Idle Speed Problem - Ford Mustang Forum
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post #1 of 13 (permalink) Old 4 Weeks Ago Thread Starter
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Idle Speed Problem

After 3 days of digging through the web and forums, im stuck. I'm a carb guy, so this is a learning experience for sure. I'm having the issue where the idle seems sluggish to drop and idles way too high. I have a SCT Tuner and a tune loaded that has the car running like a top now. The issue im having is that its idling between 1200-1450. It doesnt surge it just seems to "choose" one of those to idle at every time it settles back into idle.


Here is some history of the car. The guy i bought it off of has removed the EGR (turned off in tune), Evap (dont know why...), full exhaust long tube headers to tail pipes with no cats. Rear O2s turned off in tune, It has an Air Raid CAI, 78MM BBK Throttle Body, AC Delete and 24lb injectors. The car was an automatic car that has been converted to a manual.



In the SCT i set Idle Neu and Idle Drv to 820. I ran the tune and changed the RPM up or down one setting a few times thinking maybe it didnt program quite right because when it wasnt running right i had the idle at 1100 to keep it idling.



I installed a brand new IAC because i didnt like the look of the old one inside and that solved the fact the car wouldnt idle lower than 1100 and solved the cold start issue i was having where i would have to hold the gas for about 30 seconds and then the car would ramp up and settle down and idle on its own.


I dont have access to a scan tool at the minute but the SCT gives codes and some data. I dont know how to read the TPS in the SCT (will have to search up a user manual for the SF3/X3). The Desired Idle shows right in the data but the computer doesnt ever throw an over idle code. I replaced the TPS and tried adjusting it with no change thinking the tps was telling the car i was on the gas a bit.


I sprayed intake cleaner all over the top of the engine multiple times and no change to idle. I installed a new PCV and inspected all hoses. I went through and removed any loose vacuum leads that were capped to eliminate as much line as possible that could have a leak.


Thinking maybe TPS i installed a new one. I dont know how to read its data right from the SCT either, its showing tp_v 138 at idle and climbs as you push the throttle (supposed to be between 0v and 1v supposedly so is this 0.138v?)


I changed spark plugs, did oil change, ran some redline injector cleaner and cleaned out the throttle body.



I then looked to the manual adjustment on the throttle body and adjusted it until the plates where completely closed and then cracked them back open about 1/4 of a turn with no change. I then unplugged the IAC and it immediately stalled (thinking this is good because a significant vacuum leak would of let it still let it run?). I set the idle as low as i could without the car stalling manually. at about 600RPM it would surge 600-750 so i left it there. I shut the car off and unhooked the negative from battery for 10 mins with lights on. Started it up and it idled at 2000 for 5 minutes (engine was at operating temp). I unplugged the IAC and the car sputtered and stalled. I started the car and it went back to surging 600-750. When i plugged the IAC back in you could hear it sucking air like mad and the idle ramp back up to 2000rpm. It sounded like sticking your thumb over most of a vacuum cleaner nozzle. I then set the plates closed manually and opened them just enough to not get stuck when closing and it was back to 1250 rpm with a desired of 820 at that point.


In case i introduced a vacuum leak while messing around i again sprayed every inch of the top of the motor down liberally with intake cleaner and no change in RPM. Since all of this it has run for about 45 minutes and done quite a few stop start cycles at stop signs and again this morning idles at about 1200.



I started looking over if any hoses were hooked to the wrong places and it looks fine. I did notice that if i unhooked the hose going to the drivers valve cover the idle would drop about 100RPM and pick back up when i hooked it back up. I also pulled the PCV and plugged it with my finger to see if it would change anything and it did not.



I might have missed a few other steps i took but i pretty much followed any and all high idle "fixes" i could find between forums and youtube videos. Any help would be appreciated.

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post #2 of 13 (permalink) Old 4 Weeks Ago
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IMO you do not have a great understanding of HOW the idle/IAC strategy actually works and trying to think of it like a carb or a speed density strategy.

The first thing I would like to know is if you realize that the idle is supposed to be elevated while the car is MOVING. This is for two reason. SMOG and it allows the hydro boost power brakes to have enough pressure. Many people do not realize that the PCM needs an accurate speed in order for the IAC to correctly select slow idle or fast idle.

The next thing is how the PCM uses the TPS to decide "closed throttle" verses "part throttle". The actual TPS voltage does not matter as long as the TPS sends the SAME voltage each time the throttle is closed. The PCM will re-learn the TPS closed throttle value each time the key is cycled. So any attempt to "adjust" the TPS to read a specific voltage is a waste of time.

The BEST way to trouble shoot a suspected TPS problem is to monitor the TP-MODE PID to see IF it matches "closed throttle" or "part throttle" at the time. What you are looking for is to see if the TP-MODE does not match the actual driving mode. For example if the PCM "thinks" that the driver is holding the throttle partially open (part throttle), then it's reasonable to have a fast idle. Make sense?

Regarding the unstable surging idle. The first cause for this is to rule out throttle butterfly movement when the throttle is supposed to be closed. The usual cause for this is the throttle butterfly is not solidly resting on the idle stop screw. This can allow the throttle to move as vacuum increases/decreases. This further worsen the problem as TP-MODE switches back and forth between "closed" and "part" throttle. This in turn causes the PCM to switch back and forth between fast and slow idle. Hence the reason WHY a stable TPS value is critical to a stable idle.

Here's some more information that can help.

Trouble shooting IAC idle problems 1996-2004:
https://www.allfordmustangs.com/foru...9-post200.html

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post #3 of 13 (permalink) Old 4 Weeks Ago Thread Starter
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Quote:
Originally Posted by wmburns View Post
IMO you do not have a great understanding of HOW the idle/IAC strategy actually works and trying to think of it like a carb or a speed density strategy.

The first thing I would like to know is if you realize that the idle is supposed to be elevated while the car is MOVING. This is for two reason. SMOG and it allows the hydro boost power brakes to have enough pressure. Many people do not realize that the PCM needs an accurate speed in order for the IAC to correctly select slow idle or fast idle.

The next thing is how the PCM uses the TPS to decide "closed throttle" verses "part throttle". The actual TPS voltage does not matter as long as the TPS sends the SAME voltage each time the throttle is closed. The PCM will re-learn the TPS closed throttle value each time the key is cycled. So any attempt to "adjust" the TPS to read a specific voltage is a waste of time.

The BEST way to trouble shoot a suspected TPS problem is to monitor the TP-MODE PID to see IF it matches "closed throttle" or "part throttle" at the time. What you are looking for is to see if the TP-MODE does not match the actual driving mode. For example if the PCM "thinks" that the driver is holding the throttle partially open (part throttle), then it's reasonable to have a fast idle. Make sense?

Regarding the unstable surging idle. The first cause for this is to rule out throttle butterfly movement when the throttle is supposed to be closed. The usual cause for this is the throttle butterfly is not solidly resting on the idle stop screw. This can allow the throttle to move as vacuum increases/decreases. This further worsen the problem as TP-MODE switches back and forth between "closed" and "part" throttle. This in turn causes the PCM to switch back and forth between fast and slow idle. Hence the reason WHY a stable TPS value is critical to a stable idle.

Here's some more information that can help.

Trouble shooting IAC idle problems 1996-2004:
https://www.allfordmustangs.com/foru...9-post200.html

So reading what you say and what is in that guide. When i adjust the idle screw so that the plates are basically completely closed (just open enough to not get sticky when opening) i get 1200ish RPM idle with a desired of 820. If i unplug the IAC i stall immediately. It is the black vent style so according to your write up it should still run at a lower rpm? I will have to try to get it on an actual scanner for the rest of the info. The surging was with IAC disconnected and manually setting an idle with the screw to the lowest possible setting before stalling. If i set the screw to idle at this 600ish rpm range with IAC unplugged, when i plugged it in it idled around 2000rpm so i adjusted it back to basically closed. This is also at idle in neutral with no accessories on including AC or Defrost.


Currently i have no speedo. It appears the spot on the trans has a cover over it and has no sensor so i will look at that. Didnt think missing a speedo sensor would effect idle.
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post #4 of 13 (permalink) Old 4 Weeks Ago
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I don't believe this thread stated the model year of the Mustang.
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Originally Posted by Jesse Coles View Post
So reading what you say and what is in that guide. When i adjust the idle screw so that the plates are basically completely closed (just open enough to not get sticky when opening) i get 1200ish RPM idle with a desired of 820.
Just wondering. Did you shut off the ignition after adjusting the idle stop screw? This step is necessary to allow the PCM to re-learn the TPS closed throttle position.

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Originally Posted by Jesse Coles View Post
If i unplug the IAC i stall immediately. It is the black vent style so according to your write up it should still run at a lower rpm?
This depends upon where in the adjustment range the IAC is at the start of the test. So having the motor die for a black vented IAC is not a fail. The take away should be the fact that for the black vented IAC the idle must drop below the point of it being a good idle. This proves two things. That the IAC is capable of lowering the idle and the the IAC is allowing bypass air to pass at the time.

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Originally Posted by Jesse Coles View Post
Currently i have no speedo. It appears the spot on the transmission has a cover over it and has no sensor so i will look at that. Didnt think missing a speedo sensor would effect idle.
You miss understand. Not have a speedometer itself isn't going to affect the idle. What matters is IF the PCM knows the car's speed. Again here's where an ODB2 scanner can help to quickly answer these sort of questions.

Here's some information on a very affordable ($30) Windows based ODB2 scanner.

ForScan ODB2 scanner w ELM327 USB
https://www.stangnet.com/mustang-for...elm327-usb.57/
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post #5 of 13 (permalink) Old 4 Weeks Ago Thread Starter
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Quote:
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I don't believe this thread stated the model year of the Mustang.

I'm sorry, it is a 2000.


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Originally Posted by wmburns View Post
Just wondering. Did you shut off the ignition after adjusting the idle stop screw? This step is necessary to allow the PCM to re-learn the TPS closed throttle position.

When i did this step what i did was shut off the car, open the screw a bit and try to start it. I repeated this until it ran without the IAC plugged in and then adjusted it back until the motor was almost at stall condition. I plugged the IAC in and it jumped to 2000. Then i did shut the car off and start it again and it went right to 2000 rpm. Now at this point i didnt let it idle for long, maybe a minute or two. Car was already at operating temp long before this point.


By now i was getting rather frustrated and i just reset the screw back to basically closed, then started the car again. It was back to 1200 rpm at this point so i disconnected the battery again (to relearn idle as some videos and forum posts instructed). after 5 mins i reconnected it and went home for the night with no change.



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Originally Posted by wmburns View Post

You miss understand. Not have a speedometer itself isn't going to affect the idle. What matters is IF the PCM knows the car's speed. Again here's where an ODB2 scanner can help to quickly answer these sort of questions.

The pcm would need the speed sensor to know the cars speed would it not? or does it fall back to ABS sensors for that? I just grabbed a Launch V+ scanner and will try it out tonight and report readings i get.


I can rebuild modified engines all day long, bust book times on most everything i do, align a car like no bodys business, adjust a 4bbl carb to perfection but this FI is so frustrating to me lol.
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Originally Posted by Jesse Coles View Post
I'm sorry, it is a 2000.





When i did this step what i did was shut off the car, open the screw a bit and try to start it. I repeated this until it ran without the IAC plugged in and then adjusted it back until the motor was almost at stall condition. I plugged the IAC in and it jumped to 2000. Then i did shut the car off and start it again and it went right to 2000 rpm. Now at this point i didnt let it idle for long, maybe a minute or two. Car was already at operating temp long before this point.


By now i was getting rather frustrated and i just reset the screw back to basically closed, then started the car again. It was back to 1200 rpm at this point so i disconnected the battery again (to relearn idle as some videos and forum posts instructed). after 5 mins i reconnected it and went home for the night with no change.






The pcm would need the speed sensor to know the cars speed would it not? or does it fall back to ABS sensors for that? I just grabbed a Launch V+ scanner and will try it out tonight and report readings i get.


I can rebuild modified engines all day long, bust book times on most everything i do, align a car like no bodys business, adjust a 4bbl carb to perfection but this FI is so frustrating to me lol.



Are you attempting all this with the tune loaded up or did you revert back to the stock tune before attempting to do a base idle reset?


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Are you attempting all this with the tune loaded up or did you revert back to the stock tune before attempting to do a base idle reset?
This is all with the loaded tune. Without the tune i get EGR and O2 codes. Figured that would put the car into "limp in" base default mode.

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Quote:
Originally Posted by Jesse Coles View Post
When i did this step what i did was shut off the car, open the screw a bit and try to start it. I repeated this until it ran without the IAC plugged in and then adjusted it back until the motor was almost at stall condition. I plugged the IAC in and it jumped to 2000. Then i did shut the car off and start it again and it went right to 2000 rpm. Now at this point i didnt let it idle for long, maybe a minute or two. Car was already at operating temp long before this point.
In my experience it does take the PCM time to re-learn idle trim values after a battery disconnect. This does not happen immediately. Why is this important? Because it should be expected to have a poor idle after a battery disconnect (or DTC code clear).

Pro tip: Try to avoid disconnecting the battery during trouble shooting. This will likely slow down or confuse trouble shooting because it increases the number of variables to deal with. At this point it not entirely clear to me if this is the cause of the 2000 RPM idle.

IF the motor is idling at 2000 RPM's AND disconnecting the IAC drops the idle to near stall levels, THEN this proves that the IAC is raising the idle. Likely because the PCM is "commanding" it. The next step is to figure out WHY.

Previously you stated that you had access to TPS voltage. Use the scanner and make note of the TPS volts after ignition power on. Later when the idle is too high compare the TPS voltage to the initial start value. Is it the same, lower, or higher? If higher, likely this means the PCM "thinks" that someone is pressing on the throttle (part throttle mode). This could be the reason why the PCM is calling for a fast idle. The next trick is to figure out why the fast idle is so fast. Again, this could be a function of the PCM needing to re-learn.
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post #9 of 13 (permalink) Old 4 Weeks Ago Thread Starter
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Originally Posted by wmburns View Post
In my experience it does take the PCM time to re-learn idle trim values after a battery disconnect. This does not happen immediately. Why is this important? Because it should be expected to have a poor idle after a battery disconnect (or DTC code clear).

Pro tip: Try to avoid disconnecting the battery during trouble shooting. This will likely slow down or confuse trouble shooting because it increases the number of variables to deal with. At this point it not entirely clear to me if this is the cause of the 2000 RPM idle.

IF the motor is idling at 2000 RPM's AND disconnecting the IAC drops the idle to near stall levels, THEN this proves that the IAC is raising the idle. Likely because the PCM is "commanding" it. The next step is to figure out WHY.

Previously you stated that you had access to TPS voltage. Use the scanner and make note of the TPS volts after ignition power on. Later when the idle is too high compare the TPS voltage to the initial start value. Is it the same, lower, or higher? If higher, likely this means the PCM "thinks" that someone is pressing on the throttle (part throttle mode). This could be the reason why the PCM is calling for a fast idle. The next trick is to figure out why the fast idle is so fast. Again, this could be a function of the PCM needing to re-learn.
Thanks for all of your info so far. I obviously caused myself more work then i should have following these different google results for fixes. I ran the scanner i got quickly when i was out for lunch and didnt get into it enough to see what is available where. I did see "Absolute Throttle Position" in there and it was at 13.3% at idle but i was in graphing options and i think there is more options in the non-graphing section i will go through tonight still.

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So to keep anyone who may care up to date. Between my 2 devices i still dont get enough info. What i may have determined though is a few people saying that since the car had a manual swapped into it and the auto ecu still being used, that the auto had different idle characteristics than the manuals did and that since the ecu thinks the tranny is in total failure, it may be affecting rpms in an attempt to save the tranny. So i am getting a custom tune written to delete the auto functions from the ECU to hopefully sold that. I had to adjust the butterfly on the throttle body to completely closed to get it to idle around 850 (desired 670) but with still no codes. The idle is still intermittent where it starts and runs at 850 one time then 1250 another and then when driving, a few stops will idle at 850 and then others 1200ish and then others 1400ish. It never really changes or surges it just picks one. The rpms also drop to which ever it picks REALLY slowely which is what my tuner thinks is coming from the auto strategy on the ECU.


I will keep this up to date as i go in case others have the same issue as i find solutions.
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So, i think the tuner was wrong. For some reason the car started surging and stalling. Had to open the throttle adjustment to keep it running at 800 rpm. Im assuming this was just a REALLY long relearn time. Once this happened it would vary much less but still take way too long to come down to 800. I found where the previous owner clipped a wire to install a wideband af sensor and it was the power lead to the O2 heaters. Repaired that and all was good. It comes down slower than a carbed car but that is to be expected. It does however now always settle to the same RPM and does so within a couple seconds. So i think Case Solved. Thanks Guys.
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Originally Posted by Jesse Coles View Post
So, i think the tuner was wrong. For some reason the car started surging and stalling. Had to open the throttle adjustment to keep it running at 800 rpm. Im assuming this was just a REALLY long relearn time. Once this happened it would vary much less but still take way too long to come down to 800. I found where the previous owner clipped a wire to install a wideband af sensor and it was the power lead to the O2 heaters. Repaired that and all was good. It comes down slower than a carbed car but that is to be expected. It does however now always settle to the same RPM and does so within a couple seconds. So i think Case Solved. Thanks Guys.
Glad you found it and thanks for the update


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Quote:
Originally Posted by Jesse Coles View Post
So, i think the tuner was wrong. For some reason the car started surging and stalling. Had to open the throttle adjustment to keep it running at 800 rpm. Im assuming this was just a REALLY long relearn time. Once this happened it would vary much less but still take way too long to come down to 800. I found where the previous owner clipped a wire to install a wideband af sensor and it was the power lead to the O2 heaters. Repaired that and all was good. It comes down slower than a carbed car but that is to be expected. It does however now always settle to the same RPM and does so within a couple seconds. So i think Case Solved. Thanks Guys.
Will the RPM’s drop fast enough where it will engine brake if you let off the gas when you’re cruising? Or does it just cruise on when you let off of the gas?

Last edited by 90lxwhite; 2 Weeks Ago at 03:59 PM.
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