
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