'66 with EFI conversion miss/stumble problem

Discussion in 'Fox 5.0 Mustang Tech' started by The rent man, Oct 12, 2010.

  1. '66 Fastback...stock 302 roller...primarily '89 Mustang EFI components....Crown Vic left hand air intake....external Hypertech HY-4018 fuel pump at tank....T5 trans. Motor was professionally rebuilt and car ran fine until sent out for paint...15 months later the engine has an intermittent low RPM miss/stumble that stops abruptly at about 1500-2000 RPM when accelerating, is most noticeable in 3-5th gear. If the car wasn't EFI, it feels almost like it's stumbling and then the secondaries open up and it runs perfect, then when you shift, it starts all over again. On occassion, it doesn't do it at all.

    So far I have:
    Removed upper intake and cleaned everything, new Fel-pro gasket
    Checked for any external vacuum leaks
    Cleaned Idle control motor & throttle body
    Cleaned mass air flow sensor
    Reset throttle position sensor from 1.3v to .99v as instructed
    Base idle re-set procedure
    New ignition coil (old unit read low by 1/2)
    New fuel filter @ tank (old unit seemed restricted, fuel lines were new metal prior to engine)
    Re-lube ignition control module backing
    New temp sensor to computer
    Check fuel pressure...reads steady 32lb @ idle
    More new plugs, wires, cap & rotor
    Check computer for codes....none current or stored
    Both oxygen sensors....one in each header
    Rebuilt Bosch design III injectors 19 lb.
    New fuel pressure regulator
    I have played with the timing up and down the scale, played with the plug gap up and down....nothing helps. Here is some more component info if anyone really knows this stuff good enough to check compatibility.
    Computer: A9S....seems to be a California EEC-IV unit....similar to the A9L
    Maff sensor: F1ZF-12B579-AA
    MAP sensor: E8EF-12A644-A2A

    If anyone has any realistic ideas on what could be wrong with this thing please help me!
    Thanks, Dave
  2. Sounds ignition related.
    Did you use all real ford parts for replacements? That includes the cap and rotor.
    The TFI module original?
    Is it worse when the car is warmed up?
  3. Thanks for responding.

    It feels ignition related to me too.
    All replacement parts are usual Auto Zone items. The new ignition coil was barely within correct range based on the manual. I checked another new one at the counter and it read the same. Both still better than the old one so I thought sure that was it. No change. Second set of Bosch wires and Autolite 25 plugs. Second cap and rotor. No change.
    If the TFI is the ignition control module that is bolted to the side of the dist, it's motorcraft and looks ancient, probably the same one that was on the donor car. This was going to be one of the next things I tried but am tired of guessing with no results.
    Doesn't seem to matter, hot or cold.

    Thanks, Dave
  4. Have you checked for spark leak?
  5. What type of "spark leak" are you refering to?
  6. Dave, autozone parts are horrible.
    They never last.
    Not saying this is your issue, but ALL electrical items imo have to be ford, even if it costs a few more bucks. This includes using for racing wires, cap and rotor.
    A lesson i learned too many times.

    If you land up changing the tfi, do not even consider autozone as an option.
  7. +1

    AZ parts, especially electrical parts are junk.
  8. Either inductance due to wires touching or leaking to ground (near the boots is a common spot). Leaky wires can act up at certain RPM's and not others, and low RPM while under load is one time they can.

    A spray bottle with water can help check for leaks. If one sprays the wires in the dark (be careful of hot, moving engine parts, etc), the leaks can often be seen.
  9. What exactly does the miss/stumble feel like? A true misfire is a shocking lack of power - if you think about it, at least 1/8 of your power is gone if you're misfiring on one cylinder. That is extremely noticable and hard to miss diagnose.

    As you can see from my signature, I've got a EFI swap as well, and I've played the game you are playing only to find in the end that the cheapo Summit wires I had were leaking sparks from the boot to the head. Extremely hard to see at night and impossible to see during the day.

    To see it at night I had to get the car good and hot after a hard drive and then park it in a very dark place and let it idle. Then I would have to look down into the engine bay and look for very, very faint traces of light - every now and again I could see them coming from the boot to the head at several plugs.

    Two things helped immensely. First, get some dielectric grease and put it inside your spark plugs boots to assure you've got a very good conduction path to the plug. Second, order yourself some Accel boots for the wires - they have great electical insulation properties.

    One other issue I once had gave me similar symptoms - a poor fitting rotor cap from Autozone that I put on during a tune up. The cap didn't fit well and would actually flex noticably in your hands; I suspect that it actually physically lifted off the dizzy during acceleration. The Ford part is much sturdier and a replacement Ford cap solved the issue.

    Good luck...
  10. If you do find it to be the wires, do the safe bet.
    Get ford racing wires.
    Not only are they fairly inexpensive but the only ones i've ever ruined, were my fault.