99 Mustang GT High Rev On Cold Start

Onesick99GT

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Nov 20, 2018
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Hi Guys,

Hopefully together we can figure out an issue I'm having with my mustang. I've identified the culprit but don't know how to fix it. My mustang will idle up around 2k rpm when the engine is cold. It bounces back and forth for a couple minutes slowly coming down to around 1200 rpm. My SCT tuner has it set at 1200 rpm (due to large cams) along with my set screw so those are fine. I've checked and tested my maf sensor, tps sensor, throttle body linkage, clutch cables and have replaced all of my rubber hosing in case I had a vacuum leak. BTW I have a brand new maf, throttle body, tps and IAC that I replaced looking for the issue I'm having. My mustang never used to do this before the addition of my power adders. It would start up and hit the proper idle instantly at start with no warming up needed. I'm sick of the people saying that's normal when its cold because its NOT. Even cold the engine should not rev up like that. So last night I was playing with my IAB with the car running and noticed when I unplugged it during idle my rpms were more consistent and it idle'd exactly how it should. It also sounded better. Mind you I already replaced this item and did a ground check and voltage check to make sure the harness and wiring are good which they are. So I believe my problem lies within this IAC. When my mechanic added everything to my car he said the problem was the same and the only way to keep it from idling high at start was to block off the IAB with a plate and drill a small hole to allow some air to bypass and it would calm down the start idle. I have since removed the plate he installed before I had it tuned. I felt like removing the complete IAB would just eliminate the issue all together but I also feel like my PCM and Tuner should be able to modify and adjust the amount of air coming in to reduce it from revving to 2k rpm on cold starts. After its warm it has no issues. So I'm stuck still trying to figure out my next step. I'm currently learning my sct tuner and I'm thinking of adjusting my custom tune by going into the settings to find out which part of the tune modifies the IAB. But I'm not sure what to do or how to find it.

Mods
Trickflow Twisted Heads 44cc
Edelbrock Intake Manifold and Heads
75mm Throttle body and Plenum
Stage 3 Comp Cams .050 lift
BBK Fuel Rails
Accel Coils
39 lb injectors
255 LPH Walbro Fuel pump
Long tube headers
Steeda Pulleys
Cold Air Intake
90 Ford Peformance Maf Sensor Changed to SCT BA3000 Maf Sensor
High Performance Spark Plugs
Optimum Battery
Bigger Alternator
SCT Tuner X3
More mods but not sure those matter

Short Video Of My Mustang GT Click Here

Mustang Engine.jpg
 
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wmburns

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So last night I was playing with my IAB with the car running and noticed when I unplugged it during idle my rpms were more consistent and it idle'd exactly how it should.
^^^This all but proves that the idle stop screw is set too high or there's un-metered air entering the intake.

The IAC can only ADD bypass air. It can not subtract air. So the IAC can't work correctly unless the idle drops when the IAC is disconnected.

Here's some more information on how to trouble shoot IAC/idle issues.

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

Onesick99GT

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Nov 20, 2018
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wmburns,
Quote "^^^This all but proves that the idle stop screw is set too high or there's un-metered air entering the intake.

The IAC can only ADD bypass air. It can not subtract air. So the IAC can't work correctly unless the idle drops when the IAC is disconnected."


The idle set screw is fine. I'm agreeing that there is un-metered air entering the intake. When I start my mustang and hold down the mileage reset button my car goes in test mode. I cycle through until I reach the rpm gauge. It shows the rpms bouncing from like 800 rpm to 1200 rpm when its warm and idling as it should. Even though I have a large cam this indeed confirms I have too much air coming in to the throttle body through the AIC. BTW when I disconnect the AIC the idle drops and it will stall unless I hold down the throttle to keep it running. So its working as it should but when I disconnect the AIC and hold the throttle down to the desired rpm I want, it sounds so much better and idles better that way. I spoke with a tune shop and supposedly they said the maf sensor which is a Ford Performance 90 MAF has issues with the SCT tuner so it probably wasn't tuned correctly. He told me to buy the sa-b3000 SCT 90 MAF sensor and replaced the one I have before I bring it to him for a tune and he can fix it.

On a side note my mustang bucks at low rpms in each gear and seems to run perfectly at WOT(wide open throttle). He also said tuning it will fix that issue as well. Sounds like I'm getting too much air in the intake at start.
 

wmburns

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Aug 14, 2009
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Without KNOWING what the IAC duty percentage is I do not agree that one can positively state that the IAC is working correctly. The point I'm trying to make is that it's possible that the IAC is being asked to add too much air. Would also help to know what the TPS_mode is (C/T, P/T, or WOT).

It's also possible that the porpoise idle is caused by the throttle butterfly not firmly seating on the idle stop screw. The situation when the motor vacuum is high it causes the butterfly to fully close there by cutting off the air. This causes the idle to drop. The vacuum drops, This releases the hold on the butterfly allowing it to open. This allows more air to enter. The motor recovers.

Wash, rinse, repeat.

The throttle body manufacturer should have a specific procedure to follow to establish the idle screw setting. This works hand in hand with the TPS to control the idle MODE. That is why it could help with trouble shooting if we knew if the TPS is stable or not.

The other problem is that the black vented IAC is affected by changes in vacuum. The black vent is designed to open to allow cranking air to enter. It's not the best design which is why Ford dropped it in later model years. The non-vented IAC requires the PCM to "command" the IAC to open enough to allow the motor to start. I could see cases where a larger cam "could" affect the IAC value causing the black vent to open when it shouldn't.

If the original throttle body has a small bleed hole and the after market unit does not, this can make the situation worse.

If this were my car I would use my propane wand to probe around the IAC black vent to see if there's any unmetered air entering at that point.

I would also use an ODB2 scanner to monitor TPS_mode, IAC duty cycle, MAF flow, and RPM's to see if there's a relationship between the PID's while the idle is going up and down. An intake vacuum gauge could also be a big help.
 
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Onesick99GT

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Nov 20, 2018
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Without KNOWING what the IAC duty percentage is I do not agree that one can positively state that the IAC is working correctly. The point I'm trying to make is that it's possible that the IAC is being asked to add too much air. Would also help to know what the TPS_mode is (C/T, P/T, or WOT).

It's also possible that the porpoise idle is caused by the throttle butterfly not firmly seating on the idle stop screw. The situation when the motor vacuum is high it causes the butterfly to fully close there by cutting off the air. This causes the idle to drop. The vacuum drops, This releases the hold on the butterfly allowing it to open. This allows more air to enter. The motor recovers.

Wash, rinse, repeat.

The throttle body manufacturer should have a specific procedure to follow to establish the idle screw setting. This works hand in hand with the TPS to control the idle MODE. That is why it could help with trouble shooting if we knew if the TPS is stable or not.

The other problem is that the black vented IAC is affected by changes in vacuum. The black vent is designed to open to allow cranking air to enter. It's not the best design which is why Ford dropped it in later model years. The non-vented IAC requires the PCM to "command" the IAC to open enough to allow the motor to start. I could see cases where a larger cam "could" affect the IAC value causing the black vent to open when it shouldn't.

If the original throttle body has a small bleed hole and the after market unit does not, this can make the situation worse.

If this were my car I would use my propane wand to probe around the IAC black vent to see if there's any unmetered air entering at that point.

I would also use an ODB2 scanner to monitor TPS_mode, IAC duty cycle, MAF flow, and RPM's to see if there's a relationship between the PID's while the idle is going up and down. An intake vacuum gauge could also be a big help.
How can you find out which mode the TPS is in(C/T,P/T or WOT). And how do you find out if the TPS is stable or not? I'll check for a small bleed hole today and let you know what I find. And how do I use the ODB2 scanner to monitor the TPS mode, IAC duty cycle and MAF flow and RPM's. I'll be looking at this later today. I'll take a video on cold start to show you what its doing. I'm still learning so anything you help me out with is much appreciated.
 

wmburns

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Aug 14, 2009
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How can you find out which mode the TPS is in(C/T,P/T or WOT). And how do you find out if the TPS is stable or not?.
With and ODB2 scanner that is capable of monitoring operational PID's. This information was included within the links already provided. Will include it here again for your convenience.

To find out IF a PID is stable simply graph the PID over time and note the rate of change.

ForScan ODB2 scanner w ELM327 USB
https://www.stangnet.com/mustang-forums/resources/forscan-odb2-scanner-w-elm327-usb.57/
 

Onesick99GT

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Update! My MAF sensor had a piece missing off of it after I replaced it with a bama one. I can almost bet it came like that when I ordered it from American muscle. After installing it, I no longer have a high idle but it wont stay running due to it idling too low. I also have an error code 0122 low voltage on the tps. I'll be checking the voltage today and adjusting the TPS to see if I can get it to idle better. I may end up replacing the TPS again for good measure then re-adjust. I took video and pictures that I was going to post but at this point I'm fairly confident I can fix it since I know what I'm dealing with.
 

wmburns

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The 1996+ model year does not need to have the TPS "adjusted". Why? Because the PCM re-learns the initial TPS setting at each start up. So the P0122 DTC code is more likely associated with a wiring problem OR the TPS value dropping below the initial learned value which is the sort of thing that could happen if the TPS is rotating after the initial start up (loose physical mounting).

If this were my car the first thing would be to verify that the TPS isn't moving or changing position. Next double check the electrical connector. The final test is to data log the TPS voltage to be sure it's stable.

P0122 - Throttle Position (TP) Circuit Low Input

The TP sensor circuit is monitored by the PCM for a low TP rotation angle (or voltage) input through the comprehensive component monitor (CCM). If during key ON engine OFF or key ON engine running the TP rotation angle (or voltage) changes below a minimum calibrated limit, the test fails

  • TP sensor not seated properly
  • TP circuit open to PCM
  • VREF open to TP sensor
  • TP circuit short to GND
  • Damaged TP sensor
  • Damaged PCM
A TP PID (TP V PID) reading less than 3.42% (0.17 volt) in key ON engine OFF, continuous memory or key ON engine running indicates a hard fault.

Note: With a voltage less than 0.17 volt is waaaaayyyyy lower than what can be accounted for by an "adjustment". Think "loose wire", dirty connection. Open,
 

Onesick99GT

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I tested the tps voltage and it reads constant .48 to .51. When I removed the tps it's spring loaded and functioning correctly upon inspecting. I did notice the throttle affects the turning inside the tps. I'm assuming the tps communicates with the pcm at how open the throttle is. I'm lost at this point.
It won't stay idling now unless I hold the throttle down. Almost as if it's in limp mode. Very hesitant when pushing the throttle down.
No codes aside from p1504 and p0720. These were from last night. I've since cleared the codes. Only code showing is p1000.
 
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wmburns

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As a reminder that it does take some time for the PCM to re-learn new idle/IAC trim settings after a battery disconnect or PCM clear.

The P1504 DTC code would indicate that the IAC isn't actually functioning at all. If so this would certainly explain why it won't idle. Anytime a DTC calls out a "Circuit Malfunction" it means there is something sooooooo wrong with the basic circuit operation that the PCM isn't able to perform any diagnosis. Think wiring or total IAC device failure.

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

P1504 - Idle Air Control (IAC) Circuit Malfunction

This DTC is set when the PCM detects an electrical load failure on the IAC output circuit.

  • IAC circuit open
  • VPWR to IAC solenoid open
  • IAC circuit short to PWR
  • IAC circuit short to GND
  • Damaged IAC valve
  • Damaged PCM

  • The IAC solenoid resistance is from 6 to 13 ohms.
 

Onesick99GT

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I cleared the codes and the only one showing is p1000. I tried starting the mustang about 10 times and even held the gas down for a few minutes thinking the pcm needs to adjust. It tried idling up a couple times above 2000 rpm when I was partially throttling to keep it at 1000 rpm as if the pcm was trying to learn the proper idle. What should I do at this point.
How long does it take for the pcm to learn. I feel like it should idle enough after start to drive it but it immediately dies. There is no pause in between the start up rev to dying at zero rpm. I’ll post the video I took last night of the start up idle and immediately dying.

I’ll test the ohms and report back
 
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wmburns

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Regarding how long it takes to re-learn new idle trim. IF all the systems are working it only takes a few minutes of idle at full operating temperature.

What has been done to rule out an upper intake vacuum leak?

If this were my car I wouldn't be looking to replace yet another IAC as the next step. Test. Confirm key on +12 at the IAC. Take a test light across the IAC. There should be some changes in the brightness of the light as the throttle is moved. Suspect problems if the light never lights.

Better yet. Monitor the IAC duty percent to confirm that it changes. Compare to the test light. As the IAC duty percent changes the brightness of the test light should also change.

A full function Windows based ODB2 scanner adapter can be acquired for less than the cost of yet another IAC.

Has the idle stop screw been adjusted or set per the instructions? If set too low then there's won't be enough adjustment range for a working IAC to maintain the idle. But it would help to know what the IAC duty percent is. For example IF the IAC is very high, then it's possible that the idle stop screw is set too low. But before attempting to alter the idle stop screw it's best to confirm that the PCM is actually "in control" of the IAC and that the IAC is responding.
 

Onesick99GT

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I checked the hoses today to make sure there were no cracks and everything was snug. The hoses are new and was replaced earlier this year. I did a duty test on the IAC. When off it continued to run up the numbers. When I turned the key it went from 40% to 60% and back and forth from the two numbers. I also checked it by using 1.5 volt and it read 11.22 on the multimeter. I believe I tested another setting not sure which one and it read 24. BTW when I was checking the fuses under my steering wheel a fuse was blown labeled 7 owner 13. It was a 15 amp fuse which I replaced. It was located closer to the bottom right side when looking towards the hood at the fuse box. Replacing it did absolutely nothing. I re-ran the codes and the only code showing was p1504 after replacing the IAC.

Which full function Windows based ODB2 scanner adapter do you recommend. I will gladly order it.
 

wmburns

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Replacing it did absolutely nothing. I re-ran the codes and the only code showing was p1504 after replacing the IAC.

Which full function Windows based ODB2 scanner adapter do you recommend. I will gladly order it.
The information was given earlier in this thread. Here it is again for your convenience.

ForScan ODB2 scanner w ELM327 USB
https://www.stangnet.com/mustang-forums/resources/forscan-odb2-scanner-w-elm327-usb.57/

1999-2004 MY fuse panel schedule:
https://www.allfordmustangs.com/forums/4-6l-tech/336452-1997-mustang-wont-ignite.html#2984838

The Information for the DTC P1504 was included in previous post.
 

Onesick99GT

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[QUOTE="

This DTC is set when the PCM detects an electrical load failure on the IAC output circuit.

  • IAC circuit open
  • VPWR to IAC solenoid open
  • IAC circuit short to PWR
  • IAC circuit short to GND
  • Damaged IAC valve
  • Damaged PCM

  • The IAC solenoid resistance is from 6 to 13 ohms.
[/QUOTE]

Are we able to rule out any of the above with the test I completed and information provided. Could the IAC have the wrong value? I seen that in a different post of yours. And I order the scanner with adapter. Will I need the software to run it or does it have it already.
 
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Onesick99GT

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Got to test the continuity tonight from the IAC to the 21 pin connector on the PCM. I didn't know which input was the 21pin so I tested each one until I came across the 21pin connector sounding the solid alarm. Its getting 12 volts at the red wire and has good continuity to the 21 pin connector. Wanted to give an update on that. On a side not I tested the following fuses underneath my driver side dash panel and here is what I found.

F2.37 0 with or without fuse in with key turned on.
F2.34 1.0 Volt with Fuse in/-6.98 without fuse in with key turned on both times. The IAC has 12 volts with the fuse in and has 1.0 volt without the fuse.
F2.21 -.230 volts. I was completely confused on this.
F2.5 1.2 volts with fuse and -6.60 without fuse
F2.13 This was the fuse blown I found last night and replaced unrelated to problem I believe.

After re-connecting the PCM I had no check engine lights BUT now my odometer isn't displaying anything. Its in the dark when it was working before. I do remember earlier this year that dashes were being displayed and when I hit bumps or turned off the car and back on it would display the mileage. It started working just fine a few months ago. Not sure if this is related to anything.

Here is the video from tonight after finishing up the continuity test. It appears the codes were deleted somehow. Maybe from disconnecting the ground to the PCM in order to remove it. Not sure.
View: https://www.youtube.com/watch?v=1CzS51SeaD4

Is there anything else I should check. Should I check the fuses in the engine compartment and which ones if I need to.
 

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wmburns

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Are you positive there isn't an after market alarm system on this car?

Otherwise I would be concerned that the motor has jumped time.

The other concern I have is there's a strange sound I'm hearing. Perhaps a loose spark plug. Re-torque to Ford factory specs.
 

Onesick99GT

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It has an alarm system. Did they come factory with one? I see it flashing sometimes when I start it up. And how can a motor jump time? I believe your hearing detonation in the engine. It putters out as if its getting too much air or fuel. Before it was having this problem it ran really rich. You could smell the fuel inside the car. Ill check the spark plugs next time I'm over there and re-torque them.
 
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