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  1. #1
    FEP Power Member Ourobos's Avatar
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    Default Random RPM dip after Mass Air conversion

    This makes two 86s in a row that this has happened, never figured out the last one before it was sold.. I have a bone stock 86 5.0, did a proper mass air conversion :

    https://www.stangnet.com/2002/01/23/...ir-conversion/

    Fires right up, but now there's a random dip in the idle. It can be idling at 800, quick dip it 200 or so then right back to 800. Might happen every minute, 30 seconds or five minutes. Any ideas? Here's a video of it.

    https://youtu.be/rUyOUr8j58g
    1986 CHP SSP Coupe

  2. #2
    FEP Super Member erratic50's Avatar
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    Get a can of throttle body cleaner.

    with the engine off remove the air intake tube and open the throttle body wide open using the accelerator linkage on the bottom.

    spray the holy buzzjesus out of the back side of the throttlebody. Use 2/3 can.

    now start the car and shoot cleaner in while revving it up until most of it is gone. Go to idle and drown it out, let it sit.

    Pull the negative battery cable for an hour so the ECU loses its ideas on what the engine wants to run, etc.

    Readjust idle set screw according to shop procedures.
    Readjust TPS according to correct shop procedures.

    Mark the distributor then loosen the hold down. Pull the advance stub then hook up a timing light and start the car. set timing to 10 before TDC. once you are at 10 reconnect the snub and snug the holddown down.

    Or if you prefer a lot better performance..... make sure you only run premium fuel, but turn it up by adding more base timing. There's an easy 25 HP and 40 lbs of torque to be had from most 5.0HO motors just by removing the intake silencer and adding base advance. Advance the timing until the motor just starts speeding up then back just a hair. Shut the car off and try to start it. It it kicks back against the starter or runs hot it's too advanced.

  3. #3
    FEP Power Member Ourobos's Avatar
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    I plan on upgrading the intake soon and will check all that, but whybwoukd it not do this when speed density? Almost seems electronic, like it's losing signal or something is changing momentarily then it recovers.

    Thanks
    1986 CHP SSP Coupe

  4. #4
    FEP Power Member Ourobos's Avatar
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    Well, the 86s did not have cruise control (neither of mine did / do). Apparently the VSS wires that are ran to the harness need that signal form the VSS sensor.. I'll have to purchase a sensor and new speedometer cable it seems.. I think that will fix it.. Thanks
    1986 CHP SSP Coupe

  5. #5

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    What PCM are you running?

  6. #6
    FEP Power Member Ourobos's Avatar
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    A9L. 5 speed car
    1986 CHP SSP Coupe

  7. #7
    FEP Senior Member dtmilsap's Avatar
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    86 Capri's 5.0 have cruise control, I would guess depending model of Mustang they would have it too.

    Sent from my SAMSUNG-SM-G900A using Tapatalk

  8. #8
    FEP Power Member Ourobos's Avatar
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    Yeah these two didn't.. One was an optionless 86, and this is an SSP which don't have cruise.. I plan on crawling under it today to verify no VSS sensor. I will then put one in place if so and update if that solved it.

    Thanks
    1986 CHP SSP Coupe

  9. #9

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    I don't believe this would related to the vss. The vss produces no voltage at 0 mph

  10. #10
    FEP Power Member Ourobos's Avatar
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    How would the PCM know that though, if it has no signal from the lack of the VSS? Lack of signal does not equal zero voltage, correct?
    1986 CHP SSP Coupe

  11. #11

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    The vss creates its own ac voltage. But only when the gear is spinning, the faster it spins the higher the frequency. If your issue is at idle parked it's not the vss. Your issue seems like a sudden loss of power. I would check all hots and grounds to pcm, any splices in harness you made to send vref to maf. Battery cables ignition switch. It also could be a sharp fluctuation in a/f mixture. Try running it with the maf unplugged and see if it helps

  12. #12
    FEP Power Member Ourobos's Avatar
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    Nah, had ZERO issues when speed density the same day, and everything is soldered and crimp connected.. It's happened twice in a row with two non-cruise cars that had no VSS. I also have a friend who had the same problem. Another bud who had cruise and a VSS had no problems. Internet if full of similar issues, as the conversion write up are for VSS equipped 87+ cars.

    I was thinking, the FMS conversion harness does not require splicing into these. I am going to remove those wires first, and if that doesn't solve it, I am going to add a VSS with wiring and grab a new speedo cable as well.

    I crawled under the car and there is no wiring on the transmission harness, there is a looped pink wire, but that's it. Should be a green and a orange wire.

    Will update, might be a few days - I plan on doing all this as I do an intake, exhaust and smog delete upgrade.
    1986 CHP SSP Coupe

  13. #13
    FEP Super Member erratic50's Avatar
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    Sounds like an adventure...... I didn't have any major issues going to an A9L when I converted my 86. My car has cruise from the factory. I run a 1993 cobra 70MM MAF and a 65MM tb and a Typhoon on mine.

  14. #14
    FEP Power Member Ourobos's Avatar
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    Yeah, I think that's the deciding factor, thanks bro
    1986 CHP SSP Coupe

  15. #15
    FEP Power Member Ourobos's Avatar
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    Wanted to update.. I installed a VSS, no change.

    Being as the car sat for nearly 10 years, I installed new Motorcraft coil and TPS along with a complete tune up.

    NO change.

    Installed a new distributor and TFI module today, problem solved. Guessing hall effect / PIP in distributor was on it's way out.

    Thanks
    1986 CHP SSP Coupe

  16. #16
    FEP Super Member erratic50's Avatar
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    Very good guess! Well done!

  17. #17

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    Weird i'm having issues with my maf conversion as well. Won't idle and no throttle response

  18. #18

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    I wonder if the 86 dizzy didn't have the vane with the different cut to signify cyl 1?

  19. #19
    FEP Super Member erratic50's Avatar
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    I've never had any problems converting an 86 to MAF while retaining the factory dizzy. The only thing I'll say is that the timing advance curves vary widely between the different ECUs. What you could get away with on one vs the another is very different. Just my experience.

    VM1 to A9L ..... even base timing is pretty much direct. On both I was able to run 13.5 based advance and no more. It would ping at WOT even with premium. (1/2 million mile 86 engine with 65MM, typhoon, headers, etc.)

    When I jumped up to an X3Z I will call it death by 1000 cuts. The timing up top is a lot less advanced. Several degrees. I tried to put more base in. I was able to get away with 14.5 with premium fuel but could not move up base advance any more. I picked up an obvious ping mid-range at 15. No surprise there with mildly worked E6 heads still in place with the Typhoon and other stuff.

    I have up at least 1 car length in the 1/4 mile but drivability drastically improved. Runs WAY better low/mid and part throttle cruising.

    I'll eventually repair my A9L and tune for the injectors and revisit this stuff.

    Bear in mind that my cam is probably hopelessly retarded by now which costs you some degrees of base advance but also affords a guy some RPM.

    With the A9L it pulling to 6100-6150. With the X3Z it is all done by just past 6000. Give me a little more room before the rev limiter when I power shift it I guess.

  20. #20
    FEP Power Member 306gt's Avatar
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    My son's 86 did this after we converted it. I did alot of reading on it. Ford used a diode on the 86 cars in the wiring going to the I.A.C valve. The diode is located right before it plugs into the valve. Now the valve did not have diode in them for 1986. 87-93 cars had the diode in the I.A.C. valve not in the harness. If you use a Replacement I.A.C. valve it will have the diode as well as your harness. You will have 2 diodes. Not good. On my son's car I ended up using a 86 iac valve and left the diode in the harness. It cured the idle hunting. Some guys will bypass the harness diode and use the 87-93 I.A.C. valve. I think the valve with the diode in it has a x stamped on it.
    85 G.T. All motor
    337 c.i.d 11.44-120 mph

    1984 1/2 G.T. 350 (13.01-106 mph)

    1984 G.T. (Daughters car)

    1986 G.T. (Son's car) (12.99-105 mph)

  21. #21
    FEP Power Member 306gt's Avatar
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    Read this thread carefully on four eyed pride
    (Wanted Dead or alive hanging/ surging idle solved) it goes into detail about it.
    85 G.T. All motor
    337 c.i.d 11.44-120 mph

    1984 1/2 G.T. 350 (13.01-106 mph)

    1984 G.T. (Daughters car)

    1986 G.T. (Son's car) (12.99-105 mph)

  22. #22
    FEP Super Member xctasy's Avatar
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    Quote Originally Posted by 306gt View Post
    Read this thread carefully on four eyed pride
    (Wanted Dead or alive hanging/ surging idle solved) it goes into detail about it.

    Pure Tech Mr 306GT



    "http://vb.foureyedpride.com/showthread.php?134024-Wanted-dead-or-alive-Hanging-Surging-Idle-SOLVED!!"
    http://vb.foureyedpride.com/showthre...ng-Idle-SOLVED!!

    60 posts, three pages. Read every one.

    Quote Originally Posted by erockk View Post
    it wired into the connector coming off of the idle air controller.


    mine was in the wiring harness (not my engine but the best picture i could find)



  23. #23
    FEP Super Member xctasy's Avatar
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    Quote Originally Posted by Ourobos View Post
    Wanted to update.. I installed a VSS, no change.

    Being as the car sat for nearly 10 years, I installed new Motorcraft coil and TPS along with a complete tune up.

    NO change.

    Installed a new distributor and TFI module today, problem solved. Guessing hall effect / PIP in distributor was on it's way out.

    Thanks



    The 1986 Passenger car Port EFi was sequentially injected, but still speed density.

    Ford attempted to do a lot of extra, complicated stuff without the customary Crank Position sensor it only used in the 79-84 Duraspark III equipped Feedback engines like the Truck 335 series 351M, the Lincoln/Mercury 5.0 Variable Venturi, Lincoln/Mercury/Thunderbird 5.0 Central Fuel Injected.

    Ford made a variation on the MISAR Mircroprocessor Ignition Sensor and Automatic Advance Regulator system. That was a GM system with a Crank Position Sensor. Even back in the mid 70's, GM with its Oldsmobile Toronado and Cadillac Seville's first Gen Olds 350 cube V8, they went with a Bendix Corp analog computer 2bbl throttle body. It was Port Bank Fire EFi in 1975-1/2 to 1979 and some Cali 1980's. Some non EFi 1977-1978 Oldsmobile 403s used it as well. It used a totally solid state distributor with no centrifugal advance, and a crank sensor. Unlike GM, Ford got ride of the CPS, and yet again, Ford was Way Out There in doing all the Sequential control with just a Dual Synch SEFi ignition in 1986.

    Typically, as it was with all early EFi systems, you have to go on a witch hunt to find what signal source is missing.



    For the SEFi 5.0, the permanent magnets in the stock TFi distributor are able to loose magnetism, and the signal to the ECM is not always clean. Replacements have an even worse reliability record. The distributor is "dual synch" where one part of the signal acts as a cam synchroniser, to pulse the injectors sequentially. The other makes a simple Solid State ignition spark.

    https://www.w8ji.com/distributor_stabbing.htm


    Drag Radial Performance

    Here's a Picoscope waveform of what Ford TFI ignition signals look like. The blue signal is the PIP sensor inside the distributor, short for Profile Ignition Pickup. This is called the PIP signal, and it's how the distributor tells the ignition module and the PCM when a cylinder is at 10 degrees before Top Dead Center. One out of every 8 pulses is narrower than the other 7 are. This is called Signature PIP, and represents Cylinder #1 is at 10 degrees before TDC.

    Once the PCM knows which cylinder is #1, it can fire injector #1 at the correct time. To fire the other 7 injectors, it doesn't need a special cylinder identification. The PCM just counts from 1 to 8 in the firing order - 1-3-7-2-6-5-4-8.

    The green signal is called SPOUT, short for SPark OUTput. The computer receives the PIP signal, modifies the timing of this signal to reflect the ignition advance timing it wants the coil to fire at, and sends this modified signal back to the ignition module. The ignition module then fires the coil based on this SPOUT signal.

    If the SPOUT signal is not present, the ignition module creates it's own SPOUT signal that is identical to the PIP signal, and fires the coil based on that instead. This is what happens when either the PCM loses communication with the ignition module, or when you unplug the SPOUT connector to set your timing. Unplugging SPOUT literally cuts the wire going from the PCM to the ignition module.

    The notches you see in the top of the PIP and SPOUT signals are reflections of the one signal's edge in the other. Presence of these notches indicates the ignition module is receiving a SPOUT signal OK. The TFI ignition system can be standalone. It does not need the PCM in order to fire the coil. It only needs the PCM to alter the timing of the coil firing, to create the proper ignition advance. Without the PCM, the ignition will fire at 10 degrees BTDC every time, if that's where your base timing is set to.

    These ignition signal waveforms were captured using a Picoscope 4423 4 channel digital storage oscilloscope. It's basically the world's fastest graphing voltmeter. Each one of these blue PIP signals lasts for about 11.32 milliseconds at an idle speed of 672 RPM. A typical voltmeter will only update about 4 times per second, or once every 250 milliseconds, which is nowhere near fast enough to capture these events.

    At 672 RPM, one PIP signal lasts about 11.32 milliseconds, or 11/1,000 of a second. At 6,000 RPM, one PIP signal will last for approximately 1.25 milliseconds, or 1/1,000 of a second. A scope could capture that as if it were in slow motion. This scope can measure signals as fast as 1/2 of one nanosecond. Nothing gets past this scope. If you're at 6,000 RPM and one single PIP signal is missing, causing one cylinder to misfire one time, a scope can catch it in the act, which is why a scope is perfect for diagnostics.

    At Drag Radial Performance, we specialize in these classic 5.0L Mustangs. Despite their age, we're moving the hobby forward in technology, picking up where these cars were left off 20 years ago, and applying today's technology in tuning and diagnostics to them. It's a great time to be rolling in a 5.0!
    Pictures in the links. Be careful not to open them in an insecure environment

    "https://fordsix.com/proxy.php?image=https%3A%2F%2Fhosting.photobucket. com%2Fimages%2Fcc501%2Fxecute6%2Fpip_43143986_1918 953674864500_3318110304881082368_o.jpg%3Fwidth%3D1 920%26height%3D1080%26fit%3Dbounds&hash=409a54750b ea0679cb3ed31043505b85"


    See "https://www.facebook.com/DragRadialPerformance/posts/heres-a-picoscope-waveform-of-what-ford-tfi-ignition-signals-look-like-the-blue-/1919035208189680/"

    "https://scontent.fchc1-1.fna.fbcdn.net/v/t1.6435-9/p600x600/43143986_1918953674864500_3318110304881082368_n.jp g?_nc_cat=108&ccb=1-5&_nc_sid=730e14&_nc_ohc=V0hl2C84zecAX-sbbsi&_nc_ht=scontent.fchc1-1.fna&oh=782ca51f667ae5372d689ba04af8b3dc&oe=616C7 A10"

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