2003 Grand Marquis problems

[Eric H]

It's probably a bad coil. I work on these every day (In Crown Vic trim) and a misfire is almost always a coil, or Coolant from a leaky intake down a plug hole, but you would have noticed that.

You can test them easily enough by holding the end close to a ground while running, a good one will arc out the end of the boot and then some with a healthy snap, a weak one often won't have enough kick to make it to the end of the boot.

Just do one at a time until you find the weak one, you'll probably set some codes doing this but that's easily fixed.