Blown coil on 99 Grand Marquis LS.

The coils themselves do fail occasionally, but the most common failure is the spark plug boot. The boot is a servicable part and should be changed at ~80K miles or so just like spark p;ug wires.

yes, its ok to just disconnect the battery for a few minutes to clear the code.

???

I still have the blown part, and it doesn't seem to be serviceable.

Do you mean I can take the rubber part and its connector (the big spring) from the coil and just replace those?

Thanks all of you...

Just the rubber part. The spring will pull off too but I don't believe it's available as a separate part. Grab the coil in one hand, the boot in the other near the coil, twist and pull. Look closely at the rubber around the plug opening. If it's bad, there will likely be a carbon track (uneven grey\black color). If not there look at the side of the boot. You may see a tiny burnt spot with a grey corona where the spark was leaking through to the cylinder head. If there are signs of liquid (oil\coolant) on the boot, the moisture contaminated the spark plug well and caused the spark to bypass the plug. If you know how to use an ohm meter, you can check the windings in the old coil. Check across the connector terminals, then from the spring contact to the metal insert in the screw hole. I don't recall the spec, but they should both show continuity. The coil could very well be bad, but I've seen the boots fail WAY more often.

I've had several vehicles with these coils and only replaced one (most miles on anyone vehicle was 147,000). There is better than a 50/50 chance that the actual coil was OK, and just the plug boot was bad (or the plug itself). The plug boot is the short connecting wire between the coil and the plug. They fail at least as often as the coils and when they fail it is difficult to tell that failure from a coil failure. Ford has an elaborate diagnostic procedure for this, but the amount of labor need to complete the diagnosis is greater than the cost of just replacing the coil, boot, and plug. Therefore most shops just replace all three anytime a miss-fire for a particular cylinder is indicated. I always recommend replacing the plug boots whenever you replace the plugs. They are cheap compared to replacing even one coil, boot, and plug combination.

Just disconnecting the battery would have reset the light, but then just waiting for a few days would have reset it as well. Once the problem was fixed, the code would have eventually cleared when the PCM could no longer detect the misfire.

Ed

I didn't have my DBM with me (coil is at work), but I couldn't see any traces of either contaminants, carbon track, so on...

I'll try to think about bringing my DVM to work (or the coil at home :)