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Discussion Starter #1
I have an 03 gt automatic. When I turn the car on it idles around 1k rpms and when I put it in gear it jumps up to around 2.5k rpms. I can drive for a minute or two but then it shuts off. When I crank it back up it runs fine. If I get down on it though it will start idling high, so then I put it in neutral to turn it off and back on. Once again it runs fine after that. I have changed out the IAC, PCV, Throttle position sensor, and I've cleaned the throttle body. If you have any suggestions or solutions please let me know!
 

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What are your mods, and is your car tuned? Did you get an OEM or aftermarket IAC? Aftermarket IAC (From Autozone, etc.) are known to have a lot of problems, even from factory. Also, have you cleaned the MAF and checked for vacuum leaks?
 

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When I crank it back up it runs fine. If I get down on it though it will start idling high, so then I put it in neutral to turn it off and back on. Once again it runs fine after that.
^^^^This is a sign that the throttle is NOT returning to the SAME position each time the throttle is closed. Possible causes are:
  • Broken/missing throttle return spring
  • incorrectly adjusted idle stop set screw (either too high or low)
  • Binding throttle linkage
  • loose or worn throttle linkage
  • loose TPS sensor. IE the sensor itself is moving.
  • An incorrect speedometer can contribute to idle issues.

For the 96+ model year the PCM re-leans the TPS closed throttle value at each start up. The hanging idle occurs when the TPS value creeps up after start up. The problem clears up when the motor is shut off and back on because the PCM re-leans a new closed throttle TPS value.

To confirm this is your problem, monitor the TPS value with a VOM or monitor the TP-MODE PID with an ODB2 scanner. If using the VOM method, make note of the TPS value when the motor is first started. Compare the TPS value when the problem occurs.

For the ODB2 method, look at the TP-MODE PID to see if it's "P/T" (part throttle) when the idle is too high.

Then some checking for a vacuum leak should be done. A high idle value of 2500 RPM's is too high and may indicate a vacuum leak (assuming that the base problem isn't just binding throttle linkage).

See the following thread on how the TPS works and how to trouble shoot IAC/idle issues.

http://www.allfordmustangs.com/foru...ng-idle-idling-rough-post2260649.html#2260649

Bottom line: For correct idle determination the TPS must give the SAME value every time the throttle is closed.
 

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Discussion Starter #5
I have a JLT cold air intake, bbk x-pipe, flowmaster mufflers, and its tuned by Bama Performance. I have cleaned the throttle body and checked for vacuum leaks. I think it is some type of electrical or computer problem.
 

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I have a JLT cold air intake, bbk x-pipe, flowmaster mufflers, and its tuned by Bama Performance. I have cleaned the throttle body and checked for vacuum leaks. I think it is some type of electrical or computer problem.
Having access to a hand held tuner could make short work of this. Soooo much easier to monitor PCM values using an ODB2 scanner. Also have access to other PID's that will give important clues such at IAC duty%, MAF flow, and most importantly the actual TPS value.

Some tuners perform only PCM modification duties. However, it should be a simple matter to upgrade and add ODB2 monitoring to the tuner.

Good luck.
 

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I'm also attempting to battle with an idle issue on my completely stock 03 Mach 1 which is showing no computer codes. - I cleaned the MAF sensor with CRC's product. This did not improve my condition. I cleaned the IAC value with carb cleaner but saw no improvement. I replaced the IAC valve with an aftermarket from the auto parts store and now I made the idle condition worse. It's idling extremely high and even stalled twice coming to a stop. I also added Seafoam to the tank.
 

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Discussion Starter #8
Yeah I am going to stop by a performance garage and see if they have any luck figuring it out. Im basically shooting in the dark.
 

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Picking back up - I cleaned and reinstalled the original IAC valve and found the rubber elbow/boot on the PVC line was compressed - not sure if that's my issue. When I return the stores IAC valve, I'll try and pick up a replacement elbow. I'll keep you posted
 

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Picking back up - I cleaned and reinstalled the original IAC valve and found the rubber elbow/boot on the PVC line was compressed - not sure if that's my issue. When I return the stores IAC valve, I'll try and pick up a replacement elbow. I'll keep you posted
You mean the elbow by the plenum (behind the throttle body) that goes to the PCV valve on the passenger side valve cover? Mine had caved after many years and miles of deterioration and caused a rough idle and a weird click-like sound by the passenger side head at idle. I ordered a complete new OEM one from Latemodel Restoration for like $36 dollars and my problem went away. Ford wanted $60 for just the elbow..

I looked around at every parts store, and none of them had a pre-molded vacuum elbow, so Autozone gave me a radiator hose with the same dimensions, but it did not hold up well, so if you're having that problem get the OEM replacement from LMR!
 

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The part I am talking about can be found here.
 

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That is the part which connects the plenum to the PCV valve but for my 03 Mach 1 the PCV valve is in the driver side valve cover - Ford part number is 6A664. They do not sell the elbow separate. I purchased Dorman HELP part 46025 which is a PVC end assembly for a Ford. I had to cut it down. I didn't purchase the complete part from Ford because I wasn't curtain if the problem was just a worn elbow. Could the PCV valve be bad? This Mach 1 has an electric heated PCV valve.
 
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