Help me create the "Surging Idle Checklist"

Discussion in 'Fox 5.0 Mustang Tech' started by jrichker, Apr 10, 2005.

  1. 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.

    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

    Ignition switch wiring

    Fuel pump, alternator, ignition & A/C wiring

    Computer,. actuator & sensor wiring

    Fuse panel layout

    Vacuum routing
    #381 jrichker, Aug 28, 2013
    Last edited: Aug 28, 2013
  2. 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 for more help.
  3. I have removed the information I posted as it was misleading and incorrect. The reason the car idled with the Ford MAF is because the EC was in "limp" mode and ignoring the O2 sensors. I discovered this while reviewing the data log and chasing down the problem with the driver side O2 not reading. I'm up to step nine and still surging.
    #383 fox bodies rule, Aug 31, 2013
    Last edited: Aug 31, 2013
  4. Briefly scanned threw the last few pages, But could you explain why not to mount the air filter in the engine bay?
  5. under the hood air is hot, intake doesn't like the turbulence created by "prop wash" (engine cooling fan) there may be more
  6. My opinion is its more temp related than anything. The air filter should knock down most of the turbulence created by the fan. But Id hazard a guess that the amount of turbulence that could theoretically get into the intake tract could also depend on the type of filter being used.
  7. 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.
    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.
  9. Yes, they are happy to assist you. Here's the webpage info.
  10. Yeah i bought the specific one for my filter being in the engine bay.. Was just wondering why it was a no no
  11. 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?
  12. Does it idle at 16? If yes then your good
  13. Yes, I guess my question is if the ignition time HAS to be at 10* to properly set the base idle? And if you HAVE to reset the base idle anytime you change the ignition timing?

    Is it normal for the upper intake to physically shake a lot during idle? It is erratic and inconsistant (but I have a big cam with a lot of lift). You can feel it in the cab as well, that's how much it physically shakes. However, when the upper intake has these shaking fits the idle itself does NOT fluctuate, nor does the sound of the exhaust or lope change at all. I guess it would best be described as an engine shudder that doesnt affect the idle at all really.

    One last thing, if I manually rev up the engine with the TB under the hood and let it snap back closed, it will surge and then even out. Is that normal? For instance, if I open it up for one big rev (VROOM) and let it snap back shut, it will surge then even out.
  14. Yeah mine surges a bit after snapping it like that. Its hard to say about the intake shaking? Like a broken motor mount? And i wouldnt think you would have to bring it back down to 10 as I never have had to.. Also does it shake a little less if you bring the idle up a bit more? I found mine likes to idle smooth at about 900-950rpm
  15. Yea I dunno. Watch the first 10 seconds of this video and you'll see the shaking I'm talking about. Sometimes its even worse but like I said, doesnt really affect rpm's or sound/steadiness of exhaust. People are telling me that because my cam is so big (Ed Curtis Custom, TFS 170 heads, Moss ported lower) that I may not be getting enough air at idle so I have a bypass plate on the way I'll play with. All my electronics tested good and gave good voltage, tps is at .97 and idles nice and steady 99% of the time at 750 but I'm trying to chase down this engine shudder stuff thats going on.
  16. Are you sure all cylinders are firing? I had a similar prob and it was a bad plug wire. If you have a temp gun check the header tube temps as the engine is fired up after being cold. And make sure all of them are heating up about the same.. cars can still run decent on 7 cylinders.. my car ran a 13.3 1/4 on 7
  17. No temp gun. Any other ways to tell?
  18. If we knew what ECU you are running and if you have a "tune" in it would help immensely, if it's a EEC-IV running a GUFB strategy and A9L calibration for instance I think I can answer most of your questions and even some suggestions as to what might be going on with your setup
  19. 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.
  20. Thanks. It's set around 16* right now and I didn't have any pinging... What are some other symptoms of too much timing if there is no pinging. Just cold start issues and stalling? Would it make the engine physically shake?

    I'm going to check the resistance of all my wires tonight with a dvm. Do I need to pull them completely out or can I leave them routed and just detach them from plug and cap and measure while they are still in there.

    Just got done doing a swap on my 95 F-150 5.0 auto 2wd. TFS 170 heads, ed curtis cam, moss ported lower, 1.6 rollers, and all the supporting hardware. It has the stock truck computer as far as I am aware. Haven't had it dynoed yet, trying to get all the kinks worked out first.