High Hanging Rpm Issues(2004 Gt)

nogripcal

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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.
 
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wmburns

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

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

Please look at the parts about checking the basic charging system function.

Seems like you have covered most of the bases that trip up other people.

Is the AC short cycling or low on Freon?

Is the speedometer accurate and working?

Please explain exactly what happens when the IAC electrical connection is unplugged. I'm confused if the motor runs normally or dies.

If you monitor the IAC duty percent does it change when the throttle is moved or RPM's go up/down?

Since you have an ODB2 scanner, also monitor the TP-MODE PID. What I want to know is what the PCM "thinks" the throttle mode is when the problem happens. The mode is likely to be CLOSED, PART, or WOT. What I'm trying to see does the PCM "think" the throttle is "part throttle" when the idle is high.
 

nogripcal

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Jun 16, 2017
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Here's some information on how to trouble shoot idle and IAC issues.

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

Please look at the parts about checking the basic charging system function.

Seems like you have covered most of the bases that trip up other people.

Is the AC short cycling or low on Freon?

Is the speedometer accurate and working?

Please explain exactly what happens when the IAC electrical connection is unplugged. I'm confused if the motor runs normally or dies.

If you monitor the IAC duty percent does it change when the throttle is moved or RPM's go up/down?

Since you have an ODB2 scanner, also monitor the TP-MODE PID. What I want to know is what the PCM "thinks" the throttle mode is when the problem happens. The mode is likely to be CLOSED, PART, or WOT. What I'm trying to see does the PCM "think" the throttle is "part throttle" when the idle is high.

This evening I'll run through the charging system and check it all. Im also not entirely convinced the autozone tps isnt bad. I may not have given my buddys tps enough time after clearing the KAM so ill run a voltage test on that too for .9~1v and smooth gain/drop.

I'll check the pressure on the AC and watch for short cycling.
*Side note: the AC knocks when i turn it on, i figured it was the clutch starting to go out but its been like that for 8ish months. Do you think the two problems could be related?

The speedometer works fine and is only off by 10% because i removed the tune accounting for larger tires haha.

As far as the IAC, as soon as i unplug its sensor while the car is idling it will die. It'll fight to stay alive maybe 2 seconds.

Ill double check the duty cycle adjusting with rpms, as well as check the TP-MODE you described.

I'll try get a reply to you in the morning. Thanks a ton!
 

wmburns

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Please don't spend any time trying to "adjust" the TPS. The absolute value of the TPS does not matter. What matters is for the TPS to send the SAME value each time the throttle is closed. Read the part about the "ratch" function.

Also remember that the PCM learns the new TPS value at each start up. Soooooo after swapping a TPS the only thing necessary is to turn off and back on the key.

That is one of the reasons for monitoring the TP-MODE PID as this is the throttle mode as the PCM understands it.

The PCM maintains two IAC idle trim values. AC on and AC off. So if there's an AC problem such that the AC can't run long enough for the PCM to re-learn new AC on idle trim values, then it makes sense for the idle to be poor with the AC on.
 
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nogripcal

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I only had a bit of time this morning to check with my reader and the closest my reader has to a TP mode is the Absolute Throttle Position which is a 0-1000 value. At idle it sits at 190ish and fluctuates higher when I press the gas.

The IAC duty cycle goes up with rpms when I press the gas pedal and stays high when it hangs then drops back down to 40% at standard idle.

Another symptom is awful gas mileage, I don't know it that might help figure out where the leak could be. But over half a tank only got me 90 miles. Just food for thought.

The idle and hang issue does get significantly worse with the AC on but it's pretty bad even without it.

What you're saying about the TPS makes perfect sense.

Thank you for the help I'll update you with the AC side of things tomorrow
 

nogripcal

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I only had a bit of time this morning to check with my reader and the closest my reader has to a TP mode is the Absolute Throttle Position which is a 0-1000 value. At idle it sits at 190ish and fluctuates higher when I press the gas.

The IAC duty cycle goes up with rpms when I press the gas pedal and stays high when it hangs then drops back down to 40% at standard idle.

Another symptom is awful gas mileage, I don't know it that might help figure out where the leak could be. But over half a tank only got me 90 miles. Just food for thought.

The idle and hang issue does get significantly worse with the AC on but it's pretty bad even without it.

What you're saying about the TPS makes perfect sense.

Thank you for the help I'll update you with the AC side of things tomorrow
 

wmburns

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IF LTFT is 1.00 then I agree that a vacuum leak is unlikely. However, I personally have never encounter a vehicle that had exactly 1.00 LTFT. If this were my car I would confirm that the PCM has fuel control. Perhaps create a small vacuum leak and CONFIRM that the LTFT and STFT actually changes in response to the created vacuum leak.

Also does it make any difference if the MAF electrical connector is disconnected?
 

nogripcal

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Alrighty just ran a bunch of tests and heres what ive figured out:

AC system isnt short cycling, its staying on just fine though it was low on freon so i added more(based on ambient temp) and got that settled.

Afterward i cleared my KAM and let it relearn its idle for about 10 minutes with AC on and 10 mins with it off, then about 20 minutes of driving with AC on one way and off the other. At this point the engine is still trying to die if the AC is on while the ECU tries to push the idle below where its hanging up at 1700 RPM. it usually over corrects the idle to about 500 RPM and almost dies before coming back to 750 RPM.
*Note when its fighting the hanging RPMs the STFT only fluctuates 3% over or under 100% on both banks.

I started data-logging at this point to watch LTFT and STFT and see any changes.

I found the TP-MODE you discussed earlier, and at idle it stays at -1.00, if my foots on the throttle at all its at 0 (assuming thats part throttle?) and if i go WOT it goes to 1.00. As soon as my foot comes out from max throttle it instantly drops to 0 and as soon as my foot comes all the way off the pedal(even if the rpms are hanging) the value returns to -1.00.

I induced a consistent vaccum leak at the EVAP hose at idle, and STFT dropped to 80% and after about 5 mins of rising LTFT hit 120% to balance it. seems to be working well.

While driving around regularly the LTFT and STFT seem to both stay within 5% of 100. the one condition i could get STFT to act up was under moderately strong throttle. it would always sink to 80-85% during acceleration and then when acceleration stopped it could go over to 110%. Then it would balance back to 100% bouncing around by +/-5%.
**Does this make sense? it would seem that the MAF should account for the extra air and the fuel trim shouldn't get that off right?

One last thing, I tried the "copper cap trick", while it did fix the hanging rpm issue, it made my standard idle rise to 1000 RPM with AC on and 1400 RPM with the AC off.

Also the car dies as soon as the MAF is unplugged.
 

wmburns

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One last thing, I tried the "copper cap trick", while it did fix the hanging rpm issue, it made my standard idle rise to 1000 RPM with AC on and 1400 RPM with the AC off.
^^^^IMO this is IMPORTANT. Important enough to perhaps be the smoking gun.

Think about it. IF the IAC bypass line has been blocked and this CAUSES the idle to still be high this all but PROVES there's air getting past the throttle body from SOMEWHERE.

This is a 2004 model year correct? As such this should have the non black vented version of the IAC. Are you absolutely positive you are using the correct IAC for the application? For the 2004 model year the PCM has more control over the idle using the IAC. Therefore the IAC line should be the only source of bypass air. We are back to the inconvenient truth that there just has to be air entering from someplace other than the IAC.

My vote is this is NOT a vacuum leak. Why? Because the LTFT's look so good. The fuel control is solid. That's why I'm thinking the extra air is in fact metered air. Say for example through the throttle body butterfly valve. I'm thinking:
  • Butterfly valve not seating fully allowing air to pass.
  • incorrect throttle body used for the application. For example the tune is expecting a throttle body with no bleed holes and a version with bleed holes has been used.
  • Other sources of metered air could be the PVC system. Perhaps totally block off the PCV line for a short test to see if the idle improves.
It seems to me that we are running out of "easy" fixes. But my experience also tells me that often when faced with a difficult head scratcher that defies good trouble shooting that it's time to go back and double check some of the base assumptions. By that, look at some part/system that was assumed to be good.

I'm also a big believer in Occam's Razor. Which means that we shouldn't be too quick to look for ever increasing complex solutions to a problem. Or at the least be open to the possibility that something simple has been overlooked.

https://en.wikipedia.org/wiki/Occam's_razor

What ODB2 scanner are you using?
 

nogripcal

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Im positive the IAC is the correct one, its the single hole one with just a motor and port on top of it for the electrical input. Is there a surefire way i can test its function, maybe with a volt meter? I know it dies if i unplug the electrical connector but past that I dont have a good way to check that its operating 100%.

The throttle body has no bleed holes or sticky spots in its rotation that i can see and the blade looks to be very flush with the body. It seats fully while running and off. Even when closed slowly from both the gas pedal and throttle body itself.

Short of checking that PCV valve and spraying the hose with carb cleaner i havent done much with it. Ill try blocking it off today and check operation/idle. If this helps would that indicate a bad PCV valve or something else?

Im using the SCT X4 Tuner i got for the car btw.

Side note, I took it to work today and initially it drove perfectly normal, I assume because my LTFT's were still adjusted up to compensate for the vacuum leak i made.
 

wmburns

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Short of checking that PCV valve and spraying the hose with carb cleaner i havent done much with it. Ill try blocking it off today and check operation/idle. If this helps would that indicate a bad PCV valve or something else?
I don't think that you understand exactly what/where the PCV is leaking. It is NOT leaking externally. If it were, it would be drawing in un-metered air.

I'm saying maybe the PCV is leaking INTERNALLY. Such that it's drawing in metered air from the engine crank case. Are you assuming that just because the PCV valve rattles when shaken that it actually seals when a vacuum is applied?

Myself personally I would just replace the PCV valve. After all it is a maintenance item.

The alternative is some sort of base motor problem such that excessive blow by is getting past the piston oil control rings. Perhaps a compression test and/or a cylinder leak down test is in order.
 
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nogripcal

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Update for the day:

I replaced the PCV valve as suggested and even though I already cleaned the MAF, went ahead and decided to replace it too.

Results were no improvement, actually worse hanging at higher rpm now but now I'm getting legitimate data about the fuel trim.

I don't know if maybe it was a partial issue with both since I changed them at the same time but air data was not being read correctly and now is. I cleared the KAM and after driving around for about 30 minutes and noticing it now hangs at 1750RPM, here's the new data:
At idle the LTFT is anywhere from 110%-112%. Under load it decreases to maybe 102%ish. Also the 2nd bank averages about 4% higher on STFT/LTFT than bank 1 always.

Checking with propane, I still couldn't find a vacuum leak and did triple check the areas I worked in to make sure I didn't make the leak myself.

I'm going to reclean this new MAF with some MAF cleaner just to make sure it isn't that but still no codes, the LTFT is just way high.
 
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nogripcal

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Another update, I'm watching my O2 sensors and they seem to jump around wildly regardless of driving/idle anywhere from .6v to -.5v which seems really odd to me. They generally fluctuate between .06v(really low?) and -.46v though. The negative portion is really throwing me off.
 

wmburns

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In some of the links provided did you see:

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

LTFT staying within the +/- 10 percent range is generally considered OK.

Regarding the MAF. This is a new MAF right? Seems silly to think that there's a problem with a brand new unit. Seems more plausible to me there's something wrong with HOW the unit is installed. For example some cold air intakes are not very good. Others are just too big for stock motors. The stock Ford air tube and MAF holder are the best for trouble shooting.

Have you considered a fuel injector "issue"? If so consider a fuel injector cleaning/testing service such as InjectorRX.com. Once cleaned and tested they are as good as new. Cheaper than new. Best of all is the report documenting the change. This will let you know if there was even a problem to begin with.

Have you data logged fuel pressure?
 

BGberg31

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Another update, I'm watching my O2 sensors and they seem to jump around wildly regardless of driving/idle anywhere from .6v to -.5v which seems really odd to me. They generally fluctuate between .06v(really low?) and -.46v though. The negative portion is really throwing me off.
Did you ever find a solution???