2002 Mustang GT Deluxe manual Idle issues. - Ford Mustang Forum
 
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post #1 of 8 (permalink) Old 03-16-2014 Thread Starter
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2002 Mustang GT Deluxe manual Idle issues.

Hello, I read through the sticky post on idle problems and did not find an answer to my problem, which has been evolving.

It started with my idle hanging around 1000-1200 RPM then returning to normal, I removed and cleaned the IAC, which caused the idle to start bouncing between 500-900 RPM. I then replaced the IAC which solved the idle hunting but it started pulling codes for not having any more adjustment. I hunted down vacuum leaks and found a big one going to the PCV valve and fixed it, I also took the time to clean the PCV valve at the time. So it no longer codes.

What it is doing now is whenever I take the revs above 2000 RPM the idle will hang around 2100 RPM for around 10 seconds and then drop and die unless I blip the throttle and keep it running. If it does die, it will start right back up and run just fine. I thought I got a bad IAC so I went and got another one, but it did the same thing. If I put the old IAC back in it will buzz/rattle at idle now, but there is no hanging problem. With the new IAC in the car if I drive it and keep the revs below 2000 RPM you would never know there was a problem. If you compare the new and old IAC's they look identical on the outside but on the inside, manifold side, they are different. The new one has a shaft/spring assembly where my old one does not. I asked the guy at NAPA about it but he swears that I have the right part. Oh, my car is all stock except what looks like a K&N air filter (prior to me).

Any help/suggestions would be awesome... Thanks!

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post #2 of 8 (permalink) Old 03-17-2014
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Does the old IAC have the black vent? Does the new IAC have the black vent? If they are different, likely this is part of the problem.

Did you disconnect the battery after changing the IAC? Remember it's necessary for the PCM to re-learn new Idle/trim settings after making repairs. I could see this being part of the problem if the wrong IAC is used and the PCM is not reset.


2003 GT Convertible (sold & missed)
2000 GT coupe (Craigslist project. Fixed. Now my DD). Windsor to Romeo swap.
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post #3 of 8 (permalink) Old 03-17-2014 Thread Starter
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There are no black vents on either IAC, and I have done the battery disconnected for 10 min and idle without touching the gas for 10 min each time I changed it.
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post #4 of 8 (permalink) Old 03-18-2014
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Confirm that the battery holds a charge and that the battery terminals are clean and tight.

Confirm that you have read the following post.

https://www.allfordmustangs.com/forum...9.html#2260649

A hanging idle around 1000-1200 RPM until stopped is normal. Hanging idle above 2000 RPM is not.

Suggest starting by CONFIRMING the throttle return springs. There should be two. The throttle must be free of binding.

Have you attempted to adjust the idle using the throttle adjustment screw? Does the throttle rest on the idle stop screw?

A high idle could be caused by:
  • Vacuum leak
  • bad TPS
  • Bad IAC

A low idle can be caused by:
  • vacuum leak between the MAF and TB butterfly
  • dirty MAF
  • Poor cylinder power balance

If this were my car I would focus on worn, loose, binding throttle linkage as a possible cause for the changing symptom.

An ODB2 scanner that can monitor operational data could make short work of this.

2003 GT Convertible (sold & missed)
2000 GT coupe (Craigslist project. Fixed. Now my DD). Windsor to Romeo swap.
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post #5 of 8 (permalink) Old 03-26-2014 Thread Starter
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I do have access to a scanner. The emission status is green when turned on, all self tests solid. I have included photos from the live data after a drive. I have went through the vacuum lines again rechecking them and adding hose clamps to some to ensure they were not leaking. I replaced the PCV valve and cleaned the MAF sensor for good measure. The butterfly and springs all appear solid (only 90K miles on the car). The TPS is linear but with my foot off the throttle it is at 18% and it always returns to 18% when going on and off the throttle. I noticed the LOAD% appears to be odd to me (I am a industrial mechanic, not a auto mechanic) sitting stopped at idle and out of gear it is around 30%. When moving with even light acceleration on a level road will send the LOAD% into the 90s. The IAT is saying 107 deg when it is only in the 40's outside. The readings from the MAF sits steady at around .5 lb/min at idle and goes up to around 6 lb/min when on the throttle (didn't look too closely). One other clue that may help identify the problem that I noticed, is that when the idle hangs at around 2100 RPM it will stay there or surge from 1500 RPM to 2100 RPM as long as the car is in motion... in other words, if I rev it high and get the idle to hang, I can take it out of gear and take my foot off the gas/clutch and coast and the idle will not settle down... until I get the car to a complete stop then the idle will return to normal. I could not find a way to monitor the IAC, I am reading the manual now to see if it is a reading I can add to the personalized view. I also noticed that the scanner had a hard time staying connected to the computer, it keeps loosing communication. Holding or moving the OBD2 plug did not appear to make a difference. Thank You!
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post #6 of 8 (permalink) Old 03-26-2014
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Compare the LTFT B1 and B2 values. Are the close to each other? Do NOT focus on the STFT values as they change too fast to be of any real value to a human.

The TPS and load values are normal. They don't go to zero at idle.

Really need to know what the IAC percent ant TPS values while the idle is hanging.

2003 GT Convertible (sold & missed)
2000 GT coupe (Craigslist project. Fixed. Now my DD). Windsor to Romeo swap.
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post #7 of 8 (permalink) Old 03-28-2014 Thread Starter
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This scan tool I borrowed does not seem to be able to display what the IAC is up to, but I am still learning how to use it. As for the LTFT... I got a data log of it happening. I started the log and took the RPM over the magic 2000 RPM. In frame 7 you can see I have taken my foot off the gas pedal and I am coasting, I am still doing about 55 MPH and the engine is surging from around 1500 to just over 2000. As I slow down to a stop the surging settles and builds up to about 2300 RPM and will drop to idle from there once fully stopped. What I notice is that the LTFT Bank 2 goes higher suddenly. Prior to frame 5 it averages about 2%. I also noted that Bank 2 Sensor 1 % dropped about the same time. So... O2 sensor maybe? Thank You!!!
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post #8 of 8 (permalink) Old 03-28-2014
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I'm confused. The opening posts gave me the impression the complaint is a LOW idle. The data log gives me the impression that the problem is a HIGH hanging idle.

Stated to compare the LTFT bank 1 and bank 2 numbers. Note that the LTFT B2 is always higher than B1 for the same time slice. This is typical with an exhaust leak on the Bank 2. This could also indicate a bad O2 sensor.

Is the vehicle speed sensor accurate? Is the speedometer accurate?

Note also that when the TPS is indicating closed throttle (18.43%), the MAF airflow is still rather high (1.8 - 1.9). Typical idle MAF values are 0.75 (your reported MAF at idle is 0.5). This tells me either:
  • vacuum leak.
  • The throttle isn't closing all the way (binding).
  • the throttle idle stop screw is set too high allowing too much air into the motor. This prevents the IAC from lowering the idle.
  • the throttle idle stop screw is set so low that the butterfly isn't resting on the stop.
  • the bleed holes in the TB butterfly are too large or the wrong butterfly valve has been used for the application.
  • The MAF is not accurate
  • The IAC is allowing too much air to bypass. Either because it's stuck open or is being commanded too high.

When the throttle is closed it MUST rest on the idle stop screw. The throttle MUST return to the SAME position each time it's close. The logs tend to indicate that the throttle is returning to the same position (TPS 18%). However, it's important that it's also resting on the idle stop screw.

The logs do NOT rule out a vacuum leak. Why? Because a typical 4.6 at idle the MAF flow is around 0.75. Yours is 0.5. This may indicate additional air is entering from another location.

Do you have an aftermarket throttle body?

2003 GT Convertible (sold & missed)
2000 GT coupe (Craigslist project. Fixed. Now my DD). Windsor to Romeo swap.

Last edited by wmburns; 03-28-2014 at 08:25 PM.
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