Why did #5 die?

Discussion in 'SVT Tech Forum' started by Desert Stallion, Dec 29, 2004.

  1. Desert Stallion

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    Okay, what started out as a simple misfire has turned into a friggin' mechanical pergatory. Here's the scoop...
    -Drivin' along minding my own business, all of a sudden I have a harsh vibration start. I figure I tossed a wheel weight or something, pull over only to discover it's the motor that's developed a hard miss. I limp home and do some basic trouble shooting, decide I've got bad plug wires because one I had an intermittent break in and the others all had high resistance.

    -I rebuild the broken wire, but no change.

    -I measure the coil packs and discover they're all out of spec, so I replace them as well as finishing rebuilding the rest of the plug wires. No change.

    -Now the car has all new ignition stuffs, plugs, wires, coil packs.

    -Just to shotgun parts, I had a set of 24lbs injuctors laying around, so I swapped them in. No change.

    -Now the car has new ignition and injectors, plus good fuel pressure.

    -I run a compression test, and all cylinders are between 175 and 190psi, the lowest being #3, and the miss is on #5, opposite sides of the motor, go figure.

    All this, and the computer is throwing NO CODES. Not even a check engine light for the motor going lean/rich.

    So, new ignition, new injectors, good fuel pressure, good compression throughout the motor, no vacuum leaks that I could find and a strong 19 inches at idle, bearing in mind that the car is running on 6-7 cylinders right now since #5 being gone is somehow affecting #6.

    So WTF is going on?
     
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  2. Desert Stallion

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    Okay, update:

    -Put the scope on the injector harness, and it's fine. Got a good, steady pulse on all 8 injectors, so they're firing.

    -Pulled the valve cover (in 30 minutes might I add, I'm getting waaayyyy too good at taking this engine apart) and I didn't find anything amiss. No broken valve springs, no damaged/missing followers, nothing out of the ordinary.

    -Ran another compression test for *****s and giggles, and still have good compression, showing that the valves aren't damaged.



    Okay, so there's nothing physically wrong short of an imploded piston with that cylinder, and even there it would've shown up in the compression test. It's not fuel, or air, so it's gotta be spark still, but I don't see where the fault is.

    Can the EDIS module fail in a way that would only take out #5 without affecting #3? Because #3 still works from what I can tell. If I pull the plug wire on it the motor responds by dropping RPM's and dogging down like it should. If I pull the wire for #5 nothing happens. The coil is still firing though, and the wire is new, as is the spark plug(s) that I've tried.

    I've still got the valve cover off, anything else I should be looking for?
     
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  3. BensBACobra

    BensBACobra New Member

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    try the general or technical threads, this is the SVT boards.
     
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  4. Desert Stallion

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    That's cool, already started a thread in the 4.6 section. Just figured that since this is about a 4V 4.6L Cobra motor that I'd start in the SVT section.
     
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  5. hersheyman

    hersheyman New Member

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    I think we all assumed that you drive a 5.0 due to the picture next to your title. I really would not begin to know what to do on one of these 32valve 4.6 mod engines. A pushrod 302 is a little simpler than a dual overhead cam modular engine. Good luck to you. Let us know how it goes.
     
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