coil pack?

[tmbfi]

My '92 Ford Explorer has been misfiring for a while and the mileage has dropped form 20 mpg to 15 mpg. I replaced the wires and plugs (#5 was badly fouled ...probably lower intake leak). For kicks I put my timing light on each of the wires while the engine was running and the light seemed to be firing intermitantly depending on which one it was hooked up to. I thought when coil packs went bad they just stopped working completely...any thoughts?

 

[Runnin'OnEmpty]

Tmbfi, the Explorer uses a "lost spark" ignition system that fires each spark plug on every crank revolution. This is twice the number of sparks that is needed, and the spark on the exhaust stroke is much weaker than the one on the power stroke. That's why a timing light will pick up all sorts of crazy readings.

A better test is to check the resistance of the individual coils in the pack with an ohmmeter.

Maybe you simply had a bad plug or wire on number 5?

 

[Opera House]

Where is it lost to?

If you notice on schematics, the DIS ststem has three coils and six plugs. Basically the plugs are in series and the coil which is placed in the middle of them has to produce more voltage than older systems. This can cause voltage breakdown in all of the components. The current in the two plug wires of the same coil should be the same unless there is a voltage breakdown in the coil or the wires before the pickup. If the wires and plugs are recent it is reasonable to expect a coil or at least dirt on the top of the coil. Some aftermarket wires don't seat well on the coil. If there are internal engine problems, it is not unusual to have a plug fail in a couple thousand miles. I have a brown coating that forms on #4 abd it needs to be replaced anywhere from 3-15K. Even a shorted plug should make the timing light work.

 

[Runnin'OnEmpty]

Yep, that's correct. Just to add a little more to the mix; the exhaust stroke spark takes less energy because there's no compression when it fires. No compression equals less resistance for the spark to jump the plug gap.

Also, the plugs have reversed polarity on the different banks. This might affect a timing light that was hooked up to the driver's side bank. (I don't really know, since I've never tried it....???)

If the #5 plug fouling was caused by a defective coil, I would suspect that the #1 plug would also show signs of misfires, since both cylinders are fired by the same coil. If #1 is OK, then suspect something else. As OperaHouse said, these systems are very sensitive to arcing.