Disclaimer: I spent extensive time reading all posts I could find similar to this issue (I know about the infamous 4.6 hang issue) and replacing what I could, but still to no avail.
Also a MASSIVE thank you to all the folks here that helped me get this far just from lurking and reading the forums.
First I’ll start with the car: It’s a 2004 Mustang GT 5 Speed manual with the 4.6L 2v. I have a K&N FIPK intake, muffler delete (about 1 year ago and still have resonators/cats installed), Exedy stage 2 clutch and lightweight flywheel, and an X4 Bama tuner.
The problem started abruptly about a month ago (partly why I don’t believe it’s the standard RPM hanging issue). I started the car and it did the typical rev up and settled to about 750 rpm. As I started driving I noticed it wanted to stick at about 1500 rpm in gear with no gas, so my idle speed went from almost bogging in 1st gear to about 15mph. If I tried to reduce the rpm in gear with the brake then put the car in neutral it would jump back to 1500-1700 rpm and hang until I came to a complete dead stop at which point it would hang for 3-4 seconds and then drop below 750rpm and then settle back out. I know there’s a moving and stopped idle which are around 1000/750 but this was way too high. I figured it was cold or something and would go away. Since then it’s continued to exhibit these symptoms regardless of my attempted ‘fixes’.
The main consistent issues are the following:
- No check engine light for help.
- Any time the car is revved in neutral (even slightly) it will raise/lower rpm to 1500-1700 (occasionally 2700 rpm) and hang there for about 5-8 seconds.
- It doesn’t want to drop below that rpm in any gear without use of the brake, and as soon as it’s put in neutral it goes back to that 1500-1700 range until the car is at 0mph for 3-4 seconds.
- If the AC is on, after I come to a stop the rpms will hang then drop low enough to kill the car (assuming because of more load on engine belt?)
Attempted fixes:
- Removed tune to see if it fixed the issue, it didn’t but the rpms wouldn’t drop as low after hanging so I left the tune off.
- Started small with cleaning Throttle body, Air intake, and MAF then new IAC valve and TPS sensor. No change.
- Double checked TPS and IAC with a friends off his mustang that worked just fine, no change on mine.
- Assumed vacuum leak so I brought to a mechanic to be fogged. He ran a diagnostic and fogged it and couldn’t find any leak. He suggested warped intake leaking in the back or EGR valve.
- Took the plunge and bought a new intake manifold/75mm throttle body/75mm plenum/egr/thermostat/gaskets and spent a weekend replacing it all (put in new spark plugs while I was in there). Doubled checked the new throttle body screw and adjusted it so IAC duty cycle was 40% at idle. Still no improvement.
- Pulled PCV valve, appears to be fine and still rattles, next plan is to replace egr vacuum modulator.
Some other information/thoughts:
The LTFT is right at 1.00 for both banks, which make a vacuum leak less likely it would seem. If I disconnect the IAC wire plug the car will die once again operating normal. I also can’t get any engine bay vacuum lines to change idle with carb cleaner. I’m thinking it could be the back end of the evap line but I plugged its port on the plenum and still seem to have hanging idle.
I’m planning on using the copper cap trick to block up part of the IAC tube but wanted to use that as a last resort since this problem seems a bit more extreme since the rpm hang is higher/longer and the car likes dying with the ac on after if hangs. Could it just be revving higher than the 'standard issue' because of the lighter flywheel?
Any thoughts/suggestions/guidance would be much appreciated.
Also a MASSIVE thank you to all the folks here that helped me get this far just from lurking and reading the forums.
First I’ll start with the car: It’s a 2004 Mustang GT 5 Speed manual with the 4.6L 2v. I have a K&N FIPK intake, muffler delete (about 1 year ago and still have resonators/cats installed), Exedy stage 2 clutch and lightweight flywheel, and an X4 Bama tuner.
The problem started abruptly about a month ago (partly why I don’t believe it’s the standard RPM hanging issue). I started the car and it did the typical rev up and settled to about 750 rpm. As I started driving I noticed it wanted to stick at about 1500 rpm in gear with no gas, so my idle speed went from almost bogging in 1st gear to about 15mph. If I tried to reduce the rpm in gear with the brake then put the car in neutral it would jump back to 1500-1700 rpm and hang until I came to a complete dead stop at which point it would hang for 3-4 seconds and then drop below 750rpm and then settle back out. I know there’s a moving and stopped idle which are around 1000/750 but this was way too high. I figured it was cold or something and would go away. Since then it’s continued to exhibit these symptoms regardless of my attempted ‘fixes’.
The main consistent issues are the following:
- No check engine light for help.
- Any time the car is revved in neutral (even slightly) it will raise/lower rpm to 1500-1700 (occasionally 2700 rpm) and hang there for about 5-8 seconds.
- It doesn’t want to drop below that rpm in any gear without use of the brake, and as soon as it’s put in neutral it goes back to that 1500-1700 range until the car is at 0mph for 3-4 seconds.
- If the AC is on, after I come to a stop the rpms will hang then drop low enough to kill the car (assuming because of more load on engine belt?)
Attempted fixes:
- Removed tune to see if it fixed the issue, it didn’t but the rpms wouldn’t drop as low after hanging so I left the tune off.
- Started small with cleaning Throttle body, Air intake, and MAF then new IAC valve and TPS sensor. No change.
- Double checked TPS and IAC with a friends off his mustang that worked just fine, no change on mine.
- Assumed vacuum leak so I brought to a mechanic to be fogged. He ran a diagnostic and fogged it and couldn’t find any leak. He suggested warped intake leaking in the back or EGR valve.
- Took the plunge and bought a new intake manifold/75mm throttle body/75mm plenum/egr/thermostat/gaskets and spent a weekend replacing it all (put in new spark plugs while I was in there). Doubled checked the new throttle body screw and adjusted it so IAC duty cycle was 40% at idle. Still no improvement.
- Pulled PCV valve, appears to be fine and still rattles, next plan is to replace egr vacuum modulator.
Some other information/thoughts:
The LTFT is right at 1.00 for both banks, which make a vacuum leak less likely it would seem. If I disconnect the IAC wire plug the car will die once again operating normal. I also can’t get any engine bay vacuum lines to change idle with carb cleaner. I’m thinking it could be the back end of the evap line but I plugged its port on the plenum and still seem to have hanging idle.
I’m planning on using the copper cap trick to block up part of the IAC tube but wanted to use that as a last resort since this problem seems a bit more extreme since the rpm hang is higher/longer and the car likes dying with the ac on after if hangs. Could it just be revving higher than the 'standard issue' because of the lighter flywheel?
Any thoughts/suggestions/guidance would be much appreciated.