Customer Concern: 1999 Ford F-250 5.4L has an engine miss and the Check Engine light is on with code P0304 in memory.
Tests/Procedures
1. Check for coolant dripping onto the #4 coil (passenger side rear cylinder) from the hose fitting directly above it. If coolant has dripped onto the coil, tighten the clamp and replace the coil, as it is almost always damaged by the coolant leak.
2. If there is no indication of a coolant leak, check the primary resistance across the two terminals of the coils. The resistance should be 0.5-1.5 ohms on a room-temperature coil. If the engine is warm, the resistance will be slightly higher — up to 2.5 ohms on a coil installed in a hot engine.
3. If the resistance checks OK, pull the coils, check the coil and boot for signs of arcing to the engine, and if OK, swap them with another coil (one of the easier ones to get at) and see if the problem moves with the coil.
Source: From the Identifix Hotline Archive