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Greetings, all. I just finished a lengthy and very frustrating diagnosis effort on my 2000 Focus LE (2.0 SPI, manual transmission) to try to figure out why it's behaving so poorly. Under load it begins to miss (jolt and loose power) but under light acceleration or idle it runs fine. It seems to also miss when trying to maintain speed (like on the freeway). It feels like little jolts. I'm suspecting a weak spark.

The spark is produced by a coil just like it was in the 'good old days'. We don't use points anymore, but the PMC is actually doing the exact same thing the points used to do, which is interrupting the current flow to the primary coil windings. Coils work by passing current through a primary winding. This winding acts like a big electromagnet, and a signiicant magnetic field is generated in the secondary windings. When the PCM (or the points) suddenly stops the flow of electricity in the primary windings, the magnetic field collapses. This change in magnetic field intensity causes a very large voltage to be generated in the secondary windings, which are hooked to your spark plugs. High voltage + little gap = big spark. Add fuel and air and boom, your piston goes flying toward the bottom of its stroke.

This remedial lesson in coil operation is humbly presented as the preamble to my question. My coil pack is new, so I'm assuming (for the time being) that it's functioning properly. The only other variable that has any effect on spark intensity is the current through the primary windings. I'm wondering if a poor ground at the PCM could be causing a lower than expected voltage to pass through the primary windings, which would in turn cause a lower voltage generated in the secondaries and therefore a weaker spark. Does anyone have any feedback on this theory? It's easy enough to test. I'm going to run a 10 or 12 gauge stranded copper wire directly from the negative battery terminal to the ground points for the PCM and see if it fixes anything.

Does anyone know of a good way to test for a weak spark? I know there are spark testers, but they only work while you're in your driveway. Does anyone know if the spark at idle is the same as one would expect to find under load? Does anyone have any feedback on how the Ford coil pack works? It uses only two coils, so two cylinders fire at the same time. Since one is on the exhaust stroke nothing happens. However, I read something somewhere about how one plug fires from center to ground while the other fires ground to center. That doesn't make any sense to me, so if someone has a better explanation I'd really like to hear it.

Thanks in advance for your help!
 

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I do know that the Ford Focus coil pack is a regular customer for replacement. I'v been through two in as many months. Always best to get the Ford one it seems.
 

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First....you're not certain it really is an ignition problem. What you were speculating about...would be a rare problem. I suspect a fuel delivery problem would have the same symptoms....like low fuel pressure, or a MAFS problem.

I never heard of spark intensity being changed by the computer.....it remains the same at all rpms, and only the frequency of the sparking changes. Somebody correct me if I'm wrong about the Focus.

So.....you should be able to test the ignition system at an idle. Spray some water on the wires/coils. See if any current is leaking out. I just touch the wires, and the coil while it's idling. If I get shocked....there's a problem. [strongman]
 
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