Quote Originally Posted by twistedsymphony View Post
The engine harness should be sealed from the factory. you should be able to submerge the car in water and as long as the coils and intake stay dry it will run.
But the cylinder coils are not sealed (don't take forum posts too literally). Several of us have fallen victim to sporadic misfiring due to getting water down in one or more of the spark plug wells after power-washing then engine. It's very easy to d to yourself, and it doesn't always light the check-engine light, either.

If you are concerned that this may be happening to you, remove each coil (two Philips- and hex-head screws on each coil, but use a socket, not a screwdriver) and take a peek down in there. If you see anything wet, that may be the problem. The moisture seems to ground the charge to the engine instead of firing the spark plug. Check them all - it's easy to do. If you see water or noticeable moisture, let it dry it out and reinstall the coil.