Experiencing a persistent engine misfire, especially right from startup? You’re not alone. A misfire can cause rough idling, reduced power, and that dreaded check engine light. If your OBD-II scanner is showing a P304 code, it specifically indicates a Cylinder 4 misfire. Let’s dive into diagnosing this issue based on real-world troubleshooting steps.
Initial diagnostics often involve checking basic engine parameters. A vacuum gauge test can reveal engine health. A healthy engine should show a steady vacuum reading. In this case, a solid 20 inches of mercury (in-Hg) at idle is a good sign. However, fluctuations during throttle application, like a +/- 3-5″ swing, can indicate a problem when the engine is under load. This might suggest a misfire becoming more apparent as engine RPMs increase. Similarly, a compression test is crucial to rule out major mechanical issues within the cylinder itself. Readings between 200-220 psi across all cylinders, achieved within a few engine cranks, generally point towards good cylinder compression. Consistent readings across cylinders, as observed with 145, 175, 200, 215 psi progression during cranking, further support this.
With vacuum and compression seemingly within normal ranges, the ignition system becomes a prime suspect for a P304 code. The ignition coil, responsible for delivering the high-voltage spark to the spark plug, is a common culprit. Testing the coil’s primary resistance is a standard procedure. Specifications often call for a resistance range between specific pins. An “open” reading (often displayed as ‘oL’ on a multimeter) where resistance should be present is unexpected. However, the plot thickens when a neighbor’s coil exhibits the same “open” reading, and swapping it doesn’t resolve the misfire. This could indicate the testing method or coil specifications need closer examination, or the issue lies elsewhere in the ignition circuit.
To isolate the misfire to Cylinder 4, a cylinder drop test is invaluable. By disconnecting spark plug wires one by one while the engine is idling, you can observe the engine’s reaction. A noticeable change in idle roughness when disconnecting cylinders 1, 2, and 3 suggests these cylinders are contributing to the engine’s overall operation. However, if disconnecting Cylinder 4’s spark plug wire produces no change in idle quality, it strongly points to Cylinder 4 as the source of the misfire. Furthermore, visually inspecting the spark from each plug wire can reveal differences in spark strength. A weak or intermittent spark specifically at Cylinder 4, compared to strong, consistent sparks at other cylinders, further reinforces the diagnosis of an ignition-related issue on Cylinder 4.
Given these findings, the focus should now firmly be on the ignition components of Cylinder 4. While the coil test was inconclusive, the weak spark and cylinder drop test results strongly suggest an ignition problem specific to Cylinder 4. Further investigation should include:
- Spark Plug Inspection: Examine the spark plug for Cylinder 4 for fouling, damage, or excessive wear. Replace if necessary.
- Spark Plug Wire/Boot: Inspect the spark plug wire and boot for damage, cracks, or poor connection. Ensure a secure connection to both the spark plug and distributor/coil.
- Ignition Coil Circuit: Although the coil itself might seem problematic, the “open” reading needs further clarification. Investigate the wiring and connectors leading to the ignition coil for Cylinder 4 for any breaks, corrosion, or loose connections.
By systematically checking these ignition components specific to Cylinder 4, you can pinpoint the root cause of the P304 misfire and get your engine running smoothly again.
