car dies out when coming to a stop

mustang50v8 · May 17, 2009

i know i need to call my tuner, but this started right after the tune and happens every once in a while.

what happens is this: i can drive normally and start up normally, but when coming to a stop the car dies. i can leave the car and gear decelerate, but when i push the clutch in and the rpm's drop the car will just shut off. it doesnt happen all the time but its annoying. i'd say it has happened 5-6 times and twice i have caught it at like 450rpm and gave it a bit of gas to prevent it from dying. any ideas?

the98stang · May 18, 2009

probably related to the IAC. Clean it and reset the battery/PCM. If that doesn't work try cleaning the MAF.

slayr281 · May 18, 2009

^ Sounds good.

Do you have a speedcal? I had a similar problem on my 98 Cobra, in my case, it was related to my Speedcal and T-56 not sending signals to the PCM (because of the blown fuse).

R.J. · May 18, 2009

Try resetting your idle with this procedure and if the car does not respond to this change then replace your IAC. Also have you noticed if it only happens when you have you A/C, defrost or mix turned on?

1. Disconnect the negative terminal to your battery and turn on your headlights for about 10 minutes(there's a lot of speculation as to how long. some people will even say to disconnect the positive terminal and some people will say to leave it disconnected over night.)

2. Disconnect the wiring harness connector to the IAC

3. Turn off headlights and reconnect battery. DO NOT TOUCH THE GAS PEDAL!!

4. DO NOT TOUCH THE GAS PEDAL!! Try to start the car. If the car won't start adjust the set screw on the throttle body a 1/4 - 1/2 turn and try again. Repeat until the car will stay running. It will run rough but that is what you want.

5. Let the car sit and idle (DO NOT TOUCH THE GAS PEDAL!!!) until the car reaches operating temperature.

6. DO NOT TOUCH THE GAS PEDAL!!! Step 5 allows the computer to relearn idle settings. It's pretty cool when you follow my procedure because you can actually hear and feel the car figuring out the proper idle settings. It will go from idling rough to idling smooth as it warms up. Once the car is warmed up verify that the car is idling at around 700rpm. If the car is not idling where you want it, adjust the screw until it hits ~700rpm and then let is sit maybe another 10 minutes DO NOT TOUCH THE GAS PEDAL!!!

DO NOT TOUCH THE GAS PEDAL!!!

Turn the car off. Reattach IAC wiring harness. Now remember we adjusted the set screw on the throttle body. That means that the voltage reading from the TPS sensor has changed. It should read between .96 and .99 volts. Anything outside of that range will cause all sorts of issues including misfires and rough idle.

7. You will have to back probe the TPS connector. With the connector attached to the TPS stick a paper clip into the rubber boot on the connector where the top and middle wires go into the connector. The rubber is very flexible the paper clip will slide in between the rubber and the wire.

8. Set your multimeter to volts. Turn ignition to ON. Then place your multimeter leads on the metal pins. If it comes up as a negative that is okay as long as you are -.96 to -.99. If you get this reading then great you are done and hopefully your issues are gone. If not proceed to step 9

9. This is where and extra pair of hands come in handy. Using a large screw driver you will need to loosen the bolts that hold the TPS. DO NOT USE A SCREWDRIVER THAT IS TOO SMALL BECAUSE YOU WILL STRIP THE BOLTS. They probably will be really tight so you have to really lean on the screwdriver and use some finesse.

10. Once the bolts are loose you will need to move the TPS up or down and continue to read the voltage. Once you get it to the desired setting you can retighten the bolts. What I do is I find .97 and then as carefully as possible I tighten the TPS down. what will happen is the voltage will change usually to .96 or .98 but that it okay. Once you are done with this manually open the throttle body a few times and close it then reread the voltage and make sure it is still within our desired range.

11. Depending upon how loose your set screw was you might want to cover it in RTV to hold your setting. At this point you have CORRECTLY reset you idle.

squeak93 · May 18, 2009

The above is good info but the tps really isn't needed. The computer relearns 0 or closed every time you start the car. If adjust itself. now it doesn't hurt to check it but you don't have to be exactly on .97 or what ever. I'd have to look it up but somewhere around 1 usually lets the sensor work better but you have a HUGE window there (like .75 to 1.5 or something crazy that the puter will learn from).

that being said the rest of the info is spot on.

I always have luck clearing my kam memory by unhooking the battery and stepping on the brakes for 30 seconds. Hook it back up and the memory/cel lights or what ever are cleared.

Good luck. I would guess, just a guess that you iac is either dying or the tuner messed with the factory rpm deceal (or what ever the hell its called). Another possibility is like stated that ur idle is too low but only effects you when you have a high current draw from a/c-defrost or something like that.

mustang50v8 · May 18, 2009

i dont normally ride with the ac or defrost on. just the fan. so its not that. but i will try the method r.j. suggested. and clean the IAC

R.J. · May 18, 2009

If you try cleaning it, don't be surprised if it gets worse. Some times cleaning the IAC makes the problem worse. Due to the shape and design of the valve you will not be able to 100% clean the orifice inside the valve and might even make it more dirty when you clean the areas that you can access and move the carbon around. Again this does not happen always but I have seen cars get much worse after cleaning the IAC. If you end up replacing it make sure you go with the OEM one and check out the PM I just sent you.

pedro · Aug 31, 2016

R.J. said:
Try resetting your idle with this procedure and if the car does not respond to this change then replace your IAC. Also have you noticed if it only happens when you have you A/C, defrost or mix turned on?

1. Disconnect the negative terminal to your battery and turn on your headlights for about 10 minutes(there's a lot of speculation as to how long. some people will even say to disconnect the positive terminal and some people will say to leave it disconnected over night.)

2. Disconnect the wiring harness connector to the IAC

3. Turn off headlights and reconnect battery. DO NOT TOUCH THE GAS PEDAL!!

4. DO NOT TOUCH THE GAS PEDAL!! Try to start the car. If the car won't start adjust the set screw on the throttle body a 1/4 - 1/2 turn and try again. Repeat until the car will stay running. It will run rough but that is what you want.

5. Let the car sit and idle (DO NOT TOUCH THE GAS PEDAL!!!) until the car reaches operating temperature.

6. DO NOT TOUCH THE GAS PEDAL!!! Step 5 allows the computer to relearn idle settings. It's pretty cool when you follow my procedure because you can actually hear and feel the car figuring out the proper idle settings. It will go from idling rough to idling smooth as it warms up. Once the car is warmed up verify that the car is idling at around 700rpm. If the car is not idling where you want it, adjust the screw until it hits ~700rpm and then let is sit maybe another 10 minutes DO NOT TOUCH THE GAS PEDAL!!!

DO NOT TOUCH THE GAS PEDAL!!!

Turn the car off. Reattach IAC wiring harness. Now remember we adjusted the set screw on the throttle body. That means that the voltage reading from the TPS sensor has changed. It should read between .96 and .99 volts. Anything outside of that range will cause all sorts of issues including misfires and rough idle.

7. You will have to back probe the TPS connector. With the connector attached to the TPS stick a paper clip into the rubber boot on the connector where the top and middle wires go into the connector. The rubber is very flexible the paper clip will slide in between the rubber and the wire.

8. Set your multimeter to volts. Turn ignition to ON. Then place your multimeter leads on the metal pins. If it comes up as a negative that is okay as long as you are -.96 to -.99. If you get this reading then great you are done and hopefully your issues are gone. If not proceed to step 9

9. This is where and extra pair of hands come in handy. Using a large screw driver you will need to loosen the bolts that hold the TPS. DO NOT USE A SCREWDRIVER THAT IS TOO SMALL BECAUSE YOU WILL STRIP THE BOLTS. They probably will be really tight so you have to really lean on the screwdriver and use some finesse.

10. Once the bolts are loose you will need to move the TPS up or down and continue to read the voltage. Once you get it to the desired setting you can retighten the bolts. What I do is I find .97 and then as carefully as possible I tighten the TPS down. what will happen is the voltage will change usually to .96 or .98 but that it okay. Once you are done with this manually open the throttle body a few times and close it then reread the voltage and make sure it is still within our desired range.

11. Depending upon how loose your set screw was you might want to cover it in RTV to hold your setting. At this point you have CORRECTLY reset you idle.

Sorry to resurrect the thread. I'm having this issue on my 99 GT. There's a very rough idle (woes from 500-1200rpm) when I come to a stop, but only when my A/C is on. What could it be?

wmburns · Aug 31, 2016

Here's some information that should be more helpful for your 1999 GT.

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

Note, if the AC is low on Freon that will prevent the PCM from correctly learning the AC on IAC idle trim value. Expect a poor AC on idle if low on Freon.

car dies out when coming to a stop

mustang50v8

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slayr281

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R.J.

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squeak93

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R.J.

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pedro

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car dies out when coming to a stop

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