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Discussion Starter #1
After much searching and reading as well as changing parts and trying different tunes i am at the end of my whit and really just need some help. I have a 2004 Mustang gt it has a few bolt on mods nothing extreme however about 2 years ago it developed a bad hanging idle. It has pretty much sat in a garage since i didn't have time to mess with it and was frustrated. When i start the car it runs fine and even drives fine until i get over 1/3 throttle. At which point when i come to a stop sign or red light the idle is about 2200 rpm and holds there for about 8-15 seconds until it suddenly drops and the car dies. The car pulls hard under WOT or in other words seems to act fine. I have have had no luck at solving this problem and am really hoping someone can offer some help. The car has just been rotting in a garage for years now and i would like to do something with it.

Parts i have changed and repairs i have made// changed IAC, changed TPS sensor twice, adjusted throttle set screw to fully closed, blocked off PCV valve in case it was leaking vacuum, cleaned MAF, Then replaced the MAF, I have also unplugged the IAC while vehicle was running and the car dies right away so i can rule out bad vacuum leaks, I have purchased two different types of tunners and even had a custom mail order tune done neither helped, only trouble code in the PCM is a EGR code and that is because i removed all EGR equipment, I even have attached a data log file to this post in hopes someone might be able to make heads or tails out of it, It was taken with a SCT x4,

I am open for suggestions as at this point i have no idea what this could be...
 

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Super Moderator
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What throttle body are you running?
 

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Have you made any adjustments to the throttle blade. It sounds like your throttle blade may be hanging up.
 

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Discussion Starter #5
Have you made any adjustments to the throttle blade. It sounds like your throttle blade may be hanging up.
Yes i have, I have set the set screw to fully closed as well as tried other adjustments i have also cleaned it and used lube and extra springs on the return to make sure it wasn't sticking. Thank you for your responses i am open for anything at this point!
 

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Is there any way you can duplicate this while parked so you can see if the throttle is hanging or sticking? Im just thinking out side the box since you have been dealing with this for a while and already done alot of the things most would think to do.
 

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Discussion Starter #7
Is there any way you can duplicate this while parked so you can see if the throttle is hanging or sticking? Im just thinking out side the box since you have been dealing with this for a while and already done alot of the things most would think to do.
It is diffcult to duplicate this issue while the vehicle is parked which makes diagnosis even harder. However i can sometimes get the car to hang its idle by reeving it but i have pushed in on the throttle plate while this is going on and it did not change anything nor did the throttle plate have any movement left until fully closed. To take things even a step further i fully removed the set screw so the throttle plate was resting 100 percent closed with no obstruction.
 

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6 Cylinduh, Really Bruh?
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What brand iac did you replace with?

Have you checked the maf is getting proper voltage?

This is a tough one. Only time I ever had an issue similar to this was the idle air controller. You can read horror stories online where anything but motorcraft won't work right. I personally had luck with cheap house brands.

It's almost like something just isn't getting the right signal. Time to start probing around. Check and ensure the tps is getting the correct voltage. Try going to the stock tb if you have it. Sounds like the blade is getting possibly stuck. Never had experience with SR performance but they are a budget brand. I run bbk.

Again the only time I had this issue was the idle air. Stupid thing sucks on these Ford's. Had issues with it on my explorer and on my mustang. My explorer would blow thru them every year or two. My car only blew thru the oem luckily.
 

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Aftermarket IAC's do not play nice with these cars. I'm willing to bet money you got your IAC valve from O'Reilly right? Go pick up an OEM IAC from your local dealership, it should be about $180 including the gasket. If you're tight on cash, you can order an OEM valve from American muscle, but it's currently back ordered.

My 03 GT had this problem when I bought it, which is why the dealer sold it to me for so cheap. It idled at 2.2k rpms, ran very rich and died 3 times on the way back home. Swapped in the OEM valve the next day and it's been perfect ever since.

Sent from my SM-G955U using Tapatalk
 

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6 Cylinduh, Really Bruh?
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Verify this is the right iac for your car but i found it on rock auto and Amazon for $90 bucks. Amazon even had it on prime.

Motorcraft Cx1785 is the part number for the iac according to Rockauto. Even the Amazon reviews restates what apex says. Again verify because I know there are 2 different designs.
 

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6 Cylinduh, Really Bruh?
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Aftermarket IAC's do not play nice with these cars. I'm willing to bet money you got your IAC valve from O'Reilly right? Go pick up an OEM IAC from your local dealership, it should be about $180 including the gasket. If you're tight on cash, you can order an OEM valve from American muscle, but it's currently back ordered.

My 03 GT had this problem when I bought it, which is why the dealer sold it to me for so cheap. It idled at 2.2k rpms, ran very rich and died 3 times on the way back home. Swapped in the OEM valve the next day and it's been perfect ever since.

Sent from my SM-G955U using Tapatalk
It's funny you mention the cheap part. The guy i got my car from sold it to me cheap because of the coolant crossover crack. It was the plastic version but he thought it was going to be some ungodly repair. Old dude with 8 cars in the driveway. Also the vert top needed a new hydraulic lift cylinder. This was back in 2009.
 

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King Trashmouth
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The problem you describe is entirely related to the dashpot and idle functions, which utilize the IAC.

Looking at the log file, the desired idle RPM, throttle position, MAF signal, all look like what you would expect. Even the IACDC looks like you would expect as it goes from dashpot down to idle control. But the engine just drops dead. The commands look reasonable, which tells me the actual airflow isn't matching what it expects. There's two main factors in this.

1.) Air through the throttle body. Whatever is in your tune needs to match the actual value. Doesn't matter if you change it in the tune or with the stop screw, but the two need to agree. Setting it to a desired value is another topic altogether. But for calculating what it actually is-
Air through TB = MAF - air through IAC.
You can calculate air through IAC by taking the duty cycle and interpolating on the IAC transfer function.

2.) The IAC transfer function. If your transfer function is wrong relative to the IAC installed, it won't work right. I think this could be part of it. I see duty cycle drop and RPM shoots up. At another point it drops a little and there's no change. This could lend credence to a bad aftermarket IAC that doesn't match your values.

If you're interested, you can also log idle air integrator. That's like an idle air trim.

If I was your tuner I'd set your dashpot really soft and fairly high on idle values just to rule those out, keep fairly stock values in the IAC transfer function, idle air tables, and throttle bypass air. You'd fire it up, warm it up, and adjust the stop screw until it agreed with the tune value, then take it for a drive. With a good (preferably OEM) IAC, it ought drive pretty well.
 

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6 Cylinduh, Really Bruh?
Joined
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5,218 Posts
The problem you describe is entirely related to the dashpot and idle functions, which utilize the IAC.

Looking at the log file, the desired idle RPM, throttle position, MAF signal, all look like what you would expect. Even the IACDC looks like you would expect as it goes from dashpot down to idle control. But the engine just drops dead. The commands look reasonable, which tells me the actual airflow isn't matching what it expects. There's two main factors in this.

1.) Air through the throttle body. Whatever is in your tune needs to match the actual value. Doesn't matter if you change it in the tune or with the stop screw, but the two need to agree. Setting it to a desired value is another topic altogether. But for calculating what it actually is-
Air through TB = MAF - air through IAC.
You can calculate air through IAC by taking the duty cycle and interpolating on the IAC transfer function.

2.) The IAC transfer function. If your transfer function is wrong relative to the IAC installed, it won't work right. I think this could be part of it. I see duty cycle drop and RPM shoots up. At another point it drops a little and there's no change. This could lend credence to a bad aftermarket IAC that doesn't match your values.

If you're interested, you can also log idle air integrator. That's like an idle air trim.

If I was your tuner I'd set your dashpot really soft and fairly high on idle values just to rule those out, keep fairly stock values in the IAC transfer function, idle air tables, and throttle bypass air. You'd fire it up, warm it up, and adjust the stop screw until it agreed with the tune value, then take it for a drive. With a good (preferably OEM) IAC, it ought drive pretty well.
Damn you are a good man for looking over his log file for him. Props to you
 

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If the IAC is mis reporting its position, or if it is not sealing properly, wouldn't that produce the same symptoms while showing good log data?

I'm no tuner, but his symptoms are precisely identical to every description of this problem I've seen, and an aftermarket IAC was the culprit every time.

Sent from my SM-G955U using Tapatalk
 

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King Trashmouth
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This could lend credence to a bad aftermarket IAC that doesn't match your values.
If the IAC is mis reporting its position, or if it is not sealing properly, wouldn't that produce the same symptoms while showing good log data?

I'm no tuner, but his symptoms are precisely identical to every description of this problem I've seen, and an aftermarket IAC was the culprit every time.

Sent from my SM-G955U using Tapatalk
:serious:

Damn you are a good man for looking over his log file for him. Props to you
I think this may be the first time I've actually encountered someone looking at a log file on a forum. :crazy

This one seemed easy enough, plus I've spent waaaaaay too much time on idle/dashpot stuff.
 

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:serious:



I think this may be the first time I've actually encountered someone looking at a log file on a forum. :crazy

This one seemed easy enough, plus I've spent waaaaaay too much time on idle/dashpot stuff.
Oops. Just woke up and haven't had my caffeine fix yet. Sorry for the confusion.

Sent from my SM-G955U using Tapatalk
 

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Ive had a similar problem with my 02 gt after installing a c&l plenum and a 75mm tb and I even installed an iac restrictor plate. Ive given up on it lol. Its very common with these cars after installing a new tb. Realistically a bigger tb should have no effect on the dashpot function since the throttle plate is closed.

If you are getting a new iac if you can get it cheaper Hitachi makes the oem iac and I think so does Delphi. Look closely at the one you have as there are at least 2 different designs. One is smaller than the other and one has a vent and the other one doesnt.
 

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Discussion Starter #18
What brand iac did you replace with?

Have you checked the maf is getting proper voltage?

This is a tough one. Only time I ever had an issue similar to this was the idle air controller. You can read horror stories online where anything but motorcraft won't work right. I personally had luck with cheap house brands.

It's almost like something just isn't getting the right signal. Time to start probing around. Check and ensure the tps is getting the correct voltage. Try going to the stock tb if you have it. Sounds like the blade is getting possibly stuck. Never had experience with SR performance but they are a budget brand. I run bbk.

Again the only time I had this issue was the idle air. Stupid thing sucks on these Ford's. Had issues with it on my explorer and on my mustang. My explorer would blow thru them every year or two. My car only blew thru the oem luckily.
I will order a Motorcraft replacement IAC right now!

Aftermarket IAC's do not play nice with these cars. I'm willing to bet money you got your IAC valve from O'Reilly right? Go pick up an OEM IAC from your local dealership, it should be about $180 including the gasket. If you're tight on cash, you can order an OEM valve from American muscle, but it's currently back ordered.

My 03 GT had this problem when I bought it, which is why the dealer sold it to me for so cheap. It idled at 2.2k rpms, ran very rich and died 3 times on the way back home. Swapped in the OEM valve the next day and it's been perfect ever since.

Sent from my SM-G955U using Tapatalk
yes you are correct it is a basic parts house IAC and i will order a Motorcraft stock replacement right this second as i said i am open to any suggestions and will act accordingly!

Verify this is the right iac for your car but i found it on rock auto and Amazon for $90 bucks. Amazon even had it on prime.

Motorcraft Cx1785 is the part number for the iac according to Rockauto. Even the Amazon reviews restates what apex says. Again verify because I know there are 2 different designs.
Thank you for the part number i will use that to make sure i order the right part!

The problem you describe is entirely related to the dashpot and idle functions, which utilize the IAC.

Looking at the log file, the desired idle RPM, throttle position, MAF signal, all look like what you would expect. Even the IACDC looks like you would expect as it goes from dashpot down to idle control. But the engine just drops dead. The commands look reasonable, which tells me the actual airflow isn't matching what it expects. There's two main factors in this.

1.) Air through the throttle body. Whatever is in your tune needs to match the actual value. Doesn't matter if you change it in the tune or with the stop screw, but the two need to agree. Setting it to a desired value is another topic altogether. But for calculating what it actually is-
Air through TB = MAF - air through IAC.
You can calculate air through IAC by taking the duty cycle and interpolating on the IAC transfer function.

2.) The IAC transfer function. If your transfer function is wrong relative to the IAC installed, it won't work right. I think this could be part of it. I see duty cycle drop and RPM shoots up. At another point it drops a little and there's no change. This could lend credence to a bad aftermarket IAC that doesn't match your values.

If you're interested, you can also log idle air integrator. That's like an idle air trim.

If I was your tuner I'd set your dashpot really soft and fairly high on idle values just to rule those out, keep fairly stock values in the IAC transfer function, idle air tables, and throttle bypass air. You'd fire it up, warm it up, and adjust the stop screw until it agreed with the tune value, then take it for a drive. With a good (preferably OEM) IAC, it ought drive pretty well.
You sir are a hero and i am very grateful you have taken the time to look at that log file and provide me this feedback. As i stated above i will order the proper factory replacement IAC then i will proceed to have some tuning adjustments made on my vehicle following some of this advice you have shared so it really is appreciated. I am interested in learning how to log idle air integrator as well this might further help diagnosis this issue i will look into that tonight. I would also like to thank everyone that is responding you are all offering me mounds of advice and knowledge and it is greatly appreciated!
 

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6 Cylinduh, Really Bruh?
Joined
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5,218 Posts
Ive had a similar problem with my 02 gt after installing a c&l plenum and a 75mm tb and I even installed an iac restrictor plate. Ive given up on it lol. Its very common with these cars after installing a new tb. Realistically a bigger tb should have no effect on the dashpot function since the throttle plate is closed.

If you are getting a new iac if you can get it cheaper Hitachi makes the oem iac and I think so does Delphi. Look closely at the one you have as there are at least 2 different designs. One is smaller than the other and one has a vent and the other one doesnt.
That's interesting to me. I have a c&l plenum with a bbk 75mm TB with a eBay IAC restrictor plate and cheap rock auto IAC and no issues for years.

All I remember is in the c&l plenum i had to dremel out the hole for the tps sensor to get a proper voltage reading. It was at something nuts prior to this.
 
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