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5.0L (1979-1995) Mustang Technical discussions on 5.0 Liter Mustangs within. This does not include the 5.0 from the 2011 Mustang GT. That information is in the 2005-1011 section.

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Old 06-01-2006, 04:58 PM   #1
92hatchLX
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Default Surging idle sticky!!!! Here it is!!!

Mods, please put this as a sticky!!!

Solving idle problems:


Solving Idle Problems Part 1 The IAC motor

Does your idle rise and fall over and over again? Does your 'Stang stall when you come to a stop, or even when you put it in gear? Well if so then this series of articles if for you.
In this age of fuel injection, idle problems can literally be caused by hundreds of different things. What this series of articles is aimed at is how to fix the most common idle problems found in the Mustang.
The fuel injected Mustang uses a small motor/valve assembly that allows a specific amount of air to enter the engine to control it's idle. This valve is comonly called the idle motor, the IAC (idle air control) or the IAB (Idle air bypass). When your car is new the IAC works remarkably well. The problem arises when the car gets some miles on it (usually 75K+) and carbon fouling takes its toll.
What happens is dirt, excess air filter oil, and most notably carbon gunks up the IAC valve and doesn't allow it to either open or close properly. This can cause an really high idle, a lumpy/surging idle or no idle at all. The solution is to either replace or clean the IAC. Obviously we are going to do the later and here is how.

1. What is needed?
- 8mm or 5/16 socket and ratchet
- Can of Carburetor/throttle body cleaner

2. Locating the IAC valve:
Depending on the year of your Mustang the IAC can be in a few different locations.
- On 5.0L Fox body cars the IAC is bolted to the side of the throttle body.
- On SN-95 cars it is bolted to the intake manifold
- On 4.6L DOHC and SOHC engines the IAC is located on the upper intake manifold.
In reality once you know what one looks like (picture 2 "B" from a 94-95, fox body's have longer silver IACs) you should have no trouble finding yours. They all look basically the same except pre-94 cars were made of metal and are silver, 94-up cars are black plastic.

Picture 1

3. Removing the IAC:
This part, like the rest of the steps in this article, is simple. The IAC has an electrical plug that needs to be disconnected (picture 2 "C") Then all you have to do is remove the two 8mm-5/16" bolts (Picture 2 "A") that hold the IAC to the throttle body or intake. Watch out for the IAC motor to throttle body gasket, don't lose it!

Picture 2

4. Cleaning:
Next all you do is use the carb cleaner to clean the carbon out of both of the holes (picture 3 "C") in the IAC valve and both of the holes in the intake/throttle body.

Picture 3

5. Putting it back together:
Yet another self explanatory step. Put the IAC in place and install/tighten the 8mm bolts. Make sure you don't forget to reinstall the gasket.

6. Fire her up!
Finally you need to start the car and let it run for a few minutes to burn any leftover carb cleaner in the intake. You may have to crank the engine a little more than normal to start it for the first time and don't worry about the white puff of smoke you see coming from the exhaust because again it's just the carb cleaner.

Solving Idle Problems Part 2 Cleaning the throttle body plate

In part one we covered the most common problem that will cause idle problems, the IAC, but there is an often overlooked area of the intake tract that also regulates how much air enters the engine. That part is the throttle body plate (picture 2 "A").
The throttle body plate is a part inside the throttle that flips open when you press on the accelerator and the amount it opens depends on how much you press the accelerator.
Well just like the IAC when the throttle body gets gunked up with carbon it can stop it from closing all of the way causing idle problems. So the obvious next step? Clean it!

1. What is needed?
- Flathead screwdriver
- Can of carburetor/throttle body cleaner

2. Getting to the throttle plate
The first step is to remove the air duct that goes between the throttle body and MAF (mass airflow sensor). There's not much work to be done because all that holds that air duct is two clamps that need to be loosened with a flathead screwdriver. (or an 8mm or 5/16 socket)
On certain engines/models you may need to remove wires or small hoses from the air duct like the one pictured.

Picture 1

3. Cleaning the throttle plate
Look inside your throttle body. Do you see all of the gunked up carbon? (that black stuff) Well that's what you need to get rid of with your carb cleaner so get to it!
You're going to want to either hold the throttle open with your hand or have somebody do it for you by pressing on the accelerator so you can clean all around the throttle plate. You might also want to put a shop rag or paper towels under the the throttle body to catch all of the carb cleaner before it goes all over your fender. The Throttle body in picture 2 was taken off the car for illustrative purposes.

Picture 2

4. Putting it back together
Replace the air duct, and any wires you disconnected. Then start her up!!!
Again, just like when cleaning the IAC, don't worry if it takes a little longer to start. Also dont worry about that puff of smoke from the exhaust.


Solving Idle Problems Part 3 Resetting the base idle

The base idle is nothing more than the idle that the computer has learned at whatever angle your throttle body is set at when at idle. This technically shouldn't change by itself but it's always good to check it because you never know what the dealer or previous owner did to the car before you had it.
Once you change the setting of the base idle you will definately want to go onto step 4 in this series of articles because if you move the base setting of the idle plate you also change the tps voltage at idle.

What is needed?
- Flathead screwdriver
- You'll also need a wrench to remove your battery cable

How to do it
1. Disconnect the battery's negative terminal to reset/clear the computer's memory. Leave it disconnected for 30 minutes. WARNING: Whenever removing your battery cables ALWAYS disconnect the negative battery cable first and reconnect it last. Otherwise you could destroy your computer or cause a battery explosion.
2. Disconnect the plug going to your idle motor which is located on the front of your throttle body.
3. Reconnect your battery's negative terminal.
4. Start the engine, and set the idle to the rpm you want with the stop screw on the bottom of the throttle body.
5. Turn off the engine.
6. Reconnect the plug on the idle motor
7. Make sure all accesories (radio, blower motor, a/c, lights, etc) are off and start the engine.
8. Let engine run for two minutes.
9. Turn engine off and wait two minutes then restart engine and let idle for two minutes with all accesories on.


Solving Idle Problems Part 4 Setting the TPS voltage

If you haven't already read how to do it a million times in all of the Mustang magazines then this is your chance. Here's how to check and adjust your TPS voltage. You may be asking, "What is a TPS"? Simply put its the Throttle Position Sensor. The TPS is a sensor that tells the ECM (computer) how much gas
you are giving it, which in turn lets the computer decide how much fuel and spark to give you.

What is needed?
- Phillips head screwdriver
- Two small straight pins
- Digital multimeter

Picture 1

How to do it
The TPS (picture 2 "B") basically bolts to the throttle body with two screws. (picture 2 "A") The holes that the screws go through on the TPS are elongated so you can twist the TPS in either direction to adjust it. To adjust the TPS all you have to do is loosen (don't take them out) both screws and move the TPS until you see the right voltage.

Picture 2

To hook the multimeter up you will have to pierce the green wire with a pin so you can check how much voltage is going through it. The red or positive (+) wire on your multimeter will go to the pin in the green wire. Then
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Old 05-03-2008, 02:55 PM   #2
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Default RE: Surging idle sticky!!!! Here it is!!!

help !!! 302 motor 1995GT, street heat top end build headers 75mm throttle body all the mods. Theidle pulsates up and down till it stalls.
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Old 05-03-2008, 03:55 PM   #3
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Default RE: Surging idle sticky!!!! Here it is!!!

1. Start with the basics first, ask the EEC what is working with = DIY KOEO/KOER/Cylinder Balance Self-Test

2. Make sure your base idle is set correctly = Summarized/Corrected Base Idle Reset Procedure

3. Make sure you don't have any vacuum leaks = Engine Vacuum Leaks

4. DO NOT adjust the TPS to the mythical .9999 vdc setting, it's not required as long as the closed throttle value is between .7 - 1.0 vdc.

LUK what shows up from doing those chores.
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Old 05-03-2008, 06:30 PM   #4
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Default RE: Surging idle sticky!!!! Here it is!!!

Where was this copied and pasted from?
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Old 05-03-2008, 06:58 PM   #5
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Default RE: Surging idle sticky!!!! Here it is!!!

*sigh* Sorry, I can longer lurk on these forums, I have to comment on this subject.

Why do people keep doing write up's on this subject that outline adjusting the TPS? Measure it's voltage at idle, if it's between .7 and 1.0 vdc the computers happy with it and all is well with it. It [the EEC-IV] is self adjusting for all intents and purposes, it'll adapt unless it is outside that range.

Also 99% of surging idle issues happen after airflow has been increased(anything from adding a bigger MAF, TB, heads, intake, or cam). There is a point were the EEC-IV simply cannot adapt to the increased air-flow. The other 1% is a Vac leak or a failed sensor.

Guess what, the correct answer is to fix the EEC-IV and make it understand there is more air flow now via the "Thottle Body Air Flow" Scalar, it's measured in Kg/Hr. Increase it in .1 intervals until the surge goes away. And while your at it input the correct MAF Transfer Tables and the Injector Slope High and Low values(= to injector rating +/- .25)
Everything else is quick and dirty fix which tricks the computer at best. So stop screwing around with these hoaky fixes and FIX the cause, not the symptoms with an EEC-IV tuner. If you wanna play with big EFI horse power parts your gunna have to pay.
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Old 05-03-2008, 08:00 PM   #6
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Default RE: Surging idle sticky!!!! Here it is!!!

joel and stern make a lot of good points,

dont trick the computer actually fix whats wrong with it and tune correctly
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Old 07-06-2009, 10:22 AM   #7
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It takes two seconds to set it as close to .99 as possible. You won't, but get it as close while your there.
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Old 07-06-2009, 10:36 AM   #8
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Quote:
Originally Posted by cholericfc View Post
It takes two seconds to set it as close to .99 as possible. You won't, but get it as close while your there.
What for?..... you don't need to set the TPS to a specific value, it does not cause any gains or solve anything. Just like suggesting to set the lifters preload between .005" - .006".
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Old 07-06-2009, 10:36 AM
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