Another High Idle Problem 200 Rpm And Higher...(already Replaced Iac)

Taker730444

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Dec 27, 2017
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Ok I have a new issue with my recently acquired 1996 mustang gt.

Has a high idle with the IAC plugged in say ooo around 2000 - 2500 RPM. When I unplug it it drops to normal 900 RPM. So I installed a new IAC. No change some issue. Now if I leave the IAC plugged in and remove the hose from it that goes to the breather the idle drops to around 900 RPM.

Next issue TPS unplug it I get no response nothing not even a fart from the car...

Now lastly I need a wireing diagram. For the black wire with the white
20171229_214003.jpg
stripe in the pic below on the middle black conector with the arrow pointing at the wire. Mine is melted.....
 
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wmburns

SN Certified Technician
Aug 14, 2009
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Has a high idle with the IAC plugged in say ooo around 2000 - 2500 RPM. When I unplug it it drops to normal 900 RPM.
First problem. 900 RPM with the IAC disconnected is NOT normal RPM. With the IAC disconnected the RPM's need to be loooooower or the motor die out right. An IAC disconnected RPM of 900 means that 900 is a low as it can go. IE, there is no adjustment room left.

Please review the following information. Pay special attention to the part about how the IAC works with "bypass" air. For your issue look for a vacuum leak or someone attempted to adjust a low idle problem with the throttle valve idle stop screw.

Troubleshoot IAC idle problems 1996-2004
https://www.stangnet.com/mustang-forums/resources/troubleshoot-iac-idle-problems-1996-2004.13/

Next issue TPS unplug it I get no response nothing not even a fart from the car...
The TPS is not used the same way after 1996 model year. It is really only used to determine 3 states. Closed throttle, part throttle, and WOT. Since in your your case the IAC has no adjustments room left I suspect that is the reason why you don't notice anything when the TPS is disconnected. But in any case it would not be expected to notice a huge immediate difference when the TPS is disconnected. In a properly working car the expected results should be a hanging idle in the 1100-1800 RPM range.

Now lastly I need a wireing diagram. For the black wire with the white stripe in the pic below on the middle black conector with the arrow pointing at the wire. Mine is melted.....
In almost all cases a black wire with a white stripe is a GROUND wire. If this were my car I would be checking all of the body grounding points I could find. For example:
  • Battery negative
  • Radiator core support
  • engine grounding strap going between the left hand motor mount and frame rail.
  • behind center console.
If interested in getting a full set of Ford wiring diagrams with a service manual for your car I maybe able to help. PM if interested.

Finally here's some information that may prove VITAL in locating and fixing the bad ground on your car.

Howto perform charging system voltage drop test
https://www.stangnet.com/mustang-fo...perform-charging-system-voltage-drop-test.56/

Using the voltage drop method it's possible to test for a bad connection right down to the last bolt if necessary.
 

Taker730444

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Dec 27, 2017
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Have not run thru aome of the testing you listed above but I did pull 7 trouble codes... so possible bad pcm or ccrm....

P0161, 0141, 0122, 1131, 1504, 0172, 0175
 

wmburns

SN Certified Technician
Aug 14, 2009
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Houston Texas
IMO it's possible that the high idle is because of the current DTC codes.

It's also very possible that a poor PCM ground could cause everyone of the listed DTC codes. If this were my car I would double/triple check the ability of the grounds to carry current. In addition, clean and repair any connectors/wires that are already showing signs of over heating.

Based upon the large DTC list, I would also check for possible blown fuses.

P0161 - HO2S Sensor Circuit Malfunction (HO2S-22)

P0141 - HO2S Sensor Circuit Malfunction (HO2S-12)

P0122 - Throttle Position (TP) Circuit Low Input

P1131 - Lack of HO2S-11 Switch, Sensor Indicates Lean

P1504 - Idle Air Control (IAC) Circuit Malfunction

P0172 - System to Rich (Bank 1)

P0175 - System to Rich (Bank 2)
 

Taker730444

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Dec 27, 2017
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Ok well I talked to the kid I got the car from and found out some bad info now... The PCM ground that is suppose to be on the radiator support was cut and he didn't know where it went so he connected it to the batt. pos. term. So that might explain my issue. Also probably related to what he did as I mentioned above in my first post about the melted b/w ground wire(noted above in pic) so far the wire has 90% of it installation melted off and the copper is dull and showing heat damage. so far none of the other wires have any major damage in the bundle other than being stuck to the b/w wire. But I still have a long way to follow this wire through the stereo area to where ever it goes under the drivers side dash.
 

wmburns

SN Certified Technician
Aug 14, 2009
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Houston Texas
The purpose of the grounds on the radiator core support is to supply a return ground path for all of the other grounds spread through out the car. There are a bunch of them.

EXPECT problems if this radiator core support grounding is not as Ford designed it. There must be a solid grounding bond between the radiator core support and battery negative. That is one reason why Ford put TWO grounds on the core support close to each other.

I can't recommend strongly enough for you to review the information in how to perform a voltage drop test. Learning how to perform a voltage drop test is the gold standard to test any circuit's ability to carry large currents without excessive voltage drop.

Remember. voltage drop = heat. More voltage drop = more heat.
 

gwith99

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Jul 14, 2020
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Taker, Did you ever find the issue? Having same type of problem and can't figure it out for the life of me. Have read tons of threads and tried everything I know of. Thanks