Surging Idle Help!

BossmanPJ said:

Just got my top end rebuild completed a week or so ago. Fixed a lot of vacuum leaks and other known issues. Car is stock with the exception of a cold air intake and exhaust. Having said that.... the car runs amazing at driving speeds and anything above idle. Has no throttle hesitation or any drive ability issues at all.

Here is the issue. I have checked over the surging idle checklist but my car is doing strange things. If I unplug the IAC connector while the engine is running, the idle straightens out a bit. Still not perfect but noticeably better. If i plug it back in, the car sounds like ass again. Spitting and sputtering.

If i unplug the MAF connector with the engine running, there is no change at all in the idle and i feel that is strange. It does not matter if the IAC is unplugged or not when i do this.

Does anyone have a thought on if wither could be bad? Also, i have pulled codes. They are as follows:

KOEO- 66/29
KOER- 98/66

Click to expand...

Code 29 - Vehicle Speed Sensor (VSS) is an electronic sender mounted on the speedo pickup gear on the trans. It works the cruse control for both 5 speed and auto trans cars. The VSS is used to tell the computer to speed up the idle as you slow to a stop. This helps keep the engine from stalling when you slow down for a stop sign or stop light.
Check to see if the electrical connector is plugged into it. Clean the connector & contacts with non flammable brake parts cleaner prior to replacing the sensor, as that may fix the problem. The sensor cost is under $30 and it is easy to replace.


Code 66 or 157 MAF below minimum test voltage.

Revised 10-Feb-2014 to add 95-95 Mustang code 157 and 94-95 ECC diagram

Insufficient or no voltage from MAF. Dirty MAF element, bad MAF, bad MAF wiring, missing power to MAF. Check for missing +12 volts on this circuit. Check the two links for a wiring diagram to help you find the red wire for computer power relay switched +12 volts. Check for 12 volts between the red and black wires on the MAF heater (usually pins A & B). while the connector is plugged into the MAF. This may require the use of a couple of safety pins to probe the MAF connector from the back side of it.

Computer connector for 88-93 5.0 Mustangs

Diagrams courtesy of Tmoss and Stang&2Birds

ECC Diagram for 88-90 5.0 Mustangs


ECC Diagram for 91-93 5.0 Mustangs


94-95 Diagram for 94-95 5.0 Mustangs


There are three parts in a MAF: the heater, the sensor element and the amplifier. The heater heats the MAF sensor element causing the resistance to increase. The amplifier buffers the MAF output signal and has a resistor that is laser trimmed to provide an output range compatible with the computer's load tables. Changes in RPM causes the airflow to increase or decrease, changing the voltage output.. The increase of air across the MAF sensor element causes it to cool, allowing more voltage to pass and telling the computer to increase the fuel flow. A decrease in airflow causes the MAF sensor element to get warmer, decreasing the voltage and reducing the fuel flow.

The MAF element is secured by 2 screws & has 1 wiring connector. To clean the element, remove it from the MAF housing and spray it down with electronic parts cleaner or non-inflammable brake parts cleaner (same stuff in a bigger can and cheaper too).

89-90 Model cars: Measure the MAF output at pins C & D on the MAF connector (dark blue/orange and tan/light blue) or at pins 50 & 9 on the computer. Be sure to measure the sensor output by measuring across the pins and not between the pins and ground.

91-95 Model cars: Measure the MAF output at pins C & D on the MAF connector light blue/red and tan/light blue) or at pins 50 & 9 on the computer. Be sure to measure the sensor output by measuring across the pins and not between the pins and ground.


At idle = approximately .6 volt
20 MPH = approximately 1.10 volt
40 MPH = approximately 1.70 volt
60 MPH = approximately 2.10 volt

Check the resistance of the MAF signal wiring. Pin D on the MAF and pin 50 on the computer (dark blue/orange wire) should be less than 2 ohms. Pin C on the MAF and pin 9 on the computer (tan/light blue wire) should be less than 2 ohms.

There should be a minimum of 10K ohms between either pin C or D on the MAF wiring connector and pins A or B. Make your measurement with the MAF disconnected from the wiring harness.

Actually MAF pins C & D float with reference to ground. The signal output of the MAF is a differential amplifier setup. Pins C & D both carry the output signal, but one pin's output is inverted from the other. The difference in signal between C & D is what the computer's input circuit is looking for. The difference in the two outputs helps cancel out electrical noise generated by the ignition system and other components. Since the noise will be of the same polarity, wave shape and magnitude, the differential input of the computer electronically subtracts it from the signal. Then it passes the signal on to an Analog to Digital converter section inside the computer's CPU chip.


Code 98 - basic computer internal tests not passed before it started processing the request to dump codes. Do a Key On Engine Off test before starting the engine. Wait until you see the 11 or the last of the code dump to complete before pressing any buttons if you have a hand operated scanner. The computer is operating in Limp mode, so fix any codes it dumps prior to doing anything else. Limp mode means reduced power and fuel economy.


See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/

Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

Fuel pump, alternator, ignition & A/C wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

Computer,. actuator & sensor wiring
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

Fuse panel layout
http://www.veryuseful.com/mustang/tech/engine/images/MustangFuseBox.gif

Vacuum routing
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg

did you clean the MAF sensor?

BossmanPJ said:

I did. Figured out that the IAC must be the culprit. I have 12V at the harness. With the IAC removed and 12V applied, nothing happens. No movement or sound. Ordered another.

Click to expand...

did you clean the IAC? that normally fixes the issue as it gets clogged with carbon.

BossmanPJ said:

I did. Figured out that the IAC must be the culprit. I have 12V at the harness. With the IAC removed and 12V applied, nothing happens. No movement or sound. Ordered another.

Click to expand...

If you didn't supply a ground when you tested the IAC, you didn't accomplish anything...

The IAC (or IAB) is an actuator that bypasses air around the throttle to regulate idle speed under computer control. It is spring loaded to the closed position, so under computer command, it opens the little shuttle valve to allow air to bypass the throttle butterfly. More air increases idle speed, less air slows it down.

The IAC is powered from the same circuit as the injectors and has 12 volts on the Red wire going to it anytime the ignition switch is in the run position. The computer provides a ground path to the IAC to complete the circuit. The computer turns the ground on and off many times a second to control the airflow through the IAC. If you connected an oscilloscope to the White/Lt Blue wire on the IAC, you would see a square wave that varied in width and switched on and off between 12 volts and a little less than 1 Volt.





In closed loop, the computer uses signals from the MAF, PIP, ACT, ECT, BARO and O2 sensors to calculate idle RPM and air/fuel ratio. A malfunction from any one of them could cause idle problems and the surging idle common to many Mustangs.

Here's a book that will get you started with how the Ford electronic engine control or "computer" works.

Ford Fuel Injection & Electronic Engine Control 1988-1993 by James Probst: ISBN 0-8376-0301-3.

It's about $20 from Borders.com see http://www.amazon.com/ . Select boo...s helps answer some of your questions. Joe R.

The following are diagrams courtesy of Tmoss & Stang&2birds

Fuel, alternator, A/C and ignition wiring



Complete computer, actuator & sensor wiring diagram for 88-90 Mass Air Mustangs



See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring; http://www.veryuseful.com/mustang/tech/engine/ Everyone should bookmark this site.


TFI module wiring for 94-95 Mustang GT
http://www.veryuseful.com/mustang/tech/engine/images/Mustang-94-95-IgnitionControlModule.gif

Complete computer, actuator & sensor wiring diagram for 91-93 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/91-93_5.0_EEC_Wiring_Diagram.gif

Complete computer, actuator & sensor wiring diagram for 88-90 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

Fuel, alternator, A/C and ignition wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

O2 sensor wiring harness
http://www.veryuseful.com/mustang/tech/engine/images/mustangO2Harness.gif

Vacuum diagram 89-93 Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg

HVAC vacuum diagram
http://www.veryuseful.com/mustang/tech/engine/images/Mustang_AC_heat_vacuum_controls.gif

TFI module differences & pin out
http://www.veryuseful.com/mustang/tech/engine/images/TFI_5.0_comparison.gif

Fuse box layout
http://www.veryuseful.com/mustang/tech/engine/images/MustangFuseBox.gif

Mustang 5.0 Lights and Radio schematic, by TMoss:
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxLights-Radio_diag.gif

87-92 power window wiring
http://www.veryuseful.com/mustang/tech/engine/images/mustang87-92 PowerWindowWiring.gif

93 power window wiring
http://www.veryuseful.com/mustang/tech/engine/images/mustang93PowerWindows.gif

T5 Cutaway showing T5 internal parts
http://www.veryuseful.com/mustang/tech/engine/images/5_Speed_Cutaway_Illustrated.jpg

Visual comparison of the Ford Fuel Injectors, picture by TMoss:
http://www.veryuseful.com/mustang/tech/engine/images/Ford_Injector_Guide.jpg

Convertible top motor wiring http://www.veryuseful.com/mustang/tech/engine/images/mustang88VertTopMotorCkt.gif

Engine mounted fuel injector harness
http://www.veryuseful.com/mustang/tech/engine/images/mustangEngineHarness.gif

Location of the TPS, IAB, and the 10-pin connectors on a 5.0, picture by TMoss:
http://www.veryuseful.com/mustang/tech/engine/images/TPS_IAB_Pic.jpg

Starter circuit
http://forums.stangnet.com/attachment.php?attachmentid=21328&d=1080916057

Alternator diagram for 94-95 Mustangs.
http://www.veryuseful.com/mustang/tech/engine/images/Mustang-94-95-Alt.gif

BossmanPJ said:

I supplied ground when I tested it. Placed a wire to both positive and negative on the battery terminals and to each corresponding pin on the IAC. No movement at all. Not even a sound.

I have read several times that this is a valid test of the IAC. Is this incorrect?

Click to expand...

It won't prove much of anything; read the description of how the IAC works and how Pulse Width Modulation works.

BossmanPJ said:

I have already ordered the IAC so if that does not fix it at least it will be new. Have a buddy dropping by on Monday to assist with wire testing. I'm sure we will find the issue eventually.

There is just so much false reading out there apparently. Glad I found this place. You guys seem to know these cars inside and out.

Click to expand...

Did you ever find a fix for this problem? I'm having the exact same issue with my 1990 mustang lx. Runs fine but randomly starts idle surging, or sometimes after just revving the engine at idle.