Infamous Idle Hanging Issue....need help.

Alright....before you start flaming with me starting another thread, I have used the search option and came up with nothing similar to my issue.


OK......so, I've read all about these idle issues with our Stangs and eventually came across the IAC restrictor plate "mod". I first cleaned my IAC and MAF with MAF cleaner and Q-tips. All is spotless. Then, I bought one of these restrictor plates off of ebay for like $3.50 and installed it yesterday afternoon. I used the smallest hole (don't know exactly what it measures) and all was good....until I took it for a test drive. Now, when I'm up in the RPM's and shift through gears, it does what it's supposed to, which is make the RPM's drop quicker and I love that. It does exactly that with no problems. It's when I coast in neutral or come to a stop, that the RPM's still hang high for about 3-5 secs at like 1500-1600 RPMs! And even when it drops back down after the 3-5 secs, my idle RPM's sit at like 900-1000 RPMs instead of the normal 750-800 RPMs?? I love everything about this restrictor plate as far as dropping the RPMs faster during driving....but the hanging idle really really annoys me to no end. In addition, I have a SCT SF3 tuner and thought that I would just lower the neutral idle and it'll be fixed. Nope. I have the driving idle set at the lowest it will go (can't remember at the moment) and the neutral idle RPM's set at the lowest it will go (456 RPMs IIRC), and it stills idles at around 900-1000 RPMs. What's the deal??? Do I need to adjust something else in the tuner?? Any SCT SF3 experts out there to help?? I will get all of my current settings from the SCT at lunchtime and list them to see what I need to change.

My mods are:

262 Comp Cams
Densecharger intake - K&N Filter
MSD ignition/coilpacks/wires
Stock TB/Plenum
Stock MAF
Stock injectors
Stock everything else motor wise.


Any ideas!?

Ive i can remeber correctly there may be a throttle adjustment screw for the throttle stop on the accelerator cable on the TB. I would see if you have one and see if that can be adjusted. Also you can pull the Negative battery cable over night and let the ECU reset itself. That may help

My car idles as yours does when i sit still, but thats just cause mine is 18 years old!!! lol. But no, my reason is because of a sticky bbk TB. Check to make sure nothing is stuck in your TB first. thats where id start

RTW0223 wrote:My car idles as yours does when i sit still, but thats just cause mine is 18 years old!!! lol. But no, my reason is because of a sticky bbk TB. Check to make sure nothing is stuck in your TB first. thats where id start

lol my cars 27 years old and shell idle at 500 rpm

Mine is 5 and it doesnt idle at all..

chrisverneuil wrote:Ive i can remeber correctly there may be a throttle adjustment screw for the throttle stop on the accelerator cable on the TB. I would see if you have one and see if that can be adjusted. Also you can pull the Negative battery cable over night and let the ECU reset itself. That may help

I think I read that somewhere and a few people said NOT to mess with the screw. I have/do pull the NEG battery cable everytime I change something with the SCT.

I can be cruising at 40 MPH in 5th gear with the RPMs around 1200-1300, I push the clutch in while still coasting, and the RPMs jump up to 1700, and hang there until I come to a complete stop. Then 3-4 secs later, it comes back down to 900-1000 RPMs to idle when completely stopped. That's still not the correct neutral idle speed even though I have it set to the lowest on the SCT. It sounds like ass and it's annoying as hell. I'm already thinking about removing the plate if I can't figure this out soon. All was "normal" before the plate.

sounds just like me.... check your TB

BOOM_FIST wrote:

chrisverneuil wrote:Ive i can remeber correctly there may be a throttle adjustment screw for the throttle stop on the accelerator cable on the TB. I would see if you have one and see if that can be adjusted. Also you can pull the Negative battery cable over night and let the ECU reset itself. That may help

I think I read that somewhere and a few people said NOT to mess with the screw. I have/do pull the NEG battery cable everytime I change something with the SCT.

I can be cruising at 40 MPH in 5th gear with the RPMs around 1200-1300, I push the clutch in while still coasting, and the RPMs jump up to 1700, and hang there until I come to a complete stop. Then 3-4 secs later, it comes back down to 900-1000 RPMs to idle when completely stopped. That's still not the correct neutral idle speed even though I have it set to the lowest on the SCT. It sounds like ass and it's annoying as hell. I'm already thinking about removing the plate if I can't figure this out soon. All was "normal" before the plate.

Try this. Disconect the IAC and see how it runs. With it disconected it shouldnt idle. If it does that could be unmetered air. The jumpin in idle when the cluth is pushed in sounds like it could be unmetered air. Just something you can check

RTW0223 wrote:sounds just like me.... check your TB

The TB should be fine. I had it off less than a month ago and cleaned the crap out of it.

chrisverneuil wrote:

BOOM_FIST wrote:

chrisverneuil wrote:Ive i can remeber correctly there may be a throttle adjustment screw for the throttle stop on the accelerator cable on the TB. I would see if you have one and see if that can be adjusted. Also you can pull the Negative battery cable over night and let the ECU reset itself. That may help

I think I read that somewhere and a few people said NOT to mess with the screw. I have/do pull the NEG battery cable everytime I change something with the SCT.

I can be cruising at 40 MPH in 5th gear with the RPMs around 1200-1300, I push the clutch in while still coasting, and the RPMs jump up to 1700, and hang there until I come to a complete stop. Then 3-4 secs later, it comes back down to 900-1000 RPMs to idle when completely stopped. That's still not the correct neutral idle speed even though I have it set to the lowest on the SCT. It sounds like ass and it's annoying as hell. I'm already thinking about removing the plate if I can't figure this out soon. All was "normal" before the plate.

Try this. Disconect the IAC and see how it runs. With it disconected it shouldnt idle. If it does that could be unmetered air. The jumpin in idle when the cluth is pushed in sounds like it could be unmetered air. Just something you can check

I'll try this, probably going to take the plate out if this doesn't work.

So I flipped the plate around to the bigger hole, and it put the neutral idle back down where it's supposed to be, but now the slow falling RPMs when I shift and drive is back, the whole purpose of me getting the plate. So I guess you really can't have both unless you get your car professionally tuned. I plan on boring the smaller hole out a little bit to try and get an even medium of the two holes and see what that does. Worst case, I'll make some more plates and keep doing the trial and error thing.

More importantly, messing under the hood has made me find a few more minor issues....coolant leak!!! Woo-Hoooo!

After my H/C/I Build my engine does this as well now. Thought it was normal, mine will hang around 1300 rpms when I come to a stop, then fall in the next few seconds. I'm thinking it's the upgraded cam cuz that's what me and you have in common and we're both untuned yet.

The92Foxbody wrote:After my H/C/I Build my engine does this as well now. Thought it was normal, mine will hang around 1300 rpms when I come to a stop, then fall in the next few seconds. I'm thinking it's the upgraded cam cuz that's what me and you have in common and we're both untuned yet.

mine does that too

RTW0223 wrote:

The92Foxbody wrote:After my H/C/I Build my engine does this as well now. Thought it was normal, mine will hang around 1300 rpms when I come to a stop, then fall in the next few seconds. I'm thinking it's the upgraded cam cuz that's what me and you have in common and we're both untuned yet.

mine does that too

Your upper/lower intake could be causing the prob. They say it's a bad idea to only upgrade one part of the engine, its better to do it as a whole h/c/i upgrade. Plus you have an aftermarket t/b that you said you were having probs with. Or maybe it's that we're getting much more airflow then originally so it is normal? This is something I don't have actual facts for but I do not it's not a big enough issue to make me worry about it.

Simple fix guys. I get this in my shop all the time.

Start the car. Let is warm up. Drive it for about 20 mins. Get it good and hot. Leave it running. Unhook the IAC. Use a small flat screwdriver and back off the throttle stop screw till it barely runs. (just about to stall) then hook the iac back up and the rpms should return to idle.
Turn the ignition on with the engine off. There are 3 wires going to the tps. They are ground, 5 volt refrence and a variable output voltage. Find the variable output and check to make sure its reading between .80-.90 volts. You may have to loosen it and turn it slightly to get the correct voltage. This will tell the computer you want to idle. (your not giving it any gas)

This is the percedure that I use and it seems to work. There is a correct percedure on doing this but only works on bone stock engines. If you need further help give me a call and ill walk you through it. 443-340-3086 Bob

My 5.0 did the same thing untill I did this. I have done this to many others and fixed them too. It should take you about 30-45mins to do this. Take your time and make sure the engines warm. Otherwise youll have to do it all over again.

sorry for being a noob, but what does IAC stand for? And i just had my TPS set to .98 with chris

RTW0223 wrote:sorry for being a noob, but what does IAC stand for? And i just had my TPS set to .98 with chris

Don't worry I had the same question before.

IAC - Idle Air Control Valve

where is that located at? i am going to do this when i go home for t-giving

RTW0223 wrote:where is that located at? i am going to do this when i go home for t-giving

That you'd have to ask Bob. I asked him about mine and he said that this trick was said to work by FORD but only on there stock engines, so I don't know if it will work on yours or not. You'll have to call him, he's gone off to OC.

i have time. lol.

RTW0223 wrote:i have time. lol.

Just letting you know.

The92Foxbody wrote:

RTW0223 wrote:where is that located at? i am going to do this when i go home for t-giving

That you'd have to ask Bob. I asked him about mine and he said that this trick was said to work by FORD but only on there stock engines, so I don't know if it will work on yours or not. You'll have to call him, he's gone off to OC.

Okay to clearify, There is a procedure that works on a stock engine. The instructions that I posted will only work on modified engines. IAC is the "I"dle "A"ir "C"ontrol valve. TPS is the "T"hrottle "P"osition "S"ensor. The IAC has a single 2 wire connector on it and is located on the side of the throttle body. The TPS is located on the top of the throttle body. Im not going to be near my computer till monday night so if you need anymore info call me. 443-340-3086

okay ill keep this all in mind. I am not going to do this until i get my o2 sensors from home, but i will let you know