Help me create the "Surging Idle Checklist"

Here's what I know about MAFs. Some of this may be review, but here it is. As to actual MAF output, the MAF manufacturer should be able to provide you with a set chart or spreadsheet showing the voltage output at varying engine RPMs under normal driving load. The one I have is for a stock engine and stock MAF.

Make sure your air intake system uses the stock airbox & filter. If you use some other system, make sure the filter does not pull air from the engine compartment. The best location is inside the fenderwell and with a minimum of 9 in from any 90 degree bends and the inlet of the MAF. Turbulence within the intake tubing can disrupt airflow into the MAF sensor and cause all sorts of strange problems.

Code 66 MAF below minimum test voltage.

Revised 1-Jun-2013 to add different wiring colors for 91-93 model cars and diagram of the backside of the computer wiring connector

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.


/www.veryuseful.com/mustang/tech/engine/images/91-93_5.0_EEC_Wiring_Diagram.gif
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).

MAF resistance checks – all MAF

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.

91-93 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.


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

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.

See http://en.wikipedia.org/wiki/Differential_amplifier for more help.

http://www.stangnet.com/mustang-for...ing-idle-checklist.698148/reply?quote=8776718

robert fugate said:

My car isnt a DD so i'n not sitting in traffic with a hot engine.. I usually let my engine cool down a bit between each runs. My sample tube is specific for 24# with a air filter in the engine compartment. I've considered making a cold/air, ram air style thing coming from my fog light holes but hear mixed feelings on that.

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You would be wise to C&L support directly. If I remember correctly there are different calibrations for C&L meters depending on what type of filter and cold air arrangement you have. My preference is the stock air filer air box, which runs with all types of meters with minimal problems.

Benboi92GT said:

I went through the checklist and I think I'm good. Reset the base idle per instructions. I do have a question. Does the ignition timing HAVE to be at 10* when setting the base idle (my truck's base timing is ~16* right now)? Do you have to reset base idle EVERY time you advance or retard ignition timing? So basically I set base idle with it at 16*, is that fine?

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The timing doesn't have the be at 10 degrees, and there is no need to reset the base idle unless you radically exceed the 10-14 degree range. More than that range tends to cause a cold idle surge/stall problem.